ACT8828 datasheet(28/49 Pages) ACTIVE-SEMI

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STEP-DOWN DC/DC CONVERTERS

ACT8828

Rev 3, 24-Sep-09

Active-Semi

Innovative Power

- 28 -

www.active-semi.com

ActivePMU

TM and ActivePathTM are trademarks of Active-Semi.

2CTM is a trademark of Philips Electronics.

FUNCTIONAL DESCRIPTION

General Description

REG1, REG2, and REG3 are fixed-frequency,

current-mode,

synchronous

PWM

step-down

converters that are capable of supplying up to 1.3A,

1.0A, and 0.55A of output current, respectively.

These regulators operate with a fixed frequency of

1.6MHz, minimizing noise in sensitive applications

and allowing the use of small external components,

and achieve peak efficiencies of up to 97%.

Each step-down DC/DC is available with a variety

of standard and custom output voltages, which may

software-controlled

systems

requiring

advanced power management functions, via the I2C

interface.

Buck Regulator PFM/PWM Operating

Modes

The buck converters offer PFM/PWM operating

modes to maximize efficiency under both light and

full load conditions. The device will automatically

transition from fixed frequency PWM mode to PFM

mode when the output current is approximately

100mA. In PFM mode, the device maintains output

voltage regulation by adjusting the switching

frequency. The device transitions into fixed

frequency PWM mode when the output current

reaches approximately 100mA.

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

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

REG1, REG2, and REG3 are typically enabled and

disabled

using

the

ACT8828's

closed-loop

enable/disable control scheme, including the nPBIN

input. Refer to the System Startup and Shutdown

section for more information about this function.

Each regulator is enabled when the following

conditions are met:

1) ONx is asserted high to enable REGx,

2) REGx/ONx[ ] is set to 1 when ONx is high

In addition REG1, REG2, or REG3 may be enable

when nPBIN is pushed low via 100kΩ resistance. It

depends on sequence is set. See the Control

Sequence section for more information.

When none of these conditions are true, REG1,

REG2 and REG3 are disabled, and each regulator’s

quiescent supply current drops to less than 1μA.

Power-OK

REG1, REG2 and REG3 each feature a variety of

status bits that can be read by the system

microprocessor. If any output falls below its power-

OK threshold, typically 6% below the programmed

regulation voltage, REGx/OK[ ] is cleared 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 guide lines described below when

choosing external components.

Input Capacitor Selection

The input capacitor reduces peak currents and

noise induced upon the voltage source. A 4.7μF

ceramic capacitor for each of REG1, REG2 and

REG3 is recommended for most applications.

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