(

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1

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IN

OUT

OUT

OUT LL

IN

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V

V

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TPS54426

www.ti.com

SLVSAD6C

– AUGUST 2010 – REVISED JULY 2011

OVERVIEW

The TPS54426 is a 4-A synchronous step-down (buck) converter with two integrated N-channel MOSFETs and

auto-skip Eco-mode

™ to improve light lode efficiency. 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.

DETAILED DESCRIPTION

PWM Operation

The main control loop of the TPS54426 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 with virtually no ripple at the output.

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

one-shot timer expires. The one-shot timer 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

TPS54426 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The

TPS54426 runs with a pseudo-constant frequency of 700 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.

Auto-Skip Eco-Mode

™ Control

The TPS54426 is designed with Auto-Skip Eco-mode

™ to increase light load efficiency. As the output current

decreases from heavy load condition, the inductor current is also reduced and eventually comes to point that its

rippled valley touches zero level, which is the boundary between continuous conduction and discontinuous

conduction modes. The rectifying 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 almost the

same as is was in the continuous conduction mode so that it takes longer time to discharge the output capacitor

with smaller load current to the level of the reference voltage. The transition point to the light load operation

(1)

Soft Start and Pre-Biased Soft Start

The soft start function is adjustable. When the EN pin becomes high, 2

μA current begins charging the capacitor

which is connected from the SS pin to GND. Smooth control of the output voltage is maintained during start up.

The equation for the slow start time is shown in Equation 2. VFB voltage is 0.765 V and SS pin source current is

2

μA.

(2)

The TPS54426 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.

Copyright

© 2010–2011, Texas Instruments Incorporated

7

Product Folder Link(s) :TPS54426