RT7274 datasheet(5/22 Pages) RICHTEK

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RT7274/79/80/81

DS7274/79/80/81-01 February 2013

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Detailed Description

The RT7274/79/80/81 are high-performance 700kHz 2A

step-down regulators with internal power switches and

synchronous rectifiers. They feature an Advanced Constant

On-Time (ACOTTM) control architecture that provides

stable operation with ceramic output capacitors without

complicated external compensation, among other benefits.

The input voltage range is from 4.5V to 18V and the output

is adjustable from 0.765V to 8V.

The proprietary ACOTTM control scheme improves upon

other constant on-time architectures, achieving nearly

constant switching frequency over line, load, and output

voltage ranges. The RT7274/79/80/81 are optimized for

ceramic output capacitors. Since there is no internal clock,

response to transients is nearly instantaneous and inductor

current can ramp quickly to maintain output regulation

without large bulk output capacitance.

Constant On-Time (COT) Control

The heart of any COT architecture is the on-time one-

shot. Each on-time is a pre-determined “fixed” period

that is triggered by a feedback comparator. This robust

arrangement has high noise immunity and is ideal for low

duty cycle applications. After the on-time one-shot period,

there is a minimum off-time period before any further

regulation decisions can be considered. This arrangement

avoids the need to make any decisions during the noisy

time periods just after switching events, when the

switching node (SW) rises or falls. Because there is no

fixed clock, the high-side switch can turn on almost

immediately after load transients and further switching

pulses can ramp the inductor current higher to meet load

requirements with minimal delays.

Traditional current mode or voltage mode control schemes

typically must monitor the feedback voltage, current

signals (also for current limit), and internal ramps and

compensation signals, to determine when to turn off the

high-side switch and turn on the synchronous rectifier.

Weighing these small signals in a switching environment

is difficult to do just after switching large currents, making

those architectures problematic at low duty cycles and in

less than ideal board layouts.

Because no switching decisions are made during noisy

time periods, COT architectures are preferable in low duty

cycle and noisy applications. However, traditional COT

control schemes suffer from some disadvantages that

preclude their use in many cases. Many applications require

a known switching frequency range to avoid interference

with other sensitive circuitry. True constant on-time control,

where the on-time is actually fixed, exhibits variable

switching frequency. In a step-down converter, the duty

factor is proportional to the output voltage and inversely

proportional to the input voltage. Therefore, if the on-time

is fixed, the off-time (and therefore the frequency) must

change in response to changes in input or output voltage.

Modern pseudo-fixed frequency COT architectures greatly

improve COT by making the one-shot on-time proportional

to VOUT and inversely proportional to VIN. In this way, an

on-time is chosen as approximately what it would be for

an ideal fixed-frequency PWM in similar input/output

voltage conditions. The result is a big improvement but

the switching frequency still varies considerably over line

and load due to losses in the switches and inductor and

other parasitic effects.

Another problem with many COT architectures is their

dependence on adequate ESR in the output capacitor,

making it difficult to use highly-desirable, small, low-cost,

but low-ESR ceramic capacitors. Most COT architectures

use AC current information from the output capacitor,

generated by the inductor current passing through the

ESR, to function in a way like a current mode control

system. With ceramic capacitors the inductor current

information is too small to keep the control loop stable,

like a current mode system with no current information.

ACOT

TM Control Architecture

Making the on-time proportional to VOUT and inversely

proportional to VIN is not sufficient to achieve good

constant-frequency behavior for several reasons. First,

voltage drops across the MOSFET switches and inductor

cause the effective input voltage to be less than the

measured input voltage and the effective output voltage to

be greater than the measured output voltage. As the load

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