9 - 21
AS1329
Datasheet - Detailed Description
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 (g
m
) 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 fromV
OUT
to GND programs
the output voltage from2.5 to 5V via pin FB as:
V
OUT
= 1.23V(1 + (R
1
/R
2
))
(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 limted to approximately 850mA independent of
V
IN
or V
OUT
.
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 fromreversing 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 (C
SW
).
8.3 Powersave Operation (AS1329A, AS1329B)
In light load conditions, the integrated powersave feature removes power fromall circuitry not required to monitor V
OUT
. When V
OUT
has
dropped approximately 1% fromnomnal, the AS1329A & B powers up and begins normal PWMoperation.
C
OUT
(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 intermttent PWMis 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, seamess PWMoperation.
While the AS1329A switches to automatic powersave mode already at mediumloads, 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 (C
FF
) between pins V
OUT
and FB in some applications can reduce V
OUTp-p
ripple and input quiescent current
during powersave mode. Typical values for C
FF
range from15 to 220pF.
2. In powersave mode, the AS1329A and AS1329B draws only 30μA fromthe 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 frombattery; when pin SHDNN is high the device is switched on.
If SHDNN is driven froma logic-level output, the logic high-level (on) should be referenced to V
OUT
to avoid intermttently 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 mcrocontroller, 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 frompin OUT to pin BATT
immediately after shutdown, resulting in a momentary spike of the voltage at pin BATT. The ratio of C
IN
and C
OUT
partly determne the size and
duration of this spike, as does the current-sink ability of the input device.