WS3252
Operation switching frequency
Programmable Cable Drop Compensation
Cable drop compensation is implemented to achieve
good load regulation inWS3252. An offset voltage is
generated at INV by an internal current flowing into the
resister divider. The current is inversely proportional to
the voltage across pin COMP, as a result, it is inversely
proportional to the output load current, thus the drop due
to the cable loss can be compensated. As the load
current decreases from full-load to no-load, the offset
voltage at INV will increase. It can also be programmed
by adjusting the resistance of the divider to compensate
the drop for various cable lines used.
The switching frequency of WS3252 is adaptively
controlled according to the load conditions and the
operation modes. The operation switching frequency at
maximum output power is set to 60KHz internally. In
DCM mode, the maximum output power is given by
1
2
P
=
LP FSW IP
oMAX
2
Where Lp indicate the inductance of primary winding and
Ip is peak current of primary winding.
The change of the primary winding inductance results in
the change of the maximum output power and constant
output current in CC mode. To compensate the change
from viariations of primary winding inductance, the
switching frequency is locked by an internal loop such
that the switching frequency is
Gate Drive
The gate drive strength which is too weak leads to over
switch loss of MOSFET while too strong gate drive
output compromises in the over EMI. A good tradeoff
between output strength and dead time control is
achieved through the design of the built-in totem pole
gate. The low standby dissipation and good EMI system
design is easier to achieve through this dedicated devise.
For MOSFET gate protection, an internal 12V clamp is
added at higher than expected VCC input.
1
FSW
=
2TDemag
Since TDemag is inversely proportional to the
inductance, as a result, the product Lp and Fsw is
constant, thus the maximum output power and constant
current in CC mode will not change as primary winding
inductance changes. Up to ±10% variation of the primary
winding inductance can be compensation.
Protection Controls
Excellent system stability is achieved by the
comprehensive protection of WS3252. Including
Cycle-by-Cycle current limiting (OCP), VCC Clamp,
Under Voltage Lockout on VCC (UVLO).
Frequency Shuffling
Excellent EMI performance is achieved by using
frequency jittering in WS3252
Device restarts when VCC voltage drops below UVLO
limit. It is clamped when VCC is higher than threshold
value. The power MOSFET is shut down when VCC
drops below UVLO limit and device enters power on
start-up sequence thereafter.
Current Sensing and Leading Edge Blanking
Cycle-by-Cycle current limiting is offered in WS3252.
The switch current is detected by a sense resistor into
the sense pin. An internal leading edge blanking circuit
chops off the sense voltage spike at initial MOSFET on
state due to snubber diode reverse recovery so that the
external RC filtering on sense input is no longer required.
The current limit comparator is disabled and thus cannot
turn off the internal MOSFET during the blanking period.
PWM duty cycle is determined by the current sense input
voltage and the EA output voltage.
7/8
Steady, keep you advance
WINSEMI [ SHENZHEN WINSEMI MICROELECTRONICS CO., LTD ]
