ACT8798 datasheet(23/34 Pages) ACTIVE-SEMI

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STEP-DOWN DC/DC CONVERTERS

ACT8798

Rev 1, 16-Nov-09

Innovative PowerTM

- 23 -

www.active-semi.com

ActivePMUTM is a trademark of Active-Semi.

I2CTM is a trademark of Philips Electronics.

Active-Semi

General Description

REG1, REG2, and REG3 are fixed-frequency,

current-mode, synchronous PWM step down

converters that achieve peak efficiencies of up to

97%. REG1 and REG2 are capable of supplying up

to 1.2A of output current, while REG3 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. Each of the step-down DC/DCs are

available with a variety of standard and custom

output voltages, and each may be software-

controlled via the I2C interface by systems that

require advanced power management functions.

100% Duty Cycle Operation

REG1, REG2, and REG3 are each capable of

operating at up to 100% duty cycle. During 100%

duty-cycle operation, 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, REG2, and REG3 each feature integrated n-

channel

synchronous

rectifiers,

maximizing

efficiency and minimizing the total solution size and

cost by eliminating the need for external rectifiers.

Enabling and Disabling REG1, REG2,

and REG3

Enable/disable functionality is typically implemented

as part of a controlled enable/disable scheme

utilizing nMSTR and other system control features

the

ACT8798.

REG1

and

REG2

are

automatically enabled whenever either of the

following conditions re met:

1) nMSTR is driven low, or

2) PWRHLD is asserted high.

When none of these conditions are true, or if a

regulator’s ON[_] bit is set to [0], REG1 and REG2

are disabled, and each regulator’s quiescent supply

current drops to less than 1µA.

REG3 is enabled whenever ON3 is asserted high,

and is disabled whenever ON is asserted low or if

the REG3/ON[_] bit is set to [0].

Programming the Output Voltage

By default, REG1, REG2, and REG3 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 consumption of a microprocessor in standby

mode. Program the output voltages via the I2C

serial interface by writing to the REGx/VSETx[ ] and

REGx/VRANGE[ ] registers.

Programmable Operating Mode

By default, REG1, REG2, and REG3 each 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, REG2, and REG3 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, REG2, and REG3 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 loading on either output. This

circuitry ensures that each output powers up in a

controlled

manner,

greatly

simplifying

power

sequencing design considerations.

Compensation

REG1, REG2, and REG3 utilize current-mode

control and a proprietary internal compensation

scheme

simultaneously

simplify

external

component

selection

and

optimize

transient

performance over their full operating range. No

compensation design is required; simply follow a

few simple guidelines described below when

choosing external components.

FUNCTIONAL DESCRIPTION

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