?/DIV>
=
(2)
The range of the maximum on-time is designed as 10 ~
50 祍.
Peak Current Limiting
The switch current is sensed by one resistor. The signal
is   feed   into   CS   pin   and   an   input   terminal   of   a
comparator. A high voltage in CS pin terminates a
switching cycle immediately and cycle-by-cycle current
limit   is   achieved.   The   designed   threshold   of   the
protection point is 0.82 V.
Leading-Edge Blanking (LEB)
A turn-on spike on CS pin appears when the power
MOSFET is switched on. At the beginning of each
switching pulse, the current-limit comparator is disabled
for around 400ns to avoid premature termination. The
gate drive output cannot be switched off during the
blanking   period.   Conventional   RC   filtering   is   not
necessary, so the propagation delay of current limit
protection can be minimized.
Under-Voltage Lockout (UVLO)
The turn-on and turn-off threshold voltage is fixed
internally   at   12 V/9.5 V.   This   hysteresis   behavior
guarantees   a   one-shot   startup with proper startup
resistor and hold-up capacitor. With an ultra-low startup
current of 20 礎(chǔ), one 1 M& R
IN
is sufficient for startup
under low input line voltage, 85 V
rms
. Power dissipation
on R
IN
would be less than 0.1 W even under high line
(V
AC
=265 V
rms
) condition.
Output Driver
With   low   on   resistance   and   high   current   driving
capability, the output driver can drive an external
capacitive load larger than 3000 pF. Cross conduction
current has been avoided to minimize heat dissipation,
improving efficiency and reliability. This output driver is
internally clamped by a 16.5 V Zener diode.
Zero-Current Detection (ZCD)
The zero-current detection of the inductor is achieved
using its auxiliary winding. When the stored energy of
the inductor is fully released to output, the voltage on
ZCD goes down and a new switching cycle is enabled
after a ZCD trigger. The power MOSFET is always
turned on with zero inductor current such that turn-on
loss and noise can be minimized. The converter works
in boundary-mode and peak inductor current is always
exactly twice of the average current. A natural power
factor correction function is achieved with the low-
bandwidth, on-time modulation. An inherent maximum
off time is built in to ensure proper startup operation.
This ZCD pin can be used as a synchronous input.
Noise Immunity
Noise on the current sense or control signal can cause
significant pulse-width jitter, particularly in the boundary-
mode   operation.   Slope   compensation   and   built-in
debounce circuit can alleviate this problem. Because
the FAN6961 has a single ground pin, high sink current
at the output cannot be returned separately. Good high-
frequency or RF layout practices should be followed.
Avoiding   long   PCB   traces   and   component   leads,
locating compensation and filter components near to the
FAN6961, and increasing the power MOSFET gate
resistance improve performance.