ACT8325

Rev 2, 25-May-11

STEP-DOWN DC/DC CONVERTER

Active-Semi

Innovative Power

TM

- 9 -

www.active-semi.com

Copyright © 2011 Active-Semi, Inc.

ActivePMU

FUNCTIONAL DESCRIPTION

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 800mA. 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.

100% Duty Cycle Operation

Both REG1 and REG2 are 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 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

Each of the ACT8325's regulators features inde-

pendent pin-controlled enable control. Drive ON1 to

a logic-high to enable REG1, drive ON1 to a logic-

low to disable REG1. Similarly, drive ON2 to a

logic-high to enable REG2, drive ON2 to a logic-low

to disable REG2. When disabled, each regulator's

supply current drops to less than 1µA.

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.

Thermal Shutdown

The ACT8325 integrates thermal shutdown protec-

tion circuitry to prevent damage resulting from ex-

cessive thermal stress, as may be encountered un-

der fault conditions. This circuitry disables all regu-

lators if the ACT8325 die temperature exceeds

160°C, and prevents the regulators from being en-

abled until the IC temperature drops by 20°C (typ).

Input Capacitor Selection

The input capacitor reduces peak currents and

noise induced upon the voltage source. A 2.2µF

ceramic capacitor for each of REG1 and REG2 is

recommended for most applications.

Output Capacitor Selection

For most applications, 10µF ceramic output capaci-

tors are recommended for both REG1 and REG2.

Although the these regulators were designed to

take advantage of the benefits of ceramic capaci-

tors, namely small size and very-low ESR, low-ESR

tantalum capacitors can provide acceptable results

as well.

Output Voltage Programming

Figure 4:

Output Voltage Programming

Figure 4 shows the feedback network necessary to

set the output voltage when using the adjustable

OUTx

ACT8325

FBx