The W83321S is a high-speed N-channel synchronous buck regulator controller. W83321S adopts the PWM control structure of fixed frequency voltage mode. Both high-side and low-side MOSFETs are low-cost n-channel types. The regulator is biased by a 5V rail, and the power supply for the high-side mosfet can be powered by a separate 12V rail or by a local charge pump. Current limit is achieved by monitoring the voltage drop across the on-resistance of the low-side MOSFET. This method eliminates the requirement for additional current-sensitive resistors and avoids false triggering of OC protection when the vehicle identification number changes. Adaptive non-overlapping MOSFET gate drivers help avoid potential shoot-through issues while maintaining high efficiency.
Features Y operating range from +5V input Y 0.8V to VIN output range -0.8V Internal reference -? 1.5% over line voltage and temperature Y drives N-channel mosfet Y Simple single loop control design - Voltage mode pwm control Y Fast transient response Y Lossless, programmable over current protection - Use lower rds(on) of mosfet - no inductive Current Limit for Resistor y Small Converter Size - 250 kHz Fixed Frequency Oscillator - Internal Soft Start - Tiny Plastic Sop-8 Package

Applications y Motherboard power regulation y Subsystem power supply - pci/agp/gtl+ bus - acpi power control - sstl-2 and ddr sdram bus termination power y cable modems, set top boxes and dsl modems y dsp and core communication processors provide y storage power y Personal computer peripherals y industrial power supplies y 5v input dc-dc regulators y low voltage distributed power supplies

Internal block diagram

The MOSFET gate driver supply for the high side driver goes through the boot pin. This voltage can be provided by a separate, higher voltage source, or by a local charge pump structure, or by a combination of the two. Since the voltages at the low-side MOSFET gate and the high-side MOSFET gate are being monitored to determine the state of the MOSFET, careful consideration should be given to adding external components between the gate driver and their respective MOSFET gates. Doing so may interfere with fire protection. Current Limiting (Over Current Protection) Current limiting is achieved by sensing the voltage on the low side mosfet. The method improves the efficiency of the converter and reduces the cost by eliminating the current sensitive resistance. When the low side mosfet is turned on, a constant current of 90ua flows through the rocset, which is an external resistor connected between phase and isen, resulting in a fixed voltage drop. This fixed voltage is compared to VDS, and if the latter is higher, the chip goes into current limit mode. In current limit mode, both high-side and low-side MOSFETs are turned off. After a 25ms delay, soft start

The cycle begins. If the cause of the overcurrent still exists after the delay interval, the current limit will be triggered again. The shutdown-delay-soft-start cycle will repeat indefinitely until the overcurrent event clears. ipeak = (iocset x rocset)/rds(on) rds(on): low-side mosfet resistance
Local charge pump application circuit in vboot application

Separate VCC12 rail for Vboot application:

layout layout