AS1329 datasheet(9/20 Pages) AMSCO

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AS1329

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

8.2 Low-Noise Fixed-Frequency Operation

8.2.1

Oscillator

The AS1329 switching frequency is internally fixed at 1.2MHz allowing the use of very small external components.

8.2.2

Error Amplifier

The integrated error amplifier is an internally compensated trans-conductance (gm) type (current output). The internal 1.23V reference voltage is

compared to the voltage at pin FB to generate an error signal at the output of the error amplifier. A voltage divider from VOUT to GND programs

the output voltage from 2.5 to 5V via pin FB as:

VOUT = 1.23V(1 + (R1/R2))

(EQ 1)

8.2.3

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 limited to approximately 850mA independent of

VIN or VOUT.

8.2.4

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.

8.2.5

Anti-Ringing Control

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

damping the resonant circuit formed by the inductor and the capacitance on pin SW (CSW).

8.3 Powersave Operation (AS1329A, AS1329B)

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

dropped approximately 1% from nominal, the AS1329A & B powers up and begins normal PWM operation.

COUT (see Figure 19 on page 8) recharges, causing the AS1329A and AS1329B to re-enter powersave mode as long as the output 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 AS1329A and AS1329B turns on less frequently. When the load current increases above the

powersave threshold, the AS1329A and AS1329B will resume continuous, seamless PWM operation.

While the AS1329A switches to automatic powersave mode already at medium loads, the AS1329B will do so only at very light loads.

The AS1329C is a continuous switching device, hence the output voltage ripple is very low and no additional frequencies are produced which

may cause interference.

Notes:

1. An optional capacitor (CFF) between pins VOUT and FB in some applications can reduce VOUTp-p ripple and input quiescent current

during powersave mode. Typical values for CFF range from 15 to 220pF.

2. In powersave mode, the AS1329A and AS1329B draws only 30µA from the output capacitor(s), greatly improving converter efficiency.

8.4 Shutdown

When pin SHDNN is low the AS1329 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 to VOUT to avoid intermittently switching the device

on.

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 CIN and COUT partly determine the size and

duration of this spike, as does the current-sink ability of the input device.

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