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TPS65321A-Q1

SLVSE55 – NOVEMBER 2017

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Copyright © 2017, Texas Instruments Incorporated

Feature Description (continued)

7.3.1.3 Pulse-Skip Eco-mode™ Control Scheme

The TPS65321A-Q1 buck regulator operates in a pulse-skip mode at light load currents to improve efficiency by

reducing switching and gate-drive losses. The design of the TPS65321A-Q1 buck regulator is such that if the

output voltage is within regulation and the peak switch current at the end of any switching cycle is below the

pulse-skipping-current threshold, the buck regulator enters pulse-skip mode. This current threshold is the current

level corresponding to a nominal COMP voltage, or 720 mV. The current at which entry to the pulse-skip mode

occurs depends on switching frequency, inductor selection, output-capacitor selection, and compensation

network.

In pulse-skip mode, the buck regulator clamps the COMP pin voltage at 720 mV, inhibiting the high-side

MOSFET. Further decreases in load current or in output voltage cannot drive the COMP pin below this clamp-

voltage level. Because the buck regulator is not switching, the output voltage begins to decay. As the voltage-

control loop compensates for the falling output voltage, the COMP pin voltage begins to rise. At this time, the

high-side MOSFET turns on and a switching pulse initiates on the next switching cycle. The peak current is set

by the COMP pin voltage. The output current recharges the output capacitor to the nominal voltage, then the

peak switch current begins to decrease, and eventually falls below the pulse-skip-mode threshold, at which time

the buck regulator enters Eco-mode again.

For pulse-skip-mode operation, the TPS65321A-Q1 buck regulator senses the peak current, not the average or

load current. Therefore, the load current where the buck regulator enters pulse-skip mode is dependent on the

output inductor value. When the load current is low and the output voltage is within regulation, the buck regulator

enters a sleep mode and draws only 140-µA input quiescent current. The internal PLL remains operating when

the buck regulator is in sleep mode.

7.3.1.4 Dropout Mode Operation and Bootstrap Voltage (BOOT)

The TPS65321A-Q1 buck regulator has an integrated boot regulator and requires a small ceramic capacitor

between the BOOT pin and the SW pin to provide the gate-drive voltage for the high-side MOSFET. The BOOT

capacitor recharges when the high-side MOSFET is off and the low-side diode conducts. The value of this

ceramic capacitor must be 0.1 μF. TI recommends a ceramic capacitor with an X7R or X5R grade dielectric and

a voltage rating of 10 V or higher because of the stable characteristics over temperature and over voltage.

To improve drop out, the high-side MOSFET of the TPS65321A-Q1 buck regulator remains on for 7 consecutive

switching cycles, and is forced off during the 8th switching cycle to allow the low-side diode to conduct and

refresh the charge on the BOOT capacitor. Because the current supplied by the BOOT capacitor is low, the high-

side MOSFET can remain on before it is required to refresh the BOOT capacitor. The effective duty cycle of the

switching regulator under this operation can be higher than the fixed-frequency PWM operation through skipping

switching cycles.

7.3.1.5 Error Amplifier

The buck converter of the TPS65321A-Q1 buck regulator has a transconductance amplifier acting as the error

amplifier. The error amplifier compares the FB1 voltage to the lower of the internal soft-start (SS) voltage or the

internal 0.8-V voltage reference. The transconductance (gm) of the error amplifier is 310 µS during normal

operation. During the soft-start operation, the transconductance is a fraction of the normal operating gm. When

the voltage of the voltage on the FB1 pin is below 0.8 V and the buck regulator is regulating using an internal SS

voltage, the gm is 70 µS. For frequency compensation, external compensation components (capacitor with series

resistor and an optional parallel capacitor) must be connected between the COMP pin and the GND pin.

7.3.1.6 Voltage Reference

The voltage reference system produces a precise ±2% voltage reference over temperature by scaling the output

of a temperature stable band-gap circuit.

7.3.1.7 Adjusting the Output Voltage

A resistor divider from the output node to the FB1 pin sets the output voltage. TI recommends using 1%

tolerance or better divider resistors. Start with 10 kΩ for the R2 resistor and use Equation 1 to calculate R1. To

improve efficiency at light loads, consider using larger-value resistors. If the values are too high, the regulator is

more susceptible to noise, and voltage errors from the FB1 input current are noticeable.