ACT8342 datasheet(11/17 Pages) ACTIVE-SEMI

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ACT8342

Rev 3, 11-Sep-09

STEP-DOWN DC/DC CONVERTER

Innovative PowerTM

- 11 -

www.active-semi.com

ActivePMUTM is a trademark of Active-Semi.

FUNCTIONAL DESCRIPTION

General Description

REG1

fixed-frequency,

current-mode,

synchronous 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 operates 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 voltages, 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

applications.

Synchronous Rectification

REG1

features

integrated

n-channel

synchronous rectifier, which maximizes efficiency

and minimizes 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

enabled its output voltage tracks an internal 80µs

soft-start ramp so that it powers up in a monotonic

manner that is independent of loading.

Compensation

REG1

utilizes

current-mode

control

and

proprietary internal compensation scheme to

simultaneously

simplify

external

component

selection and optimize transient performance over

its full operating range. No compensation design is

required, simply follow a few simple guidelines

described

below

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

capacitor is recommended. Although REG1 was

designed to take advantage of the benefits of

ceramic capacitors, namely small size and very-low

ESR, low-ESR tantalum capacitors can provide

acceptable results as well.

Inductor Selection

REG1

utilizes

current-mode

control

and

proprietary internal compensation scheme to

simultaneously

simplify

external

component

selection and optimize transient performance over

its full operating 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

ACT8342

integrates

thermal

shutdown

protection circuitry to prevent damage resulting

from excessive thermal stress, as may be

encountered under fault conditions. This circuitry

disables all regulators if the ACT8342 die

temperature exceeds 160°C, and prevents the

regulators from being enabled 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

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

the following equation:

Where VFB is 0.625V

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