TPS63024x 高电流、高效单电感器降压-升压转换器

7.3.1 Undervoltage Lockout (UVLO)

To avoid mis-operation of the device at low input voltages, an undervoltage lockout is included. UVLO shuts down the device at input voltages lower than typically 1.7V with a 70 mV hysteresis.

7.3.2 Output Discharge Function

When the device is disabled by pulling enable low and the supply voltage is still applied, the internal transistor use to discharge the output capacitor is turned on, and the output capacitor is discharged until UVLO is reached. This means, if there is no supply voltage applied the output discharge function is also disabled. The transistor which is responsible of the discharge function, when turned on, operates like an equivalent 120Ω resistor, ensuring typically less than 10ms discharge time for 20uF output capacitance and a 3.3V output.

7.3.3 Thermal Shutdown

The device goes into thermal shutdown once the junction temperature exceeds typically 140°C.

7.3.4 Softstart

To minimize inrush current and output voltage overshoot during start up, the device has a Softstart. At turn on, the input current raises monotonically until the output voltage reaches regulation. During Softstart, the input current follows the current ramp charging the internal Softstart capacitor. The device smoothly ramps up the input current bringing the output voltage to its regulated value even if a large capacitor is connected at the output.

The Softstart time is measured as the time from when the EN pin is asserted to when the output voltage has reached 90% of its nominal value. There is typically a 100µs delay time from when the EN pin is asserted to when the device starts the switching activity. The Softstart time depends on the load current, the input voltage, and the output capacitor. The Softstart time in boost mode is longer then the time in buck mode. The total typical Softstart time is 1ms.

The inductor current is able to increase and always assure a soft start unless a real short circuit is applied at the output.

7.3.5 Short Circuit Protection

The TPS63024x provides short circuit protection to protect itself and the application. When the output voltage does not increase above 1.2V, the device assumes a short circuit at the output and limits the input current to 3A.

7.4.1 Control Loop Description

Figure 5. Average Current Mode Control

The controller circuit of the device is based on an average current mode topology. The average inductor current is regulated by a fast current regulator loop which is controlled by a voltage control loop. Figure 5 shows the control loop.

The non inverting input of the transconductance amplifier, gmv, is assumed to be constant. The output of gmv defines the average inductor current. The inductor current is reconstructed by measuring the current through the high side buck MOSFET. This current corresponds exactly to the inductor current in boost mode. In buck mode the current is measured during the on time of the same MOSFET. During the off time, the current is reconstructed internally starting from the peak value at the end of the on time cycle. The average current and the feedback from the error amplifier gmv forms the correction signal gmc. This correction signal is compared to the buck and the boost sawtooth ramp giving the PWM signal. Depending on which of the two ramps the gmc output crosses either the Buck or the Boost stage is initiated. When the input voltage is close to the output voltage, one buck cycle is always followed by a boost cycle. In this condition, no more than three cycles in a row of the same mode are allowed. This control method in the buck-boost region ensures a robust control and the highest efficiency.

7.4.2 Power Save Mode Operation

Figure 6. Power Save Mode Operation

Depending on the load current, in order to provide the best efficiency over the complete load range, the device works in PWM mode at load currents of approximately 350 mA or higher. At lighter loads, the device switches automatically into Power Save Mode to reduce power consumption and extend battery life. The PFM/PWM pin is used to select between the two different operation modes. To enable Power Save Mode, the PFM/PWM pin must be set low.

During Power Save Mode, the part operates with a reduced switching frequency and lowest supply current to maintain high efficiency. The output voltage is monitored with a comparator at every clock cycle by the thresholds comp low and comp high. When the device enters Power Save Mode, the converter stops operating and the output voltage drops. The slope of the output voltage depends on the load and the output capacitance. When the output voltage reaches the comp low threshold, at the next clock cycle the device ramps up the output voltage again, by starting operation. Operation can last for one or several pulses until the comp high threshold is reached. At the next clock cycle, if the load is still lower than about 350mA, the device switches off again and the same operation is repeated. Instead, if at the next clock cycle, the load is above 350mA, the device automatically switches to PWM mode.

In order to keep high efficiency in PFM mode, there is only one comparator active to keep the output voltage regulated. The AC ripple in this condition is increased, compared to the PWM mode. The amplitude of this voltage ripple in the worst case scenario is 50mV pk-pk, (typically 30mV pk-pk), with 20µF effective output capacitance. In order to avoid a critical voltage drop when switching from 0A to full load, the output voltage in PFM mode is typically 1.3% above the nominal value in PWM mode. This is called Dynamic Voltage Positioning and allows the converter to operate with a small output capacitor and still have a low absolute voltage drop during heavy load transients.

Power Save Mode is disabled by setting the PFM/PWM pin high.

7.4.3 Current Limit

The current limit variation depends on the difference between the input and output voltage. The maximum current limit value is at the highest difference.

Given the curves provided in Figure 8, it is possible to calculate the output current reached in boost mode, using Equation 1 and Equation 2 and in buck mode using Equation 3 and Equation 4.

Equation 1.

Equation 2.

Equation 3.

Equation 4.

where

• η = Estimated converter efficiency (use the number from the efficiency curves or 0.90 as an assumption)
• I_IN= Minimum average input current (Figure 8)

7.4.4 Supply and Ground

The TPS63024x provides two input pins (VIN and VINA) and two ground pins (PGND and GND).

The VIN pin supplies the input power, while the VINA pin provides voltage for the control circuits. A similar approach is used for the ground pins. GND and PGND are used to avoid ground shift problems due to the high currents in the switches. The reference for all control functions is the GND pin. The power switches are connected to PGND. Both grounds must be connected on the PCB at only one point, ideally, close to the GND pin.

7.4.5 Device Enable

The device starts operation when the EN pin is set high. The device enters shutdown mode when the EN pin is set low. In shutdown mode, the regulator stops switching, all internal control circuitry is switched off, and the load is disconnected from the input.

8.2.1 Design Requirements

The design guideline provides a component selection to operate the device within the recommended operating conditions.

Table 1 shows the list of components for the Application Characteristic Curves.

8.2.2 Detailed Design Procedure

The first step is the selection of the output filter components. To simplify this process Table 2 outline possible inductor and capacitor value combinations.

8.2.2.2 Inductor Selection

The inductor selection is affected by several parameter like inductor ripple current, output voltage ripple, transition point into Power Save Mode, and efficiency. See Table 3 for typical inductors.

Table 3. List of Recommended Inductors⁽¹⁾

INDUCTOR VALUE	COMPONENT SUPPLIER	SIZE (LxWxH mm)	Isat/DCR
1 µH	Coilcraft XAL4020-102ME	4 X 4 X 2.10	4.5A/10mΩ
1 µH	Toko, DFE322512C	3.2 X 2.5 X 1.2	4.7A/34mΩ
1 µH	TDK, SPM4012	4.4 X 4.1 X 1.2	4.1A/38mΩ
1 µH	Wuerth, 74438334010	3 X 3 X 1.2	6.6A/42.10mΩ
0.6 µH	Coilcraft XFL4012-601ME	4 X 4 X 1.2	5A/17.40mΩ
0.68µH	Wuerth,744383340068	3 X 3 X 1.2	7.7A/36mΩ

(1) See Third-Party Products Desclaimer

For high efficiencies, the inductor should have a low dc resistance to minimize conduction losses. Especially at high-switching frequencies, the core material has a high impact on efficiency. When using small chip inductors, the efficiency is reduced mainly due to higher inductor core losses. This needs to be considered when selecting the appropriate inductor. The inductor value determines the inductor ripple current. The larger the inductor value, the smaller the inductor ripple current and the lower the conduction losses of the converter. Conversely, larger inductor values cause a slower load transient response. To avoid saturation of the inductor, the peak current for the inductor in steady state operation is calculated using Equation 6. Only the equation which defines the switch current in boost mode is shown, because this provides the highest value of current and represents the critical current value for selecting the right inductor.

Equation 5.

Equation 6.

where

• D =Duty Cycle in Boost mod
• ƒ = Converter switching frequency (typical 2.5MHz)
• L = Inductor value
• η = Estimated converter efficiency (use the number from the efficiency curves or 0.90 as an assumption)
• Note: The calculation must be done for the minimum input voltage which is possible to have in boost mode

Calculating the maximum inductor current using the actual operating conditions gives the minimum saturation current of the inductor needed. It's recommended to choose an inductor with a saturation current 20% higher than the value calculated using Equation 6. Possible inductors are listed in Table 3.

8.2.2.4 Setting The Output Voltage

When the adjustable output voltage version TPS63024x is used, the output voltage is set by an external resistor divider. The resistor divider must be connected between VOUT, FB and GND. When the output voltage is regulated properly, the typical value of the voltage at the FB pin is 800mV. The current through the resistive divider should be about 10 times greater than the current into the FB pin. The typical current into the FB pin is 0.1μA, and the voltage across the resistor between FB and GND, R₂, is typically 800 mV. Based on these two values, the recommended value for R2 should be lower than 180kΩ, in order to set the divider current at 4μA or higher. It is recommended to keep the value for this resistor in the range of 180kΩ. From that, the value of the resistor connected between VOUT and FB, R1, depending on the needed output voltage (V_OUT), can be calculated using Equation 7:

Equation 7.

8.2.3 Application Curves

VOUT = 3.3 V

Figure 8. Minimum Average Input Current vs Input Voltage

PFM/PWM = High

Figure 10. Efficiency vs Output Current

PFM/PWM = Low

Figure 9. Efficiency vs Output Current

PFM/PWM = Low

Figure 11. Efficiency vs Output Current

PFM/PWM = Low

VOUT = 3.3 V

Figure 13. Efficiency vs Input Voltage

PFM/PWM = High

Figure 12. Efficiency vs Output Current

PFM/PWM = High

VOUT = 3.3 V

Figure 14. Efficiency vs Input Voltage

PFM/PWM = Low

VOUT = 2.9 V

Figure 15. Efficiency vs Input Voltage

PFM/PWM = Low

Figure 17. Output Voltage vs Output Current

VIN = 3.3 V

IOUT = 290 mA

Figure 19. Output Voltage Ripple in Buck-Boost Mode
and PFM to PWM Transition

PFM/PWM = High

VOUT = 2.9 V

Figure 16. Efficiency vs Input Voltage

PFM/PWM = High

Figure 18. Output Voltage vs Output Current

VIN = 2.8 V

IOUT = 16 mA

Figure 20. Output Voltage Ripple in Boost Mode and PFM Operation

VIN = 4.2 V

IOUT = 16 mA

Figure 21. Output Voltage Ripple in Buck Mode
and PFM Operation

VIN = 4.5 V

IOUT = 1 A

Figure 23. Switching Waveforms in Buck Mode
and PWM Operation

VIN = 2.8 V

IOUT = 0 A to 1.5 A

Figure 25. Load Transient Response Boost Mode

VIN = from 3.5 V to 3.6 V

IOUT = 1.5 A

Figure 27. Line Transient Response

VIN = 4.5 V

IOUT = 0 A

Figure 29. Start Up After Enable

VIN = 2.5 V

IOUT = 1 A

Figure 22. Switching Waveforms in Boost Mode
and PWM Operation

VIN = 3.3 V

IOUT = 1 A

Figure 24. Switching Waveforms in Buck-Boost Mode
and PWM Operation

VIN = 4.2 V

IOUT = 0 A to 1.5 A

Figure 26. Load Transient Response Buck Mode

VIN = 2.5 V

IOUT = 0 A

Figure 28. Start Up After Enable