TPS7A85 高电流 (4A)、高效 (1%)、低噪声 (4.4 µVRMS) LDO 稳压器

7.3.1 Low-Noise, High-PSRR Output

The TPS7A85 includes a low-noise reference and error amplifier ensuring minimal noise during operation. The NR/SS capacitor (C_NR/SS) and feed-forward capacitor (C_FF) are the most effective way to reduce device noise. C_NR/SS filters the noise from the reference and C_FF filters the noise from the error amplifier. The noise contribution from the charge pump is minimal. The overall noise of the system at low output voltages can be reduced by using a bias rail because this rail provides more headroom for internal circuitry.

The high power-supply rejection ratio (PSRR) of the TPS7A85 ensures minimal coupling of input supply noise to the output. The PSRR performance primarily results from a high-bandwidth, high-gain error amplifier and an innovative circuit to boost the PSRR between 200 kHz and 1 MHz.

The combination of a low noise floor and high PSRR ensure that the device provides a clean supply to the application; see the Optimizing Noise and PSRR section for more information on optimizing the noise and PSRR performance.

7.3.2 Integrated Resistance Network (ANY-OUT)

An internal feedback resistance network is provided, allowing the TPS7A85 output voltage to be programmed easily between 0.8 V to 3.95 V with a 50-mV step by tying the ANY-OUT pins to ground. Tying the ANY-OUT pins to SNS increases the resolution but limits the range of the output voltage because the effective value of R₁ is decreased. The ANY-OUT network provides excellent accuracy across output voltage and temperature; see the Application and Implementation section for more details.

7.3.3 Bias Rail

The device features a bias rail to enable low-input voltage, low-output (LILO) voltage operation by providing power to the internal circuitry of the device. The bias rail is required for operation with V_IN < 1.4 V.

An internal power MUX supplies the greater of either the input voltage or the bias voltage to an internal charge pump to power the internal circuitry. Unlike other LDOs that have a bias supply, the TPS7A85 does not have a minimum bias voltage with respect to the input supply because an internal charge pump is used instead.

The internal charge pump multiples the output voltage of the power MUX by a factor of 4 to a maximum of typically 8 V; therefore, using a bias supply with V_IN ≤ 2.2 V is recommended for optimal dc and ac performance. Sequencing requirements exist for when the bias rail is used; see the Sequencing Requirements section for more details.

7.3.4 Power-Good (PG) Function

The power-good circuit monitors the voltage at the feedback pin to indicate the status of the output voltage. When the feedback pin voltage falls below the PG threshold voltage (V_IT(PG) + V_HYS(PG), typically 89.3%), the PG pin open-drain output engages and pulls the PG pin close to GND. When the feedback voltage exceeds the V_IT(PG) threshold by an amount greater than V_HYS(PG) (typically 91.3%), the PG pin becomes high impedance. By connecting a pullup resistor to an external supply, any downstream device can receive power-good as a logic signal that can be used for sequencing. Make sure that the external pullup supply voltage results in a valid logic signal for the receiving device or devices. Using a pullup resistor from 10 kΩ to 100 kΩ is recommended. Using an external voltage detector device such as the TPS3702 is also recommended in applications where more accurate voltage monitoring or overvoltage monitoring is required.

The use of a feed-forward capacitor (C_FF) can cause glitches on start-up, and the power-good circuit may not function normally below the minimum input supply range. For more details on the use of the power-good circuitry, see the Power-Good (PG) Operation section.

7.3.5 Programmable Soft-Start

Soft-start refers to the ramp-up time of the output voltage during LDO turn-on after EN and UVLO exceed the respective threshold voltages. The noise-reduction capacitor (C_NR/SS) serves a dual purpose of both governing output noise reduction and programming the soft-start ramp time during turn-on. The start-up ramp is monotonic.

The majority of the ramp is linear; however, there is an offset voltage in the error amplifier that can cause a small initial jump in output voltage; see the Application and Implementation section on implementing a soft-start.

7.3.6 Internal Current Limit (I_LIM)

The internal current limit circuit is used to protect the LDO against high-load current faults or shorting events. During a current-limit event, the LDO sources constant current; therefore, the output voltage falls with decreased load impedance. Thermal shutdown can activate during a current limit event because of the high power dissipation typically found in these conditions. To ensure proper operation of the current limit, minimize the inductances to the input and load. Continuous operation in current limit is not recommended.

The foldback current limit crosses 0 A when V_OUT < 0 V and prevents the device from turning on into a negatively-biased output. See the Negatively-Biased Output section for additional ways to ensure start-up when the TPS7A85 output is pulled below ground.

If V_OUT > V_IN + 0.3 V, then reverse current can flow from the output to the input. The reverse current can cause damage to the device; therefore, limit this reverse current to 10% of the rated output current of the device. See the Reverse Current Protection section for more details.

7.3.7 Enable

The enable pin for the TPS7A85 is active high. The output of the TPS7A85 is turned on when the enable pin voltage is greater than its rising voltage threshold (1.1 V, max), and the output of the TPS7A85 is turned off when the enable pin voltage is less than its falling voltage threshold (0.5 V, min). A voltage less than 0.5 V on the enable pin disables all internal circuits. At the next turn-on this voltage ensures a normal start-up waveform with in-rush control, provided there is enough time to discharge the output capacitance.

When the enable functionality is not desired, EN must be tied to V_IN. However, when the enable functionality is desired, the enable voltage must come after V_IN is above V_UVLO1(IN) when a BIAS rail is used; see the Application and Implementation section for further details.

7.3.8 Active Discharge Circuit

The TPS7A85 has an internal pulldown MOSFET that connects a resistance of several hundred ohms to ground when the device is disabled to actively discharge the output voltage when the device is disabled.

Do not rely on the active discharge circuit for discharging a large amount of output capacitance after the input supply has collapsed because reverse current can possibly flow from the output to the input. This reverse current flow can cause damage to the device. Limit reverse current to no more than 10% of the device rated current for a short period of time; see the Reverse Current Protection section for more details.

7.3.9 Undervoltage Lockout (UVLO)

The undervoltage lockout (UVLO) circuit monitors the input and bias voltage (V_IN and V_BIAS, respectively) to prevent the device from turning on before V_IN and V_BIAS rise above the lockout voltage. The UVLO circuit also disables the output of the device when V_IN or V_BIAS fall below the lockout voltage.

The UVLO circuit responds quickly to glitches on V_IN or V_BIAS and attempts to disable the output of the device if either of these rails collapse. As a result of the fast response time of the input supply UVLO circuit, fast slew rate and short duration line transients well below the input supply UVLO falling threshold can cause momentary glitches during the edges of the transient; see the Application and Implementation section for more details.

7.3.10 Thermal Protection

The TPS7A85 contains a thermal shutdown protection circuit to disable the device when thermal junction temperature (T_J) of the main pass-FET exceeds 160°C (typical). Thermal shutdown hysteresis assures that the LDO resets again (turns on) when the temperature falls to 140°C (typical). The thermal time-constant of the semiconductor die is fairly short, and thus the device may cycle on and off when thermal shutdown is reached until the power dissipation is reduced.

For reliable operation, limit the junction temperature to a maximum of 125°C. Operation above 125°C causes the device to exceed its operational specifications. Although the internal protection circuitry of the TPS7A85 is designed to protect against thermal overload conditions, this circuitry is not intended to replace proper heat sinking. Continuously running the TPS7A85 into thermal shutdown or above a junction temperature of 125°C reduces long-term reliability.

7.4.1 Operation with 1.1 V ≤ V_IN < 1.4 V

The TPS7A85 requires a bias voltage on the BIAS pin greater than or equal to 3.0 V if the high-current input supply voltage is between 1.1 V to 1.4 V. The bias voltage pin consumes 2.3 mA, typically.

7.4.2 Operation with 1.4 V ≤ V_IN ≤ 6.5 V

If the input voltage is equal to or exceeds 1.4 V, no BIAS voltage is required. The TPS7A85 is powered from either the input supply or the BIAS supply, whichever is greater. For higher performance, a BIAS rail is recommended for V_IN ≤ 2.2 V.

7.4.3 Shutdown

Shutting down the device reduces the ground current of the device to a maximum of 25 µA.

8.1.1 Recommended Capacitor Types

The TPS7A85 is designed to be stable using low equivalent series resistance (ESR) ceramic capacitors at the input, output, and noise-reduction pin (NR, pin 13). Multilayer ceramic capacitors have become the industry standard for these types of applications and are recommended, but must be used with good judgment. Ceramic capacitors that employ X7R-, X5R-, and COG-rated dielectric materials provide relatively good capacitive stability across temperature, whereas the use of Y5V-rated capacitors is discouraged because of large variations in capacitance.

Regardless of the ceramic capacitor type selected, ceramic capacitance varies with operating voltage and temperature. As a rule of thumb, derate ceramic capacitors by at least 50%. The input and output capacitors recommended herein account for a effective capacitance derating of approximately 50%, but at high V_IN and V_OUT conditions (that is, V_IN = 5.5 V to V_OUT = 5.0 V) the derating can be greater than 50% and must be taken into consideration.

8.1.2 Input and Output Capacitor Requirements (C_IN and C_OUT)

The TPS7A85 is designed and characterized for operation with ceramic capacitors of 47 µF or greater (22 μF or greater of effective capacitance) at the output and 10 µF or greater (5 μF or greater of effective capacitance) at the input. Using at least a 47-µF capacitor is highly recommended at the input to minimize input impedance. Place the input and output capacitors as near as practical to the respective input and output pins to minimize trace parasitics. If the trace inductance from the input supply to the TPS7A85 is high, a fast current transient can cause V_IN to ring above the absolute maximum voltage rating and damage the device. This situation can be mitigated by additional input capacitors to dampen the ringing and to keep the ringing below the device absolute maximum ratings.

A combination of multiple output capacitors boosts the high-frequency PSRR, as illustrated in several of the PSRR curves. The combination of one 0805-sized, 47-µF ceramic capacitor in parallel with two 0805-sized,
10-µF ceramic capacitors with a sufficient voltage rating in conjunction with the PSRR boost circuit optimizes PSRR for the frequency range of 400 kHz to 700 kHz (which is a typical range for dc-dc supply switching frequency). This 47-µF || 10-µF || 10-µF combination also ensures that at high input voltage and high output voltage configurations, the minimum effective capacitance is met. Many 0805-sized, 47-µF ceramic capacitors have a voltage derating of approximately 60% to 75% at 5.0 V, so the addition of the two 10-µF capacitors ensures that the capacitance is at or above 22 µF.

8.1.3 Noise-Reduction and Soft-Start Capacitor (C_NR/SS)

The TPS7A85 features a programmable, monotonic, voltage-controlled soft-start that is set with an external capacitor (C_NR/SS).The use of an external C_NR/SS is highly recommended, especially to minimize in-rush current into the output capacitors. This soft-start eliminates power-up initialization problems when powering field-programmable gate arrays (FPGAs), digital signal processors (DSPs), or other processors. The controlled voltage ramp of the output also reduces peak in-rush current during start-up, minimizing start-up transients to the input power bus.

To achieve a monotonic start-up, the TPS7A85 error amplifier tracks the voltage ramp of the external soft-start capacitor until the voltage approaches the internal reference. The soft-start ramp time depends on the soft-start charging current (I_NR/SS), the soft-start capacitance (C_NR/SS), and the internal reference (V_NR/SS). Soft-start ramp time can be calculated with Equation 1:

Note that I_NR/SS is provided in the Electrical Characteristics table and has a typical value of 6.2 µA.

The noise-reduction capacitor, in conjunction with the noise-reduction resistor, forms a low-pass filter (LPF) that filters out the noise from the reference before being gained up with the error amplifier, thereby reducing the device noise floor. The LPF is a single-pole filter and the cutoff frequency can be calculated with Equation 2. The typical value of R_NR is 250 kΩ. Increasing the C_NR/SS capacitor has a greater affect because the output voltage increases when the noise from the reference is gained up even more at higher output voltages. For low-noise applications, a 10-nF to 1-µF C_NR/SS is recommended.

8.1.4 Feed-Forward Capacitor (C_FF)

Although a feed-forward capacitor (C_FF) from the FB pin to the OUT pin is not required to achieve stability, a
10-nF external feed-forward capacitor optimizes the transient, noise, and PSRR performance. A higher capacitance C_FF can be used; however, the start-up time is longer and the power-good signal can incorrectly indicate that the output voltage is settled. To ensure proper PG functionality the time constant defined by C_NR/SS must be greater than or equal to the time constant from the C_FF. For a detailed description, see application report Pros and Cons of Using a Feed-Forward Capacitor with a Low Dropout Regulator, SBVA042.

8.1.5 Soft-Start and In-Rush Current

Soft-start refers to the ramp-up characteristic of the output voltage during LDO turn-on after EN and both UVLOs exceed their threshold voltages. The noise-reduction capacitor serves a dual purpose of both governing output noise reduction and programming the soft-start ramp during turn-on.

The soft-start ramp is not fully linear as the error amplifier has a several millivolt offset voltage. The output voltage starts to ramp only after the voltage created by the soft-start circuit increases above this offset voltage, at which point the output rises quickly to the voltage on the NR/SS pin. After this initial jump, the voltage rises at the ramp rate determined by the soft-start function. This jump typically does not cause a problem in applications because the quick rise in the output voltage has a very small amplitude.

In-rush current is defined as the current into the LDO at the IN pin during start-up. In-rush current then consists primarily of the sum of load current and the current used to charge the output capacitor. This current is difficult to measure because the input capacitor must be removed, which is not recommended. However, this soft-start current can be estimated by Equation 3:

Equation 3.

where

• V_OUT(t) is the instantaneous output voltage of the turn-on ramp
• dV_OUT(t) / dt is the slope of the V_OUT ramp
• R_LOAD is the resistive load impedance

8.1.6 Optimizing Noise and PSRR

The ultra-low noise floor and PSRR of the device can be improved by careful selection of:

• C_NR/SS for the low-frequency range
• C_FF for the mid-band frequency range
• C_OUT for the high-frequency range
• V_IN – V_OUT for all frequencies, and
• V_BIAS at lower input voltages

The noise-reduction capacitor filters out low-frequency noise from the reference and the feed-forward capacitor reduces output voltage noise by filtering out the mid-band frequency noise. However, a large feed-forward capacitor can create some new issues that are discussed in application report Pros and Cons of Using a Feed-Forward Capacitor with a Low Dropout Regulator, SBVA042.

Note that a large output capacitor reduces high-frequency output voltage noise. Additionally, a bias rail or higher input voltage improves the noise because greater headroom is provided for the internal circuits. A high power dissipation across the die increases the output noise because of the increase in junction temperature.

A larger noise-reduction capacitor improves low-frequency PSRR by filtering any noise coupling from the input into the reference. The feed-forward capacitor can be optimized by placing a pole-zero pair near the edge of the loop bandwidth and pushing out the loop bandwidth, thus improving mid-band PSRR. Larger output capacitors and various output capacitors can be used to improve high-frequency PSRR; see Figure 7 for more details.

A higher input voltage improves PSRR by providing the device more headroom to respond to noise on the input; see Figure 2. A bias rail also improves the PSRR at lower input voltages because greater headroom is provided for the internal circuits. Table 1 lists the output voltage noise for the 10-Hz to 100-kHz band at a 5.0-V output for a variety of conditions with an input voltage of 5.6 V, an R₁ of 12.1 kΩ, and a load current of 4 A. The 5.0-V output is chosen because this output is the worst-case condition for output voltage noise. Note that the input voltage is 5.6 V, not 5.5 V as provided in the Electrical Characteristics table. The higher input voltage limits the maximum ambient temperature to below 40°C on a standard-JEDEC high-K board; see the Power Dissipation (PD) section for more information.

8.1.6.1 Charge Pump Noise

The device internal charge pump generates a minimal amount of noise, as shown in Figure 46.

Using a bias rail minimizes the internal charge pump noise when the internal voltage is clamped, thereby reducing the overall output noise floor.

The high-frequency components of the output voltage noise density curve are filtered out in most applications by using 10-nF to 100-nF bypass capacitors close to the load. Using a ferrite bead between the LDO output and the load input capacitors forms a pi-filter, further reducing the high-frequency noise contribution.

Figure 46. Charge Pump Noise

8.1.7 ANY-OUT Programmable Output Voltage

The TPS7A85 can either use external resistors or the internally-matched ANY-OUT feedback resistor network to set the output voltage. The ANY-OUT resistors are accessible via pins 2 and 5 to 11 and are used to program the regulated output voltage. Each pin can be connected to ground (active) or left open (floating), or connected to SNS. ANY-OUT programming is set by Equation 4 as the sum of the internal reference voltage (V_NR/SS = 0.8 V) plus the accumulated sum of the respective voltages assigned to each active pin; that is, 50mV (pin 5), 100mV (pin 6), 200mV (pin 7), 400mV (pin 9), 800mV (pin 10), or 1.6V (pin 11). Table 2 summarizes these voltage values associated with each active pin setting for reference. By leaving all program pins open or floating, the output is thereby programmed to the minimum possible output voltage equal to V_FB.

Table 3 provides a full list of target output voltages and corresponding pin settings when the ANY-OUT pins are only tied to ground or left floating. The voltage setting pins have a binary weight; therefore, the output voltage can be programmed to any value from 0.8 V to 3.95 V in 50-mV steps when tying these pins to ground. There are several alternative ways to set the output voltage. The program pins can be driven by using external general-purpose input/output pins (GPIOs), manually connected using 0-Ω resistors (or left open), or hardwired by the given layout of the printed circuit board (PCB) to set the ANY-OUT voltage. As with the adjustable operation, the output voltage is set according to Equation 5 except that R₁ and R₂ are internally integrated and matched for higher accuracy. Tying any of the ANY-OUT pins to SNS can increase the resolution of the internal feedback network by lowering the value of R₁; see the Increasing ANY-OUT Resolution for LILO Conditions section for additional information.

8.1.8 ANY-OUT Operation

Considering the use of the ANY-OUT internal network (where the unit resistance of 1R is equal to 6.05 kΩ) the output voltage is set by grounding the appropriate control pins, as shown in Figure 47. When grounded, all control pins add a specific voltage on top of the internal reference voltage (V_NR/SS = 0.8 V). The output voltage can be calculated by Equation 6 and Equation 7. Figure 47 and Figure 48 show a 3.3-V and a 0.9-V output voltage, respectively, that provides an example of the circuit usage with and without bias voltage.

Figure 47. ANY-OUT Configuration Circuit
(3.3-V Output, No External Bias)

Figure 48. ANY-OUT Configuration Circuit
(0.9-V Output with Bias)

8.1.9 Increasing ANY-OUT Resolution for LILO Conditions

As with the adjustable operation, the output voltage is set according to Equation 5, except that R₁ and R₂ are internally integrated and matched for higher accuracy. Tying any of the ANY-OUT pins to SNS can increase the resolution of the internal feedback network by lowering the value of R₁. One of the more useful pin combinations is to tie the 800mV pin to SNS, which reduces the resolution by 50% to 25 mV but limits the range. The new ANY-OUT ranges are 0.8 V to 1.175 V and 1.6 V to 1.975 V. The new additive output voltage levels are listed in Table 4.

8.1.10 Current Sharing

Current sharing is possible through the use of external operational amplifiers. For more details, see the reference design 6A Current-Sharing Dual LDO, TIDU421.

8.1.11 Adjustable Operation

The TPS7A85 can either be used with the internal ANY-OUT network or by using external resistors. Using the ANY-OUT network allows the TPS7A85 to be programmed from 0.8 V to 3.95 V. To extend this output voltage range to 5.0 V, external resistors must be used. This configuration is referred to as the adjustable configuration of the TPS7A85 throughout this document. Regardless whether the internal resistor network or external resistors are used, the output voltage is set by two resistors, as shown in Figure 49. Using the internal resistor ensures a 1% accurate output voltage and minimizes the number of external components.

Figure 49. Adjustable Operation

R₁ and R₂ can be calculated for any output voltage range using Equation 8. This resistive network must provide a current equal to or greater than 5 μA for dc accuracy. Using an R₁ of 12.1 kΩ is recommended to optimize the noise and PSRR.

Table 5 shows the resistor combinations required to achieve several common rails using standard 1%-tolerance resistors.

8.1.12 Sequencing Requirements

Supply and enable sequencing is only required when the bias rail is present. The start-up is always monotonic, independent of the sequencing requirements. Under these conditions the following requirements apply:

• V_BIAS and V_IN can be sequenced in any order, as long as V_EN is tied to V_IN or established after V_IN, as shown in Figure 50.

Figure 50. Sequencing Diagram

Two typical application circuits for implementing the sequencing requirements are detailed in the Sequencing with a Power-Good DC-DC Converter Pin and Sequencing with a Microcontroller (MCU) sections.

8.1.12.1 Sequencing with a Power-Good DC-DC Converter Pin

When a dc-dc converter is used to power the device and the PG of the dc-dc converter is used to enable the device, pull PG up to V_IN, as shown in Figure 51.

Figure 51. Sequencing with a DC-DC Converter and PG

8.1.12.2 Sequencing with a Microcontroller (MCU)

If a push-pull output stage is used to provide the enable signal to the device and the enable signal can possibly come before V_IN when a bias is present (such as with an MCU), convert the enable signal to an open-drain signal as shown in Figure 52. Using an open-drain signal ensures that if the signal arrives before V_IN, then the enable voltage does not violate the sequencing requirement.

Figure 52. Push-Pull Enable to Open-Drain Enable

8.1.13 Power-Good (PG) Operation

To ensure proper operation of the power-good circuit, the pullup resistor value must be between 10 kΩ and 100 kΩ. The lower limit of 10 kΩ results from the maximum pulldown strength of the power-good transistor, and the upper limit of 100 kΩ results from the maximum leakage current at the power-good node. If the pullup resistor is outside of this range, then the power-good signal may not read a valid digital logic level.

Using a large C_FF with a small C_NR/SS causes the power-good signal to incorrectly indicate that the output voltage has settled during turn-on. The C_FF time constant must be greater than the soft-start time constant to ensure proper operation of the PG during start-up. For a detailed description, see application report Pros and Cons of Using a Feed-Forward Capacitor with a Low Dropout Regulator, SBVA042.

The state of PG is only valid when the device operates above the minimum supply voltage. During short UVLO events and at light loads, power-good does not assert because the output voltage is sustained by the output capacitance.

8.1.14 Undervoltage Lockout (UVLO) Operation

The UVLO circuit ensures that the device stays disabled before its input or bias supplies reach the minimum operational voltage range, and ensures that the device shuts down when the input supply or bias supply collapse.

The UVLO circuit has a minimum response time of several microseconds to fully assert. During this time, a downward line transient below approximately 0.8 V causes the UVLO to assert for a short time; however, the UVLO circuit does not have enough stored energy to fully discharge the internal circuits inside of the device. When the UVLO circuit does not fully discharge, the internal circuits of the output are not fully disabled.

The effect of the downward line transient can be mitigated by either using a larger input capacitor to limit the fall time of the input supply when operating near the minimum V_IN, or by using a bias rail.

Figure 53 shows the UVLO circuit response to various input voltage events. The diagram can be separated into the following parts:

• Region A: The device does not turn on until the input reaches the UVLO rising threshold.
• Region B: Normal operation with a regulated output
• Region C: Brownout event above the UVLO falling threshold (UVLO rising threshold – UVLO hysteresis). The output may fall out of regulation but the device is still enabled.
• Region D: Normal operation with a regulated output
• Region E: Brownout event below the UVLO falling threshold. The device is disabled in most cases and the output falls because of the load and active discharge circuit. The device is reenabled when the UVLO rising threshold is reached by the input voltage and a normal start-up then follows.
• Region F: Normal operation followed by the input falling to the UVLO falling threshold.
• Region G: The device is disabled when the input voltage falls below the UVLO falling threshold to 0 V. The output falls because of the load and active discharge circuit.

Figure 53. Typical UVLO Operation

8.1.15 Dropout Voltage (V_DO)

Generally speaking, the dropout voltage often refers to the minimum voltage difference between the input and output voltage (V_DO = V_IN – V_OUT) that is required for regulation. When V_IN drops below the required V_DO for the given load current, the device functions as a resistive switch and does not regulate output voltage. Dropout voltage is proportional to the output current because the device is operating as a resistive switch; see Figure 25, Figure 26, and Figure 27.

Dropout voltage is affected by the drive strength for the gate of the pass element, which is nonlinear with respect to V_IN on this device because of the internal charge pump. The charge pump causes a higher dropout voltage at lower input voltages when a bias rail is not used, as illustrated in Figure 23.

For this device, dropout voltage increases exponentially when the input voltage nears its maximum operating voltage because the charge pump is internally clamped to 8.0 V; see Figure 23 and Figure 24.

8.1.16 Behavior when Transitioning from Dropout into Regulation

Some applications may have transients that place the device into dropout, especially because this device is a high-current linear regulator. A typical application with these conditions requires setting V_IN ≤ V_DO in order to keep the device junction temperature within its specified operating range. A load transient or line transient in these conditions can place the device into dropout, such as a load transient from 1 A to 4 A at 1A/µs when operating with a V_IN of 5.4- V and a V_OUT of 5.0 V.

The load transient saturates the error amplifier output stage when the pass element is fully driven on, thus making the pass element function like a resistor from V_IN to V_OUT. The error amplifier response time to this load transient (I_OUT = 4 A to 1 A at 1 A/µs) is limited because the error amplifier must first recover from saturation and then place the pass element back into active mode. During the recovery from the load transient, V_OUT overshoots because the pass element is functioning as a resistor from V_IN to V_OUT. If operating under these conditions, apply a higher dc load or increase the output capacitance to reduce the overshoot because these solutions provide a path to dissipate the excess charge.

8.1.17 Load Transient Response

The load-step transient response is the output voltage response by the LDO to a step in load current, whereby output voltage regulation is maintained; see Figure 18. There are two key transitions during a load transient response: the transition from a light to a heavy load and the transition from a heavy to a light load. The regions shown in Figure 54 are broken down in this section. Regions A, E, and H are where the output voltage is in steady-state.

During transitions from a light load to a heavy load, the:

• Initial voltage dip is a result of the depletion of the output capacitor charge and parasitic impedance to the output capacitor (region B).
• Recovery from the dip results from the LDO increasing its sourcing current, and leads to output voltage regulation (region C).

During transitions from a heavy load to a light load, the:

• Initial voltage rise results from the LDO sourcing a large current, and leads to the output capacitor charge to increase (region F).
• Recovery from the rise results from the LDO decreasing its sourcing current in combination with the load discharging the output capacitor (region G).

Transitions between current levels changes the internal power dissipation because the TPS7A85 is a high-current device (region D). The change in power dissipation changes the die temperature during these transitions, and leads to a slightly different voltage level. This different output voltage level shows up in the various load transient responses; see Figure 18.

A larger output capacitance reduces the peaks during a load transient but slows down the response time of the device. A larger dc load also reduces the peaks because the amplitude of the transition is lowered and a higher current discharge path is provided for the output capacitor; see Figure 20.

Figure 54. Load Transient Waveform

8.1.18 Negatively-Biased Output

The device does not start or operate as expected if the output voltage is pulled below ground. This issue commonly occurs when powering a split-rail system where the negative rail is established before the device is enabled. Several application solutions are possible, such as:

• Enable the device before the negative regulator and disable the device after the negative regulator.
• Delaying the EN voltage with respect to the IN voltage allows the internal pulldown resistor to discharge any voltage at OUT. If the discharge circuit is not strong enough to keep the output voltage at ground, then use an external pulldown resistor.
• Place a zener diode from IN to OUT to provide a small positive dc bias on the output when the input is supplied to the device, as shown in Figure 55.
Figure 55. Zener Diode Placed from IN to OUT
• Use a PMOSFET to isolate the output of the device from the load causing the negative bias when the device is off, as shown in Figure 56.
Figure 56. PMOSFET to Isolate the Output from the Load

8.1.19 Reverse Current Protection

As with most LDOs, this device can be damaged by excessive reverse current.

Conditions where excessive reverse current can occur are outlined in this section, all of which can exceed the absolute maximum rating of V_OUT > V_IN + 0.3 V:

• If the device has a large C_OUT, then the input supply collapses quickly and the load current becomes very small
• The output is biased when the input supply is not established
• The output is biased above the input supply

If an excessive reverse current flow is expected in the application, then external protection must be used to protect the device. Figure 57 shows one approach of protecting the device.

Figure 57. Example Circuit for Reverse Current Protection Using a Schottky Diode

8.1.20 Power Dissipation (P_D)

Circuit reliability demands that proper consideration be given to device power dissipation, location of the circuit on the printed circuit board (PCB), and correct sizing of the thermal plane. The PCB area around the regulator must be as free as possible of other heat-generating devices that cause added thermal stresses.

As a first-order approximation, power dissipation in the regulator depends on the input-to-output voltage difference and load conditions. P_D can be approximated using Equation 9:

Equation 9.

An important note is that power dissipation can be minimized, and thus greater efficiency achieved, by proper selection of the system voltage rails. Proper selection allows the minimum input-to-output voltage differential to be obtained. The low dropout of the TPS7A85 allows for maximum efficiency across a wide range of output voltages.

The main heat conduction path for the device is through the thermal pad on the package. As such, the thermal pad must be soldered to a copper pad area under the device. This pad area contains an array of plated vias that conduct heat to any inner plane areas or to a bottom-side copper plane.

The maximum power dissipation determines the maximum allowable junction temperature (T_J) for the device. Power dissipation and junction temperature are most often related by the junction-to-ambient thermal resistance (θ_JA) of the combined PCB and device package and the temperature of the ambient air (T_A), according to Equation 10. The equation is rearranged for output current in Equation 11.

Equation 10.

Unfortunately, this thermal resistance (θ_JA) is highly dependent on the heat-spreading capability built into the particular PCB design, and therefore varies according to the total copper area, copper weight, and location of the planes. The θ_JA recorded in the Electrical Characteristics table is determined by the JEDEC standard, PCB, and copper-spreading area, and is only used as a relative measure of package thermal performance. Note that for a well-designed thermal layout, θ_JA is actually the sum of the VQFN package junction-to-case (bottom) thermal resistance (θ_JCbot) plus the thermal resistance contribution by the PCB copper.

8.1.20.1 Estimating Junction Temperature

The JEDEC standard now recommends the use of psi (Ψ) thermal metrics to estimate the junction temperatures of the LDO when in-circuit on a typical PCB board application. These metrics are not strictly speaking thermal resistances, but rather offer practical and relative means of estimating junction temperatures. These psi metrics are determined to be significantly independent of the copper-spreading area. The key thermal metrics (Ψ_JT and Ψ_JB) are given in the Electrical Characteristics table and are used in accordance with Equation 12.

Equation 12.

where

• P_D is the power dissipated as explained in Equation 9
• T_T is the temperature at the center-top of the device package, and
• T_B is the PCB surface temperature measured 1 mm from the device package and centered on the package edge

8.1.20.2 Recommended Area for Continuous Operation (RACO)

The operational area of an LDO is limited by the dropout voltage, output current, junction temperature, and input voltage. The recommended area for continuous operation for a linear regulator can be separated into the following parts, and is shown in Figure 58:

• Limited by dropout: Dropout voltage limits the minimum differential voltage between the input and the output (V_IN – V_OUT) at a given output current level; see the Dropout Voltage (VDO) section for more details.
• Limited by rated output current: The rated output current limits the maximum recommended output current level. Exceeding this rating causes the device to fall out of specification.
• Limited by thermals: The shape of the slope is given by Equation 11. The slope is nonlinear because the junction temperature of the LDO is controlled by the power dissipation across the LDO; therefore, when V_IN – V_OUT increases, the output current must decrease in order to ensure that the rated junction temperature of the device is not exceeded. Exceeding this rating can cause the device to fall out of specifications and reduces long-term reliability.
• Limited by V_IN range: The rated input voltage range governs both the minimum and maximum of V_IN – V_OUT.

Figure 58. Continuous Operation Slope Region Description

Figure 59 to Figure 64 show the recommended area of operation curves for this device on a JEDEC-standard high-K board with a θ_JA = 35.4°C/W, as given in the Electrical Characteristics table.

Figure 59. Recommended Area for Continuous Operation for V_OUT = 0.9 V with Bias

Figure 61. Recommended Area for Continuous Operation for V_OUT = 1.8 V

Figure 63. Recommended Area for Continuous Operation for V_OUT = 3.3 V

Figure 60. Recommended Area for Continuous Operation for V_OUT = 1.2 V with Bias

Figure 62. Recommended Area for Continuous Operation for V_OUT = 2.5 V

Figure 64. Recommended Area for Continuous Operation for V_OUT = 5.0 V

8.2.1 Low-Input, Low-Output (LILO) Voltage Conditions

This section discusses the implementation of the TPS7A85 using the ANY-OUT configuration to regulate a 4.0-A load requiring good PSRR at high frequency with low noise at 0.9 V using a 1.3-V input voltage and a 5.0-V bias supply. The schematic for this typical application circuit is provided in Figure 65.

Figure 65. Typical Application

8.2.1.3 Application Curves

Figure 66. Output PSRR

Figure 67. Output Noise Level

8.2.2 Typical Application for a 5.0-V Rail

This section discusses the implementation of the TPS7A85 using an adjustable feedback network to regulate a 4-A load requiring good PSRR at high frequency with low noise at an output voltage of 5.0 V. The schematic for this typical application circuit is provided in Figure 68.

Figure 68. Typical Application

8.2.2.3 Application Curves

Figure 69. PSRR vs Frequency and I_OUT for V_OUT = 5.0 V with V_IN = 5.6 V

Figure 70. PSRR vs Frequency and V_IN for V_OUT = 5.0 V at I_OUT = 4.0 A