ACT8712QLCHA-T datasheet(21/36 Pages) ACTIVE-SEMI

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STEP-DOWN DC/DC CONVERTERS

Rev0, 25-Feb-08

ACT8712

Innovative Products. Active Solutions.

- 21 -

www.active-semi.com

ActivePMU

TM is a trademark of Active-Semi.

2CTM is a trademark of Philips Electronics.

General Description

REG1 and REG2 are fixed-frequency, current-

mode, synchronous PWM step down converters

that achieve peak efficiencies of up to 97%. REG1

is capable of supplying up to 550mA of output cur-

rent, while REG2 supports up to 750mA. These

regulators operate with a fixed frequency of

1.6MHz, minimizing noise in sensitive applications

and allowing the use of small external components.

REG1 and REG2 are available with a variety of

standard and custom output voltages, as described

in the Ordering Information section of this data-

sheet. REG1 and REG2 also take advantage of the

I2C interface, and may be software-controlled in

systems that require advanced power management

functions.

100% Duty Cycle Operation

Both REG1 and REG2 are capable of operating at

up to 100% duty cycle. During 100% duty-cycle op-

eration, the high-side power MOSFET is held on

continuously, providing a direct connection from the

input to the output (through the inductor), ensuring

the lowest possible dropout voltage in battery-

powered applications.

Synchronous Rectification

REG1 and REG2 both feature integrated n-channel

synchronous rectifiers, maximizing efficiency and

minimizing the total solution size and cost by elimi-

nating the need for external rectifiers.

Enabling and Disabling REG1 and REG2

Enable/disable functionality is typically implemented

as part of a controlled enable/disable scheme utiliz-

ing nMSTR and other system control features of the

ACT8712. REG1 and REG2 are automatically en-

abled whenever any of the following conditions are

met:

1) A valid input voltage is present at VIN,

2) nMSTR is driven low, or

3) ON1 is asserted high.

When none of these conditions are true, REG1 and

REG2 are disabled, and each regulator's quiescent

supply current drops to less than 1µA.

Programming the Output Voltage

By default, REG1 and REG2 each power up and

regulate to their default output voltage. Once the

system is enabled, each regulator’s output voltage

may be independently programmed to a different

value, typically in order to reduce the power con-

sumption of a microprocessor in standby mode.

Program the output voltages via the I2C serial inter-

face by writing to the REGx/VSETx[ ] and

REGx/VRANGE[ ] registers.

Programmable Operating Mode

By default, REG1 and REG2 operate in fixed-

frequency PWM mode at medium to heavy loads,

then transition 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 operation across the entire load current

range, at the expense of light-load efficiency, by

setting the REGx/MODE[ ] bit to [1].

Power-OK

REG1 and REG2 each feature a variety of status

bits that can be read by the system microprocessor.

If either output voltage is lower than the power-OK

threshold, typically 6% below the programmed

regulation voltage, REGx/OK[ ] will clear to 0.

Soft-Start

REG1 and REG2 each include matched soft-start

circuitry. When enabled, the output voltages track

the internal 80µs soft-start ramp and both power up

in a monotonic manner that is independent of load-

ing on either output. This circuitry ensures that both

outputs power up in a controlled manner, greatly

simplifying power sequencing design considera-

tions.

Compensation

REG1 and REG2 utilize current-mode control and a

proprietary internal compensation scheme to simul-

taneously simplify external component selection

and optimize transient performance over their full

operating range. No compensation design is re-

quired; simply follow a few simple guidelines de-

scribed below when choosing external components.

FUNCTIONAL DESCRIPTION

ACT8712QLCHA-T Datasheet(PDF) 21 Page - Active-Semi, Inc

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