LM2636| Datasheet

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LM2636 5-Bit Programmable Synchronous Buck Regulator Controller

March 2005

LM2636 5-Bit Programmable Synchronous Buck Regulator Controller
General Description
The LM2636 is a high speed controller designed specifically for use in synchronous DC/DC buck converters for advance d microprocessors. A 5-bit DAC accepts the VID code directly from the CPU and adjusts the output voltage from 1.3V to 3.5V. It provides the power good, over-voltage protection, and output enable features as required 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 LM2636 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 monitor 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 flexibility to make trade-offs between load transient response performance, 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 2.5% 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 implemented easily.

Features
n n n n n n n n n n n 1.3V to 3.5V 5-bit programmable output voltage Synchronous rectification Power Good flag and output enable Over-voltage protection Initial Output Accuracy: 1.5% over temperature Current limit without external sense resistor Adaptive non-overlapping MOSFET gate drives Adjustable switching frequency: 50 kHz to 1 MHz Dynamic output voltage positioning 1.256V reference voltage available externally Plastic SO-20 package and TSSOP-20 package

Applications
n Motherboard power supply/VRM for Cyrix Gxm, Cyrix Gxi, Cyrix MII, PentiumTM II, Pentium Pro, 6x86 and K6 processors n 5V to 1.3V 3.5V high current power supplies

Connection Diagrams
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10083403

Plastic SO-20 Order Number LM2636M See NS Package Number M20B

Plastic TSSOP-20 Order Number LM2636MTC See NS Package Number MTC20

PentiumTM is a trademark of Intel Corporation.

2005 National Semiconductor Corporation

DS100834

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