ACT8332 datasheet(8/13 Pages) ACTIVE-SEMI

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ACT8332

Rev0, 14-Mar-08

STEP-DOWN DC/DC CONVERTER

Innovative Power

- 8 -

www.active-semi.com

ActivePMU

TM is a trademark of Active-Semi.

FUNCTIONAL DESCRIPTION

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, as well as an adjustable output voltage option.

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,

providing a direct connection from the input to the

output (through the inductor), ensuring the lowest

possible dropout voltage in battery-powered appli-

cations.

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

REG1 is enabled or disabled using ON1. Drive ON1

to a logic-high to enable REG1. Drive ON1 to a

logic-low to disable REG1, reducing supply current

to less than 1µA

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 proprie-

tary internal compensation scheme to simultane-

ously simplify external component selection and

optimize transient performance over its full operat-

ing range. REG1 was optimized for operation with a

3.3µH inductor, although inductors in the 2.2µH to

4.7µH range can be used. Choose an inductor with

a low DC-resistance, and avoid inductor saturation

by choosing inductors with DC ratings that exceed

the maximum output current of the application by at

least 30%.

Thermal Shutdown

The ACT8332 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 ACT8332 die temperature exceeds

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

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

Output Voltage Programming

Figure 4 shows the feedback network necessary to

set the output voltage when using the adjustable

output voltage option. Select components as fol-

lows: Set RFB2 = 51KΩ, then calculate RFB1 using

the following equation:

Where VFB1 is 0.625V

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