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AS1321

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

8 Detailed Description

The AS1321 is a high-efficiency, compact step-up converter with 35µA quiescent supply current which ensures the

highest efficiency over a wide load range. With a minimum of +1.5V input voltage, the device is well suited for applica-

tions with one- or two-cells, such as lithium ion (Li+), nickel-metal-hydride (NiMH), or alkaline.

Figure 17. AS1321 - Block Diagram

The input battery is connected to the device through an inductor and an internal P-FET when pin SHDNN is low. In this

state, the step-up converter is off and the voltage drop across the P-FET body diode is eliminated, and the input bat-

tery can be used as a battery-backup or real-time-clock supply.

The built-in synchronous rectifier significantly improves efficiency.

Control Circuitry

The AS1321 integrated current-limited key circuitry provides low quiescent current and extremely-high efficiency over

by the 0.7A N-channel current limit. At each cycle, the inductor current must ramp down to zero after the on-time

before the next cycle may start. When the error comparator senses that the output has fallen below the regulation

threshold, another cycle begins.

Shutdown

When pin SHDNN is low the AS1321 is switched off and no current is drawn from battery; when pin SHDNN is high the

device is switched on. If SHDNN is driven from a logic-level output, the logic high-level (on) should be referenced to

Note: If pin SHDNN is not used, it should be connected directly to pin OUT.

In shutdown the battery input is connected to the output through the inductor and the internal synchronous rectifier P-

FET. This allows the input battery to provide backup power for devices such as an idle microcontroller, memory, or real-

time-clock, without the usual diode forward drop. In this way a separate backup battery is not needed.

In cases where there is residual voltage during shutdown, some small amount of energy will be transferred from pin

OUT to pin BATT immediately after shutdown, resulting in a momentary spike of the voltage at pin BATT. The ratio of

Zero

Crossing

Detector

AS1321

Driver

and

Control

Logic

Startup

Circuitry

10µH

+1.5 to +5.0V

Battery

Current

Limiter

+1.228V

22µF

+

–

+

–

+1.1V

+5.0V

Output

22µF

4

LX

2

BATT

6

RESETN

1

SHDNN

5

OUT

GND

3