AS1337
Datasheet - D e t a i l e d D e s c r i p t i o n
Low-Voltage Start-Up and Soft Start
The AS1337 requires a V
IN
of only 0.85V (typ) or higher to start up. The low-voltage start-up circuitry controls the inter-
nal switches and provides a soft start function, where the inductor current during start up is limited for a certain time.
This soft start and in rush current limitation feature provides also a smooth curve of V
OUT
after start up and makes the
overshoot of V
OUT
minimal. To achieve this, a 100pF capacitor as C
FF
is needed
to speed
up the feedback loop.
After a certain time a bigger inductor current is allowed and the soft start is disabled.
Low-Noise Fixed-Frequency Operation
Oscillator
The AS1337 switching frequency is internally fixed at 1.2MHz allowing the use of very small external components.
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 V
IN
or V
OUT
.
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 LX as the inductor current approaches zero. This is
accomplished by damping the resonant circuit formed by the inductor and the capacitance on pin LX.
Setting Output Voltage
A voltage divider from VOUT to GND programs the output voltage from 2.5V to 5V via pin FB as:
V
OUT
= 1.23V(1 + (R
1
/R
2
))
(EQ 1)
Powersave Operation
In light load conditions, the integrated powersave feature removes power from all circuitry not required to monitor
V
OUT
. When V
OUT
has dropped approximately 1% from nominal, the AS1337 powers up and begins normal PWM
operation.
C
OUT
recharges, causing the AS1337 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 AS1337 turns on less frequently. When the
load current increases above the powersave threshold, the AS1337 will resume continuous, seamless PWM operation.
Note:
An optional capacitor (C
FF
) between pins VOUT and FB in some applications can reduce V
OUTp-p
ripple and
input quiescent current during powersave mode. Typical values for C
FF
range from 15pF to 220pF.
Buck Mode
The AS1337A offers a low power buck mode for V
IN
> V
OUT
operation. The switching during buck mode is similar to
boost mode, but with forced reduction of some energy internal to be able to provide a buck conversion.
Revision 1.04
8 - 14
AMSCO [ AUSTRIAMICROSYSTEMS AG ]
