LM2636 5-Bit Programmable Synchronous Buck Regulator Controller
October 1999
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 di-
rectly 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 speci-
fications. Current limiting is achieved by monitoring the volt-
age drop across the r
DS_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 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
2.5% 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 imple-
mented easily.
Features
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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
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Motherboard power supply/VRM for Cyrix Gxm, Cyrix
Gxi, Cyrix MII, Pentium
™
II, Pentium Pro, 6x86 and K6
processors
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5V to 1.3V–3.5V high current power supplies
Typical Application
DS100834-1
FIGURE 1. 5V to 1.3V–3.5V, 14A Power Supply
Pentium
™
is a trademark of Intel Corporation.
© 1999 National Semiconductor Corporation
DS100834
www.national.com
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