DETAILED DESCRIPTION

PWM Operation

Low-Side Driver

High-Side Driver

PWM Frequency and Adaptive On-Time Control

Soft Start

Output Discharge Control

TPS53124

The main control loop of the switching mode power supply (SMPS) is designed as an adaptive on-time pulse

width modulation (PWM) controller. It supports a proprietary D-CAP™ Mode. D-CAP™ Mode uses internal

compensation circuit and is suitable for low external component count configuration with appropriate amount of

ESR at the output capacitor(s). The output ripple bottom voltage is monitored at a feedback point voltage.

At the beginning of each cycle, the synchronous high-side MOSFET is turned on, or becomes ON state. This

MOSFET is turned off, or becomes OFF state, after internal one-shot timer expires. This one shot is determined

by the converter’s input voltage ,VIN, and the output voltage ,VOUT, to keep frequency fairly constant over the

input voltage range, hence it is called adaptive on-time control. The high-side MOSFET is turned on again when

feedback information indicates insufficient output voltage. Repeating operation in this manner, the controller

regulates the output voltage.

represented by its internal resistance. A dead time to prevent shoot through is internally generated between

high-side MOSFET off to low-side MOSFET on, and low-side MOSFET off to high-side MOSFET on. 5-V bias

voltage is delivered from internal regulator VREG5 output. The instantaneous drive current is supplied by an

input capacitor connected between VREG5 and GND. The average drive current is equal to the gate charge at

makes the driving power which need to be dissipated from TPS53124 package.

floating driver, 5-V bias voltage is delivered from VREG5 supply. The average drive current is also calculated by

capacitor between VBSTx and LLx pins. The drive capability is represented by its internal resistance.

TPS53124 employs adaptive on-time control scheme and does not have a dedicated oscillator on board.

However, the part runs with pseudo-constant frequency by feed-forwarding the input and output voltage into the

on-time one-shot timer. The on-time is controlled inverse proportional to the input voltage and proportional to the

output voltage so that the duty ratio will be kept as VOUT/VIN technically with the same cycle time.

The TPS53124 has an internal, 1.2 ms, voltage servo softstart for each channel. When the ENx pin becomes

high, an internal DAC begins ramping up the reference voltage to the PWM comparator. Smooth control of the

output voltage is maintained during start up. As TPS53124 shares one DAC with both channels, if ENx pin is set

to high while another channel is starting up, soft start is postponed until another channel soft start has

completed. If both of EN1 and EN2 are set high at a same time, both channels start up at same time.

TPS53124 discharges the output when ENx is low, or the controller is turned off by the protection functions

(OVP, UVP, UVLO, and thermal shutdown). TPS53124 discharges outputs using an internal 40-

Ω MOSFET

which is connected to VOx and PGNDx. The external low-side MOSFET is not turned on for the output discharge

operation to avoid the possibility of causing negative voltage at the output.

This discharge ensures that, on start, the regulated voltage always start from zero volts.

Copyright © 2008, Texas Instruments Incorporated

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Product Folder Link(s): TPS53124