5-bit Programmable Synchronous Buck Regulator Controller - National Semiconductor

The LM2635 is a high speed controller designed specifically for use in synchronous DC/DC buck converters for the Pentium II or Deschutes Microprocessor A 5-bit DAC ac- cepts the VID code directly from the CPU and adjusts the output voltage from 1.8V to 3.5V. It provides the power good, over-voltage protection, and output enable features as re- quired by Intel VRM specifications. Current limiting is achieved by monitoring the voltage drop across the rDS_ON of the high side MOSFET which eliminates an expensive Current Sense resistor. The LM2635 employs a fixed-frequency voltage mode PWM architecture. To provide a faster response to a large and fast load transient, two ultra-fast Comparators are built in to moni- tor the output voltage and override the primary control loop when necessary. The PWM frequency is adjustable from 50 kHz to 1 MHz through an external resistor. The wide range of PWM frequency gives the power supply designer the flexibil- ity to make trade-offs between load transient response per- formance, MOSFET cost and the overall efficiency. The adaptive non-overlapping MOSFET Gate drivers help avoid any potential shoot-through problem while maintaining high efficiency. BiCMOS Gate drivers with Rail-to-Rail swing ensure that no spurious turn-on occur. When only 5V is available, a bootstrap structure CAN be employed to accommodate an NMOS high side Switch The precision reference trimmed to 2% over temperature is available externally for use by other Regulators Dynamic positioning of load voltage, which helps cut the number of output capacitors, CAN also be imple- mented easily. By National Semiconductor Corporation