www.austriamicrosystems.com

Revision 1.05

10 - 19

AS1344

Datasheet - D e t a i l e d D e s c r i p t i o n

8 Detailed Description

The AS1344 features a current limiting circuitry, a fixed-frequency PWM architecture, power-OK circuitry, thermal pro-

tection, and an automatic powersave mode in a tiny package, and maintains high efficiency at light loads.

Figure 32. Block Diagram with Shutdown Disconnect Switch

Automatic powersave mode regulates the output and also reduces average current flow into the device, resulting in

high efficiency at light loads. When the output increases sufficiently, the powersave comparator output remains high,

resulting in continuous operation.

For each oscillator cycle, the power switch is enabled. A voltage proportional to switch current is added to a stabilizing

ramp and the resulting sum is delivered to the positive terminal of the PWM comparator.

etic comparator disables the power circuitry, with only the low-power circuitry still active (total current consumption is

minimized).

In light loads, the output voltage (and the voltage at FB) will increase until the powersave comparator disables the

power circuitry, causing the output voltage to decrease again. This cycle is repeated resulting in low-frequency ripple at

the output.

POK Function

The POK output indicates if the output voltage is within 90% of the nominal voltage level. As long as the output voltage

is within regulation the open-drain POK output is high impedance. The POK output can be tied to any external voltage

up to a maximum of 5V via a pull-up resistance R3 (see Typical Application on page 12). If the output voltage drops

below 90% of the nominal voltage the POK pin is pulled to GND.

Note:

AS1344

+

–

PWM

Control

Slope

Compensator

1 MHz

Spread

Spectrum

Ramp

Generator

+

–

Powersave

Operation

Control

–

Shutdown

Control

PWM

Comp

Σ

+

–

1.25V

Ref

Sync Drive

Control

Good

1.13V

Amp

Shutdown

Powersave

0.9

Ω

NMOS

4.7µF

Output Voltage

5.5V to 42V

Current

Sense

PGND

5

1

EN

10

FB

6

LX

9

VOUT

GND

3

0.3

Ω

PMOS

6.8µH

7

SWOUT

8

VIN

2

POK

10µF

Input Voltage

0.9V to 3.6V

4

Short

Delay