Preliminary Datasheet
GREEN MODE PWM CONTROLLER
Operation Description (Continued)
in series with the source of the MOSFET. The peak
inductor current under normal operating conditions is
controlled by the voltage at FB pin:
Short Circuit Protection
Built-in short circuit protection can protect a SMPS
from being damaged when short circuit of output or
over load conditions happen. The feedback voltage
will keep its value above its upper limit of 4.2V as long
as short circuit exists, and then gate driver will be
turned off after fixed delay time of 23 ms. When V
CC
of AP3101 dropped under the minimum operating
voltage, the device will be turned off and the system
will try to restart. The SMPS will not recover its
normal operation until the short circuit or over load is
removed.
Green Mode Operation
ADJ pin is the non-inverting input of skip cycle
comparator. The voltage at FB pin minus 0.9V is
compared with the voltage at ADJ pin. If the voltage at
ADJ pin is larger, AP3101 will start to blank its output
pulse. In normal operation, AP3101 works at fixed
switching frequency, and V
FB
is in high level. When
load power becomes lighter, the V
FB
will decrease.
When V
FB
drops to the threshold, AP3101 will enter
the skip cycle mode and operate intermittently. The
threshold is adjustable by changing the value of the
resistor connected from pin ADJ to the ground.
AP3101
I
pk
=
(
V
FB
−
0.9) / 3
R
S
Abnormal operating conditions occur when the power
supply output is overloaded or the output voltage sens-
ing is lost. Under these conditions, the current sense
comparator threshold will be internally clamped to
0.85V. Therefore the maximum peak switch current is:
I
PK
(max)
=
0.85 /
R
S
Leading Edge Blanking
A narrow spike on the leading edge of the current
waveform can usually be observed when the power
MOSFET is turned on. A 270 ns leading-edge blank is
built-in to prevent the false-triggering caused by the
turn-on spike; so the RC filter on the current sense
input can be removed. During this period, the current
limit comparator is disabled and the gate driver can not
be switched off.
Built-in Slope Compensation
It is well known that a continuous current mode
SMPS may become unstable when the duty cycle
exceeds 50%. The built-in slope compensation can
improve the stability, so there is no need for design
engineer to spend much time on that.
Apr. 2007 Rev. 1. 3
10
BCD Semiconductor Manufacturing Limited
BCDSEMI [ BCD SEMICONDUCTOR MANUFACTURING LIMITED ]
