(

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OUT

OUT

OUT(LL)

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TPS56428

SLVSBV4A – APRIL 2013 – REVISED APRIL 2013

www.ti.com

OVERVIEW

The TPS56428 is a 4-A synchronous step-down (buck) converter with two integrated N-channel MOSFETs. It

operates using D-CAP2™ mode control. The fast transient response of D-CAP2™ control reduces the output

capacitance required to meet a specific level of performance. Proprietary internal circuitry allows the use of low

ESR output capacitors including ceramic and special polymer types. PG output can be used for sequence

operation.

DETAILED DESCRIPTION

PWM Operation

The main control loop of the TPS56428 is an adaptive on-time pulse width modulation (PWM) controller that

supports a proprietary D-CAP2™ mode control. D-CAP2™ mode control combines constant on-time control with

an internal compensation circuit for pseudo-fixed frequency and low external component count configuration with

both low ESR and ceramic output capacitors. It is stable even with virtually no ripple at the output.

At the beginning of each cycle, the high-side MOSFET is turned on. This MOSFET is turned off after internal one

shot timer expires. This one shot is set by the converter input voltage, VIN, and the output voltage, VO, to

maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. The

one-shot timer is reset and the high-side MOSFET is turned on again when the feedback voltage falls below the

reference voltage. An internal ramp is added to reference voltage to simulate output ripple, eliminating the need

for ESR induced output ripple from D-CAP2™ mode control.

PWM Frequency and Adaptive On-Time Control

TPS56428 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The

TPS56428 runs with a pseudo-constant frequency of 650 kHz by using the input voltage and output voltage to

set the on-time one-shot timer. The on-time is inversely proportional to the input voltage and proportional to the

output voltage, therefore, when the duty ratio is VOUT/VIN, the frequency is constant.

Advanced Auto-Skip Eco-mode™ Control

The TPS56428 is designed with advanced auto-skip Eco-mode™ to increase higher light load efficiency. As the

output current decreases from heavy load condition, the inductor current is also reduced. If the output current is

reduced enough, the inductor current ripple valley reaches the zero level, which is the boundary between

continuous conduction and discontinuous conduction modes. The rectifying low-side MOSFET is turned off when

its zero inductor current is detected. As the load current further decreases the converter run into discontinuous

conduction mode. The on-time is kept approximately the same as is in continuous conduction mode. The off-time

increases as it takes more time to discharge the output capacitor to the level of the reference voltage with

Equation 1.

(1)

Soft Start and Pre-Biased Soft Start

The TPS56428 has an internal 1.0ms soft-start. When the EN pin becomes high, internal soft-start function

begins ramping up the reference voltage to the PWM comparator.

The TPS56428 contains a unique circuit to prevent current from being pulled from the output during startup if the

output is pre-biased. When the soft-start commands a voltage higher than the pre-bias level (internal soft start

the first low side FET gate driver pulses with a narrow on-time. It then increments that on-time on a cycle-by-

cycle basis until it coincides with the time dictated by (1-D), where D is the duty cycle of the converter. This

scheme prevents the initial sinking of the pre-bias output, and ensure that the out voltage (VO) starts and ramps

up smoothly into regulation and the control loop is given time to transition from pre-biased start-up to normal

mode operation.

6

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