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ML4902

High Current Synchronous Buck Controller

Micro Linear Corporation 

FUNCTIONAL DESCRIPTION

The ML4902 PWM controller permits the construction of a

simple yet sophisticated power supply for Intel’s Pentium

Pro and Pentium II microprocessor families. The ML4902

and its associated circuitry can be built either as a

Voltage Regulator Module (VRM) or as a dedicated supply

on the motherboard. The ML4902 controls two N-channel

MOSFETs in a synchronous buck regulator topology to

convert a 5V input to the voltage required by the

microprocessor. The output voltage can be set between

1.8V and 3.5V, as selected by an onboard DAC. Other

features which facilitate the design of DC-DC converters

for any type of processor include a trimmed 1% reference,

special transient-response optimization in the feedback

paths, a shutdown input, input and output power good

monitors, and overcurrent protection.

OUTPUT VOLTAGE SELECTION

The inputs of the internal 2-chord 4-bit DAC come from

open collector signals provided by the processor. These

signals specify what supply voltage the microprocessor

requires. The output voltage of the buck converter is

compared directly with the DAC voltage to maintain

regulation. D3 is the MSB input and D0 is the LSB input

of the DAC, while RANGE selects the output voltage

range and the LSB voltage increment of the DAC. The

output of the DAC is between 2.121V to 3.535V in 100mV

steps when RANGE = 1, and between 1.818V to 2.071V in

50mV steps when RANGE = 0. The output voltage set by

the DAC is 1% above the processor’s nominal operating

voltage to counteract the effects of connector and PC

trace resistance, and of the instantaneous output voltage

droop which occurs when a transient load is applied. For

codes 00110 to 01111 and code 11111, the N DRV H and

N DRV L outputs are disabled.

VOLTAGE FEEDBACK LOOP

The ML4902 contains two control loops to improve the

load transient response. The output voltage is directly

monitored via the VFB pin and compared to the desired

output voltage set by the internal DAC. When the output

voltage is within +3% of the DAC voltage, the

proportional control loop (closed by the voltage error

amplifier) keeps the output voltage at the correct value. If

the output falls below the DAC voltage by more than 3%,

one side of the transient loop is activated, forcing the

output of the ML4902 to maximum duty cycle until the

output comes back within the +3% limit. If the output

voltage rises above the DAC voltage by more than 3%,

the other side of the transient loop is activated, and the

upper MOSFET drive is disabled until the output comes

back within the +3% limit. If the output voltage rises

above the DAC voltage by more than 10%, both N DRV

H and N DRV L will be disabled to turn the converter off.

During start-up, the transient loop is disabled until the

output voltage is within -3% of the DAC voltage.

ML4902

POWER GOOD (PWR GOOD)

An open drain signal is provided by the ML4902 which

tells the microprocessor when the entire power system is

functioning within the expected limits. PWR GOOD will

be false (low) if either the 5V or 12V supply is not in

regulation, when the SHDN pin is pulled low, or when the

output is not within +10% of the nominal output voltage

selected by the internal DAC.

When PWR GOOD is false, the PWR GOOD voltage

window is held to +3%; when PWR GOOD is true (high),

the window is expanded to +10%. Using different

windows for coming into and going out of regulation

makes sure that PWR GOOD does not oscillate during the

start-up of the microprocessor.

INTERNAL REFERENCE

The ML4902 contains a 3.535V, temperature

compensated, precision band-gap reference. The VREF pin

is connected to the output of this reference, and should be

bypassed with a 100nF to 220nF ceramic capacitor for

proper operation.

OVERCURRENT PROTECTION

Overcurrent sensing for the ML4902 application circuit is

typically accomplished by monitoring the voltage drop

across the synchronous rectifier MOSFETs (Q3||Q4) during

their conduction period. Alternately, current can be

sensed using a low-value, low-inductance sense resistor

connected between the most negative end of the current

recirculating element and ground. In either case, the

resulting IR drop is presented to the ML4902’s internal

overcurrent comparator via the part’s ISENSE pin. The

overcurrent comparator has approximately 250ns of

leading-edge blanking. This blanking interval allows the

ML4902 to ignore spurious circuit voltages such as

inductive transients and the synchronous rectifier’s drain-

body diode voltage during the anti-shootthrough interval.

Following this blanking interval, the comparator will turn

on if the voltage on the ISENSE pin is more negative than

–97mV.

Each time the overcurrent comparator turns on, the

PROTECT pin of the ML4902 sources a small current

(30µA) into an external RC network. If this current source

is activated over a number of cycles, the voltage on the

PROTECT pin will charge above 3.5V, signaling a

sustained overcurrent or short circuit at the load. This will

cause the N DRV H output to turn off. N DRV H will

remain off until the capacitor attached to the PROTECT

pin has discharged down to 1.5V, at which time the

converter is re-enabled. If the fault causing the

overcurrent condition has not been cleared, the

overcurrent protection cycle will repeat, and the ML4902

circuit will operate in a “hiccup” mode to protect itself,

the input supply, and the output.

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