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AS1325

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

8 Detailed Description

The AS1325 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 27. 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 AS1325 integrated current-limited key circuitry provides low quiescent current and extremely-high efficiency over

Light Loads:

Inductor current is limited by the 0.4A N-channel current limit or by the 7µs switch maximum on-time. The lower current

limit reduces the ripple of the output voltage. At each cycle, the inductor current must ramp down to zero before the

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

another cycle begins.

Higher Loads:

If after the first light load cycle the output voltage has not reached its target value of 3.3V or 5.0V, the inductor current

limit is increased to 0.7A. After the P-channel minimum on-time the next loading cycle is started if the output voltage is

still below its target value. If the target value is reached, the inductor current must ramp down to zero before the next

cycle may start. When the error comparator senses that the output has fallen below the regulation threshold, another

load cycle begins (see Figure 13 on page 7 and Figure 25 on page 9).

+5.0V Output only

Zero

Crossing

Detector

AS1325

Driver

and

Control

Logic

Startup

Circuitry

10µH

+1.5 to +3.3V or

+1.5 to +5.0V

Battery

Current

Limiter

+1.228V

22µF

+

–

+

–

+1.1V

+3.3 or

+5.0V

Output

22µF

4

LX

2

BATT

6

RESETN

1

SHDNN

5

OUT

GND

3

Anti-

Ring-

ing

Switch