ACT8309(2008) 查看數據表（PDF） - Active-Semi, Inc

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ACT8309
(Rev.:2008)

1.5A, PWM Step-Down DC/DCs in TDFN

Active-Semi, Inc 

Active-Semi

ACT8309/8310/8311

Rev PrA, 24-Jan-08

FUNCTIONAL DESCRIPTION

The patent-pending ACT8309/ACT8310/ACT8311

are current-mode, fixed-frequency PWM, synchro-

nous step-down DC/DC converters that are capable

of delivering up to 1.5A (ACT8311) with efficiency of

up to 95%. These devices feature very low-

resistance power MOSFETs and 100% duty cycle

operation, making them ideal choices for portable

applications requiring a 3.0V or 3.3V rail to be gen-

erated form a Li+ battery. These device operate

with a fixed frequency of 1.35MHz, minimizing noise

in sensitive equipment as well as optimizing both

efficiency and component size and cost.

100% Duty Cycle Operation

These devices are capable of operating at up to

100% duty cycle operation. 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

The ACT8309/ACT8310/ACT8311 each feature

integrated n-channel synchronous rectifiers, maxi-

mizing efficiency and minimizing the total solution

size and cost by eliminating the need for external

rectifiers.

Soft-Start

The ACT8309/ACT8310/ACT8311 each include

integrated soft-start circuitry. When enabled, the

output voltage track an internal 200µs soft-start

ramp so that the output power up in a controlled,

monotonic manner that is independent of loading.

Compensation

The ACT8309/ACT8310/ACT8311 each utilize cur-

rent-mode control and a proprietary internal compen-

sation scheme to simultaneously simplify external

component selection and optimize transient perform-

ance over their full operating range. No compensation

design is required, simply follow a few simple guide-

lines described below when choosing external com-

ponents.

Capacitor Selection

The input capacitor reduces peak currents and

noise induced upon the voltage source. A 10µF

ceramic capacitor is recommended for most appli-

cations.

For most applications, a 22µF ceramic output ca-

pacitor is recommended. Although the these regula-

tors were designed to take advantage of the bene-

fits of ceramic capacitors, namely small size and

very-low ESR, low-ESR tantalum capacitors can

provide acceptable results as well.

Inductor Selection

These devices were optimized for operation with

3.3µH inductors, although inductors in the 1.5µH to

4.7µH range may be used. Choose an inductors

with a low DC-resistance, and avoid inductor satu-

ration by choosing inductors with DC ratings that

exceed the maximum output current of the applica-

tion by at least 30%.

Output Voltage Setting

Figure 3 shows the feedback network necessary to

set the output voltage when using adjustable output

voltage options. output voltage options. Select com-

ponents as follows: Set RFB2 = 200kΩ, calculate

RFB1 using the following equation:

RFB1

=

RFB2

⎜⎜⎝⎛

VOUT

VFB

− 1⎟⎟⎠⎞

Where VFB is 0.8V.

Figure 3:

Output Voltage Setting

ACT8309/

ACT8310/

ACT8311 FB

VOUT

RFB1

RFB2

Enable/Disable Control

These devices may be enabled and disabled using

the EN input. Drive EN to a logic-high to enable the

regulator, drive EN to a logic-low to disable the

regulator. When disabled, each regulator’s quies-

cent supply current drops to less than 1µA.

Power-OK Output (POK)

These devices feature an open-drain “Power-OK”

indicator output. This output sinks current whenever

the output voltage is (typically) 5% below the regu-

lation voltage, and goes high-impedance when the

output voltage is above this threshold. To generate

Innovative Products. Active Solutions.

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Copyright © 2008 Active-Semi, Inc.

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