W83321S Datasheet, PDF (7/18 Pages) Winbond – N-Channel FET Synchronous Buck Regulator Controller

W83321S/W83321G
6. INTERNAL BLOCK DIAGRAM
VDD
POR
90µA
V_REF
0.8V
Soft Start
Ramp
Control
Logic
FB
COMP
+
Output
EA
-
Clamp
Oscillator
250KHz
Ramp
Generator
Gate
Control
+
Logic
-
VDD
1.5Vp-p
I_SEN
BOOT
UGATE
LGATE
GND
MOSFET Gate Drivers
Power for the high-side driver is through the BOOT pin. This voltage can be supplied by a separate,
higher voltage source, or supplied from a local charge pump structure or even the combination of the
two.
Since the voltage of the low-side MOSFET gate and the high-side MOSFET gate are being monitored
to determine the state of the MOSFET, it should be considered carefully to add external components
between the gate drivers and their respective MOSFET gates. Doing so may interfere with the shoot-
through protection.
Current Limit (Over current protection)
Current limit is realized by sensing the voltage across the low-side MOSFET while it is on. This
method enhances the converter's effeciency and reduces cost by eliminating a current sensing
resistor.
While low-side MOSFET is turned on, a constant current of 90uA is forced through R_OCSET which is an
external resistor connected between phase and ISEN, causing a fixed voltage drop. This fixed voltage
is compared against V_DS and if the latter is higher, the chip enters current limit mode. In the current
limit mode both the high-side and low-side MOSFETS are turned off. After a 25ms delay, a soft-start
Publication Release Date: January, 2006
-7-
Revision 0.51