SS

SS

C4x(nF)

VFBx(V)

C4x(nF)

0.765 V

T

(ms) =

=

I

( A)

8

A

´

´

m

m

(

)

INx

Ox

Ox

Ox(LL)

SW

INx

V

V

V

1

I

=

2

L1x

V

-

´

´

´

´ f

TPS542951

www.ti.com

SLVSBI4B – JULY 2012 – REVISED OCTOBER 2013

OVERVIEW

The TPS542951 is a 2A/3A dual synchronous step-down (buck) converter with two integrated N-channel

MOSFETs for each channel. It operates using D-CAP2™ control mode. The fast transient response of D-CAP2™

control reduces the required output capacitance 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 TPS542951 is an adaptive on-time pulse width modulation (PWM) controller that

supports a proprietary D-CAP2™ control mode. D-CAP2™ 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 when the internal

timer expires. This timer is set by the converter’s input voltage, VINx, and the output voltage, VOx, to maintain a

pseudo-fixed frequency over the input voltage range hence it is called adaptive on-time control. The timer is reset

and the high-side MOSFET is turned on again when the feedback voltage falls below the nominal output voltage.

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

induced output ripple from D-CAP™ control.

PWM Frequency and Adaptive On-Time Control

TPS542951 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The

TPS542951 runs with a pseudo-fixed frequency of 700 kHz by using the input voltage and output voltage to set

the on-time timer. The on-time is inversely proportional to the input voltage and proportional to the output voltage,

therefore, when the duty ratio is VOx/VINx, the frequency is constant.

Auto-Skip Eco-mode™ Control

The TPS542951 is designed with Auto-Skip Eco-mode™ to increase light load efficiency. As the output current

decreases from heavy load condition, the inductor current also reduces and eventually comes to the point where

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

conduction modes. The rectifying MOSFET is turned off when zero inductor current is detected. As the load

current further decreases the converter runs into discontinuous conduction mode. The on-time is kept almost the

same as it was in the continuous conduction mode because it takes longer to discharge the output capacitor with

(1)

Soft Start and Pre-Biased Soft Start

The soft start time is adjustable. When the ENx pin becomes high, 8-µA current begins charging the capacitor

which is connected from the SSx 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. VFBx voltage is 0.765-V and SSx pin source current

is 8-µA.

(2)

The TPS542951 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 becomes greater than internal feedback voltage VFBx), the controller slowly activates synchronous

rectification by starting 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-biased output, and ensures that the output voltage

(VOx) starts and ramps up smoothly into regulation from pre-biased startup to normal mode operation.

Copyright © 2012–2013, Texas Instruments Incorporated

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