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Date: 3/6/06

SP6656, 400mA Synchronous Buck Regulator with Dynamically Adjustable Voltage Output

© Copyright 2006 Sipex Corporation

THEORY OF OPERATION

The SP6656 is a high efficiency synchro-

nous buck regulator with an input voltage

range of +2.7V to +5.5V and an output that

SP6656 features a unique on-time control

loop that runs in discontinuous conduction

mode (DCM) or continuous conduction mode

(CCM) using synchronous rectification.

Other features include over-temperature

shutdown, over-current protection, an ex-

ternal feedback pin, digitally controlled en-

able and output voltage selection.

The SP6656 operates with a light load qui-

escent current of 20µA using a 0.3Ω PMOS

main switch and a 0a.3Ω NMOS synchro-

nous switch. It operates with excellent effi-

ciency across the entire load range, making

it an ideal solution for battery powered ap-

plications and low current step-down con-

versions. The part smoothly transitions into

a 100% duty cycle under heavy load/low

input voltage conditions.

On-Time Control - Charge Phase

The SP6656 uses a precision comparator

and a minimum on-time to regulate the

output voltage and control the inductor cur-

rent under normal load conditions. As the

feedback pin drops below the regulation

point, the loop comparator output goes high

and closes the main switch. The minimum

on-timer is triggered, setting a logic high for

the duration defined by:

.

where:

To accommodate the use of ceramic and

other low ESR capacitors, an open loop

ramp is added to the feedback signal to

mimic the inductor current ripple. The fol-

lowing waveforms describe the ideal ramp

operation in both CCM and DCM operation.

In either CCM or DCM, the negative going

gram) is added to FB and this creates the

FB's signal. This FB signal is applied to the

negative terminal of the loop comparator.

To the positive terminal of the loop com-

parator is applied the REF voltage of 0.8V

plus an offset voltage Vos to compensate

negative terminal. The result is an internal

ramp with enough negative going offset

(approximately 50mV) to trip the loop com-

parator whenever FB falls below regulation.

The output of the loop comparator, a rising

VOLOW, causes a SET if BLANK = 0 and

OVR_I = 0. This starts inductor charging

(DRVON = 1) and starts the minimum on-

DRVON

REF, FB

REF’

FB’

I(L1)

RAMP: DCM OPERATION

DRVON

REF, FB

REF’

FB’

I(L1)

RAMP: CCM OPERATION