TPS65320-Q1

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SLVSAY9A – DECEMBER 2012 – REVISED APRIL 2013

Because the device 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 starts to decrease, and eventually falls below the pulse-skip-mode threshold, at which time the

device enters Eco-mode again.

For pulse-skip-mode operation, the TPS65320-Q1 senses peak current, not average or load current. Therefore,

the load current where the device 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 device enters a sleep mode and draws only

140-µA input quiescent current. The internal PLL remains operating when the device is in sleep mode.

Low Dropout Operation and Bootstrap Voltage (BOOT)

The TPS65320-Q1 has an integrated boot regulator and requires a small ceramic capacitor between the BOOT

and SW pins 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 should 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 voltage.

To improve drop out, the TPS65320-Q1 operates at 100% duty cycle as long as the BOOT to SW pin voltage is

greater than 2.1 V. When the voltage from BOOT to SW drops below 2.1 V, the high-side MOSFET turns off

using a UVLO circuit, which allows the low-side diode to conduct and refresh the charge on the BOOT capacitor.

Because the supply current sourced from the BOOT capacitor is low, the high-side MOSFET can remain on for

more switching cycles than are required to refresh the capacitor, and therefore the effective duty cycle of the

switching regulator is high.

Voltage drops across the power MOSFET, inductor resistance, low-side diode, and printed circuit board

resistance are the main influence on the effective duty cycle during dropout of the regulator. During operating

conditions in which the input voltage drops and the regulator is operating in continuous conduction mode, the

high-side MOSFET can remain on for 100% of the duty cycle to maintain output regulation until the BOOT to SW

voltage falls below 2.1 V.

Pay attention in maximum-duty-cycle applications which experience extended time periods with light loads or no

load. When the voltage across the BOOT capacitor falls below the 2.1-V UVLO threshold, the high-side MOSFET

turns off, but there may not be enough inductor current to pull the SW pin down to recharge the BOOT capacitor.

The high-side MOSFET of the regulator stops switching because the voltage across the BOOT capacitor is less

than 2.1 V. The output capacitor then decays until the difference in the input voltage and output voltage is greater

than 2.1 V, which exceeds the BOOT UVLO threshold, and the device starts switching again until reaching the

desired output voltage. This operating condition persists until the input voltage and/or the load current increases.

When TPS65320-Q1 tries to turn on the high-side MOSFET continuously during the high-side off state, the

internal small low-side MOSFET turns on for a short time to charge the BOOT capacitor. Then the SW node pulls

low to recharge the BOOT capacitor for maximum-duty-cycle operation.

Error Amplifier

The buck converter of the TPS65320-Q1 has a transconductance amplifier for the error amplifier. The error

amplifier compares the VFB1 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

VFB1 pin is below 0.8 V and the device is regulating using internal SS voltage, the gm is 70 µS. The frequency-

compensation components (capacitor, series resistor, and capacitor) are added to the COMP pin to ground.

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.

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