TPS6208x 具有 DCS-Control 和贪睡模式的 1.2A、高效降压转换器

8.3.1 Power Good

The TPS6208x has a power good output which goes low when the output voltage is below its nominal value. The power good is high impedance once the output is above 95% of the regulated voltage, and is driven to low once the output voltage falls below typically 90% of the regulated voltage. The PG pin is an open drain output and can sink up to 0.5 mA. The power good output requires a pull-up resistor. When the device is off due to disable, UVLO or thermal shutdown, the PG pin is high impedance (see Table 1). The PG signal can be used for sequencing of multiple rails by connecting to the EN pin of other converters. Leave the PG pin unconnected when not used.

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8.3.2 100% Duty Cycle Low Dropout Operation

The device offers low input to output voltage difference by entering 100% duty cycle mode. In this mode, the high-side MOSFET switch is constantly turned on and the low-side MOSFET is switched off. This is particularly useful in battery powered applications to achieve longest operation time by taking full advantage of the whole battery voltage range. The minimum input voltage to maintain an output voltage is calculated as:

Equation 1.

where

• V_IN,MIN = Minimum input voltage
• I_OUT,MAX = Maximum output current
• R_DS(on) = High-side FET ON-resistance
• R_L = Inductor ohmic resistance

8.3.3 Output Discharge

The output gets discharged by the SW pin with a typical discharge resistor of R_DIS whenever the device shuts down. This is the case when the device gets disabled by enable, thermal shutdown, or undervoltage lockout. The TPS6208A differs from the TPS62080 only in its stronger discharge.

8.3.4 Soft-Start

When EN is set to start device operation, the device starts switching after a delay of about 40 μs and VOUT rises with a slope of about 10mV/μs (See Figure 27 andFigure 29 for typical startup operation). This avoids excessive inrush current and creates a smooth output voltage rise slope. It also prevents excessive voltage drops of primary cells and rechargeable batteries with high internal impedance.

If the output voltage is not reached within the soft start time, such as in the case of heavy load, the converter enters regular operation. Consequently, the inductor current limit operates as described below. The TPS6208x is able to start into a pre-biased output capacitor. The converter starts with the applied bias voltage and ramps up the output voltage to its nominal value.

8.3.5 Undervoltage Lockout

To avoid mis-operation of the device at low input voltages, an undervoltage lockout is implemented that shuts down the device at voltages lower than V_UVLO with a 120 mV typical hysteresis.

8.3.6 Thermal Shutdown

The device goes into thermal shutdown once the junction temperature exceeds typically T_JSD. Once the device temperature falls below the threshold minus hysteresis, the device returns to normal operation automatically.

8.3.7 Inductor Current Limit

The Inductor Current Limit prevents the device from high inductor current and drawing excessive current from the battery or input voltage rail. Excessive current might occur with a shorted/saturated inductor or a heavy load/shorted output circuit condition.

The incorporated inductor peak current limit measures the current in the high-side and low-side power MOSFET. Once the high-side switch current limit is tripped, the high-side MOSFET is turned off and the low-side MOSFET is turned on to reduce the inductor current. When the inductor current drops down to the low-side switch current limit, the low-side MOSFET is turned off and the high-side switch is turned on again. This operation repeats until the inductor current does not reach the high-side switch current limit. Due to internal propagation delays, the real current limit value can exceed the static current limit in Electrical Characteristics.

8.4.1 Enabling and Disabling the Device

The device is enabled by setting the EN input to a logic HIGH. Accordingly, a logic LOW disables the device. If the device is enabled, the internal power stage starts switching and regulates the output voltage to the programmed threshold. The EN input must be terminated and not left floating.

8.4.2 Power Save Mode

As the load current decreases, the TPS6208x enters Power Save Mode operation. During Power Save Mode, the converter operates with reduced switching frequency in PFM mode and with a minimum quiescent current maintaining high efficiency. Power Save Mode occurs when the inductor current becomes discontinuous. It is based on a fixed on time architecture. The typical on time is given by t_on = 500 ns × (V_OUT/V_IN). The switching frequency over the whole load current range is shown in Figure 21 and Figure 22.

8.4.3 Snooze Mode

The TPS6208x offers a Snooze Mode function. If Snooze Mode is enabled by an external logic signal setting the MODE pin to HIGH, the device's quiescent current consumption is reduced to typically 6.5 µA. As a result, the high efficiency range is extended towards the range of lowest output currents below 2 mA. See the efficiency figures in Application Curves.

If the device is operating in Snooze Mode, a dedicated, low power consuming block monitors the output voltage. All other control blocks are snoozing during that time. If the output voltage falls below the programmed output voltage by 3.5% (typ), the control blocks wake up, regulate the output voltage and allow themselves to snooze again until the output voltage drops again. Snooze Mode operation provides a clear efficiency improvement at lowest output currents. If the load current increases, the advantage of efficiency in Snooze mode is reduced. Because the dynamic load regulation operates best if Snooze Mode is disabled, it is recommended to turn off Snooze Mode when the load current exceeds 2 mA. Generally, a microcontroller operates the MODE pin.

9.2.1 Design Requirements

For this design example, use Table 2 as the input parameters.

9.2.2 Detailed Design Procedure

Table 3 lists the components used for the example.

9.2.2.1 Setting the Output Voltage

The TPS608x devices are available as fixed and adjustable output voltage versions. The fixed voltage versions are internally programmed to a fixed output voltage, whereas the adjustable output voltage version needs to be programmed via an external voltage divider to set the desired output voltage.

9.2.2.1.1 Adjustable Output Voltage Version

For the adjustable output voltage version, an external resistor divider is used. By selecting R₁ and R₂, the output voltage is programmed to the desired value.

When the output voltage is regulated, the typical voltage at the FB pin is V_FB for the adjustable devices. The following equation can be used to calculate R₁ and R₂.

Equation 2.

For best accuracy, R2 should be kept smaller than 40kΩ to ensure that the current flowing through R2 is at least 100 times larger than I_FB. Changing towards a lower value increases the robustness against noise injection. Changing towards higher values reduces the input current. For lowest input current during Snooze Mode, it is recommended to use a fixed output voltage version such as TPS62081 and TPS62082.

9.2.2.3 Inductor Selection

The main parameters for the inductor selection are the inductor value and then the saturation current of the inductor. To calculate the maximum inductor current under static load conditions, Equation 3 is given.

Equation 3.

where

• I_OUT,MAX = Maximum output current
• ΔI_L = Inductor current ripple
• f_SW = Switching frequency
• L = Inductor value

TI recommends to choose the saturation current for the inductor 20%~30% higher than the I_L,MAX, out of Equation 3. A higher inductor value is also useful to lower ripple current, but increases the transient response time as well. The following inductors are recommended for use.

Table 5. List of Recommended Inductors

INDUCTANCE [µH]	CURRENT RATING [mA]	DIMENSIONS L x W x H [mm3]	DC RESISTANCE [mΩ typ]	TYPE	MANUFACTURER
1.0	2500	3 x 3 x 1.2	35	XFL3012-102ME	Coilcraft
1.0	1650	3 x 3 x 1.2	40	LQH3NPN1R0NJ0	Murata
2.2	2500	4 x 3.7 x 1.65	49	LQH44PN2R2MP0	Murata
2.2	1600	3 x 3 x 1.2	81	XFL3012-222ME	Coilcraft

9.2.3 Application Curves

Figure 8. Efficiency vs Load Current

Figure 10. Efficiency vs Load Current

Figure 12. Efficiency vs Load Current

Figure 14. Output Voltage vs Input Voltage

Figure 16. Output Voltage vs Input Voltage

Figure 18. Output Voltage vs Load Current

Figure 20. Output Voltage vs Load Current

Figure 22. Switching Frequency vs Load Current

VIN = 3.3 V, VOUT = 1.2 V, Load Current = 10 mA

Figure 24. Typical Application (PFM Mode)

VIN = 3.3 V, VOUT = 1.2 V, Load Current = 50 mA to 1 A

Figure 26. Load Transient

VIN = 3.3 V, VOUT = 1.2 V, Load = 2.2 Ω

Figure 28. Start Up

VIN = 3.3 V, VOUT = 1.2 V, No Load

Figure 30. Shutdown

Figure 9. Efficiency vs Load Current

Figure 11. Efficiency vs Load Current

Figure 13. Output Voltage vs Input Voltage

Figure 15. Output Voltage vs Input Voltage

Figure 17. Output Voltage vs Load Current

Figure 19. Output Voltage vs Load Current

Figure 21. Switching Frequency vs Load Current

VIN = 3.3 V, VOUT = 1.2 V, Load Current = 500 mA

Figure 23. Typical Application (PWM Mode)

VIN = 3.3 V, VOUT = 1.2 V, Load Current = 2 mA

Figure 25. Typical Application (Snooze Mode)

VIN = 3.3 V to 4.2 V, VOUT = 1.2 V, Load = 2.2 Ω

Figure 27. Line Transient

VIN = 3.3 V, VOUT = 1.2 V, No Load

Figure 29. Start Up (Without Load)