www.austriamicrosystems.com

Revision 1.07

8 - 17

AS1322

Low-Noise Fixed-Frequency Operation

Oscillator

The AS1322 switching frequency is internally fixed at 1.2MHz allowing the use of very small external components.

Error Amplifier

1.23V reference voltage is compared to the voltage at pin FB to generate an error signal at the output of the error

(EQ 1)

Current Sensing

A signal representing the internal NMOS-switch current is summed with the slope compensator. The summed signal is

compared to the error amplifier output to provide a peak current control command for the PWM. Peak switch current is

Zero Current Comparator

The zero current comparator monitors the inductor current to the output and shuts off the PMOS synchronous rectifier

once this current drops to 20mA (approx.). This prevents the inductor current from reversing polarity and results in

improved converter efficiency at light loads.

Anti-Ringing Control

Anti-ringing control circuitry prevents high-frequency ringing on pin SW as the inductor current approaches zero. This

Powersave Operation (AS1322A)

In light load conditions, the integrated powersave feature removes power from all circuitry not required to monitor

operation.

load remains below the powersave threshold. The frequency of this intermittent PWM is proportional to load current;

i.e., as the load current drops further below the powersave threshold, the AS1322A turns on less frequently. When the

load current increases above the powersave threshold, the AS1322A will resume continuous, seamless PWM opera-

tion.

Notes:

2. In powersave mode the AS1322A draws only 30µA from the output capacitor(s), greatly improving converter effi-

ciency.

Shutdown

When pin SHDNN is low the AS1322 is switched off and <1µA 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

during shutdown.

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

shutdown. 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.