(

)

(

)

2

IN

OUT

OUT

OUT LL

IN

V

V

V

I

L fsw

V

-

×

=

×

× ×

1

(nF) x V

(nF) x 0.765 1.1

REF

SS

SS

t

(ms) =

=

SS

I

( A)

SS

C

1.1

C

6

´

m

´

TPS54229E

www.ti.com

SLVSAZ7A – SEPTEMBER 2011 – REVISED MARCH 2012

OVERVIEW

The TPS54229E is a 2-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.

DETAILED DESCRIPTION

PWM Operation

The main control loop of the TPS54229E 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

TPS54229E uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The

TPS54229E 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.

Auto-Skip Eco-Mode™ Control

The TPS54229E 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, 6

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

6

μA.

(2)

The TPS54229E 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

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.

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

normal mode operation.

Copyright © 2011–2012, Texas Instruments Incorporated

7

Product Folder Link(s) :TPS54229E