APW1172
Operating waveforms (Cont.)
3. Current Limit :
- V
IN
= 12V,V
OUT
= 3.3V
- C
IN
= 22µF, C
OUT
= 220µF, L = 15
µH
4. Load Transient :
- V
IN
= 12V,V
OUT
= 3.3V
- C
IN
= 22µF, C
OUT
= 220µF, L = 15
µH
I
OUT
I
OUT
V
OUT
V
OUT
COMP
Ch1 : V
OUT
,2V/div
Ch2 : COMP,2V/div
Ch3 : I
OUT
,2A/div
Time : 2ms/div
Ch1 : V
OUT
,200mV/div,offset 3.3V
Ch2 : I
OUT
,1A/div,100mA-3A
Ch2 rising time : 4us
Ch2 falling time : 4us
Time : 10us/div
Functional Description
Power-On-Reset
A Power-On-Reset circuit monitors input voltages at
VCC pin to prevent wrong logic controls. The POR
function initiates immediately by the inductor current
with it’ limit after the supply voltage exceed firstly it’
s
s
threshold voltage after powering on.
Output Voltage Regulation
An error amplifier working with a temperature-compen-
sated 1.235V reference. The error amplifier designed
with high bandwidth and DC gain provides very fast
transient response and less load regulation. It compares
the reference with the feedback voltage and amplifies
the difference in it’ output called error signal. The
s
error signal feeds into the input terminal of PWM
comparator and compared with internal saw tooth wave.
It generates a PWM control signal by the PWM
comparator. The PWM signal feeds into the logic circuit
and turns on or off the pass element. The Buck type
output stage regulates the correct output voltage
depends on the previous mechanism.
Current Limit
The APW1172 monitors the current flow through the
pass element and limits the maximum output current
to prevent damages during overload or short-circuit
conditions.
Over-Voltage Protection (OVP)
The over voltage protection is realized by using an
Copyright
©
ANPEC Electronics Corp.
Rev. A.4 - Aug., 2005
11
www.anpec.com.tw
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
