ACT8740 datasheet(18/40 Pages) ACTIVE-SEMI

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STEP-DOWN DC/DC CONVERTER

ACT8740

Rev PrB, 25-Feb-08

Innovative Products. Active Solutions.

- 18 -

www.active-semi.com

ActivePMU

TM is a trademark of Active-Semi.

2CTM is a trademark of Philips Electronics.

General Description

REG1 is a fixed-frequency, current-mode, synchro-

nous PWM step-down converters that achieves a

peak efficiency of up to 97%. REG1 is capable of

supplying up to 350mA of output current and oper-

ates with a fixed frequency of 1.6MHz, minimizing

noise in sensitive applications and allowing the use

of small external components. REG1 is available

with a variety of standard and custom output volt-

ages, and may be software-controlled via the I2C

interface by systems that require advanced power

management functions.

100% Duty Cycle Operation

REG1 is capable of operating at up to 100% duty

cycle. During 100% duty-cycle operation, the high-

side power MOSFET is held on continuously, pro-

viding a direct connection from the input to the out-

put (through the inductor), ensuring the lowest pos-

sible dropout voltage in battery-powered applica-

tions.

Synchronous Rectification

REG1 features an integrated n-channel synchro-

nous rectifier, which maximizes efficiency and mini-

mizes the total solution size and cost by eliminating

the need for an external rectifier.

Enabling and Disabling REG1

Enable/disable functionality is typically implemented

as part of a controlled enable/disable scheme utiliz-

ing nMSTR and other system control features of the

ACT8740. REG1 is automatically enabled when-

ever either of the following conditions are met:

1) nMSTR is driven low, or

2) ON1 is asserted high.

When none of these conditions are true, REG1 is

disabled, and its quiescent supply current drops to

less than 1µA.

Programming the Output Voltage

By default, REG1 powers up and regulates to its

default output voltage. Once the system is enabled,

REG1’s output voltage may be programmed to a

different value, typically in order to reduce the

power consumption of a microprocessor in standby

mode. Program the output voltage via the I2C serial

interface by writing to the REG1/VSET1[ ] register.

Programmable Operating Mode

By default, REG1 operates in fixed-frequency PWM

mode at medium to heavy loads, then transitions to

a proprietary power-saving mode at light loads in

order to save power. In applications where low

noise is critical, force fixed-frequency PWM opera-

tion across the entire load current range, at the ex-

pense of light-load efficiency, by setting the

REG1/MODE[ ] bit to [1].

Power-OK

REG1 features a power-OK status bit that can be

read by the system microprocessor. If the output

voltage is lower than the power-OK threshold, typi-

cally 6% below the programmed regulation voltage,

REG1/OK[ ] will clear to 0.

Soft-Start

REG1 includes internal soft-start circuitry, and en-

abled its output voltage tracks an internal 80µs soft-

start ramp so that it powers up in a monotonic man-

ner that is independent of loading.

Compensation

REG1 utilizes current-mode control and a proprie-

tary internal compensation scheme to simultane-

ously simplify external component selection and

optimize transient performance over its full operat-

ing range. No compensation design is required,

simply follow a few simple guidelines described be-

low when choosing external components.

Input Capacitor Selection

The input capacitor reduces peak currents and

noise induced upon the voltage source. A 2.2µF

ceramic input capacitor is recommended for most

applications.

Output Capacitor Selection

For most applications, a 10µF ceramic output ca-

pacitor is recommended. Although REG1 was de-

signed to take advantage of the benefits of ceramic

capacitors, namely small size and very-low ESR,

low-ESR tantalum capacitors can provide accept-

able results as well.

Inductor Selection

REG1 utilizes current-mode control and a proprietary

internal compensation scheme to simultaneously

FUNCTIONAL DESCRIPTION

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