TPS56921

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SLVSBL4 – OCTOBER 2012

OVERVIEW

The TPS56921 is a 17-V, 9-A, synchronous step-down (buck) converter with two integrated n-channel

MOSFETs. To improve performance during line and load transients the TPS56921 implements a fixed frequency,

peak current mode control which also simplifies external frequency compensation. The output voltage of the

TPS56921 can be set by either VSENSE with divider resistors (Adjusting the Output Voltage by External

as low as the 0.8V reference voltage in external regulation mode or 0.72V in internal regulation mode. It operates

in CCM (Continuous current mode) at any load conditions unless the COMP pin voltage drops below the COMP

pin start switching threshold (typically 0.25V).

The wide switching frequency of 200 kHz to 1600 kHz allows for efficiency and size optimization when selecting

the output filter components. The RT/CLK pin is dual-functional with two modes. In RT mode, the switching

frequency is adjusted using a resistor to ground on the RT/CLK pin. In CLK mode, the TPS56921 can be

controlled by the RT/CLK pin to synchronize the switching cycle to the falling edge of an external system clock.

The TPS56921 has been designed for safe startup into pre-biased outputs. The default start up is when VIN is

typically 4.0V. The EN pin has an internal pull-up current source that can be used to adjust the input voltage

under-voltage lockout (UVLO) with two external resistors. In addition, the EN pin can be floating for the

TPS56921 to operate with the internal pull up current. The total operating current for the TPS56921 is

approximately 770

μA when not switching and under no load. When the TPS56921 is disabled, the supply current

is typically less than 2

μA.

The integrated MOSFETs allow for high efficiency power supply designs with continuous output currents up to 9

amperes. The MOSFETs have been sized to optimize efficiency for lower duty cycle applications.

The TPS56921 reduces the external component count by integrating the boot recharge circuit. The bias voltage

for the integrated high-side MOSFET is supplied by a capacitor between the BOOT and PH pins. The bootstrap

voltage is monitored by a BOOT to PH UVLO (BOOT-PH UVLO) circuit allowing PH pin to be pulled low to

recharge the boot capacitor. The TPS56921 can operate at 100% duty cycle as long as the boot capacitor

voltage is higher than the preset BOOT-PH UVLO threshold which is typically 2.1V.

The TPS56921 has a power good comparator (PWRGD) with hysteresis which monitors the output voltage

through the VSENSE pin. The PWRGD pin is an open drain MOSFET which is pulled low when the VSENSE pin

voltage is less than 92% or greater than 106% of the internal reference voltage and asserts high when it is 94%

to 104% of the internal reference voltage.

The slow start (SS) pin is used to minimize inrush currents or provide power supply sequencing during power up.

A small value capacitor or resistor divider should be coupled to the pin for slow start or critical power supply

sequencing requirements.

The TPS56921 is protected from output overvoltage, overcurrent and overheat conditions. The TPS56921

minimizes excessive output overvoltage transients by taking advantage of the overvoltage circuit power good

comparator. When the overvoltage comparator is activated, the high-side MOSFET is turned off and prevented

from turning on until the VSENSE pin voltage is lower than 104% of the Vref. The TPS56921 implements both

high-side MOSFET overload protection and bidirectional low-side MOSFET overload protections which help

control the inductor current and avoid current runaway. If the overcurrent condition has lasted for more than the

hiccup wait time, the TPS56921 will shut down and re-start after the hiccup time. The TPS56921 also shuts down

if the junction temperature is higher than thermal shutdown trip point. When the junction temperature drops 10°C

typically below the thermal shutdown trip point, the built-in thermal shutdown hiccup timer is triggered. The

TPS56921 will be restarted under control of the slow start circuit automatically after the thermal shutdown hiccup

time is over.

Furthermore, if the overcurrent condition has lasted for more than the hiccup wait time which is programmed for

512 switching cycles, the TPS56921 will shut down itself and re-start after the hiccup time which is set for 16384

cycles. The hiccup mode helps to reduce the device power dissipation under severe overcurrent conditions.

Copyright © 2012, Texas Instruments Incorporated

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