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ML4900CT

High Current Synchronous Buck Controller

Micro Linear Corporation 

FUNCTIONAL DESCRIPTION

The ML4900 PWM controller permits the construction of a

simple yet sophisticated power supply for Intel’s Pentium

Pro microprocessor which meets the guidelines of Intel’s

Application Note AP-523. This can be built either as a

Voltage Regulator Module (VRM) or as dedicated

motherboard circuitry. The ML4900 controls two N-

channel MOSFETs in a synchronous buck regulator circuit,

to convert a 5V input to the voltage required by the

microprocessor. The output voltage can be any set to any

one of 15 output voltages from 2.1V to 3.5V, in steps of

100mV, 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.

4-BIT DAC

The inputs of the internal 4-bit DAC come from open

collector signals provided by the Pentium Pro. 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. The output voltage set by the DAC is 1% above

the Pentium Pro'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. The output of the DAC therefore

ranges from 2.121V to 3.535V in 100mV steps. For code

1111, the N DRV H output is disabled, and the output

voltage is zero.

VOLTAGE FEEDBACK LOOP

The ML4900 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 4-bit 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 ML4900 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. During start-up, the transient loop is

disabled until the output voltage is within -3% of the DAC

voltage.

POWER GOOD (PWR GOOD)

An open drain signal is provided by the ML4900 which

tells the microprocessor when the entire power system is

ML4900

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 ML4900 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

When the output of the buck converter sees an overcurrent

condition (IOUT exceeds the current limit set point ISET),

the ML4900 will operate in a “hiccup” mode until the

overcurrent condition has been removed.

During an overcurrent condition, a current sink within the

ML4900 draws a small current (35µA) out of the PROTECT

pin for the time during which IOUT > ISET. If this current

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

the PROTECT pin will drop below 4V, signalling a

sustained overcurrent or short circuit at the load. This will

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

remain in an off state until the capacitor attached to the

PROTECT pin has charged back to 4.4V, at which time the

converter is re-enabled and tries to resume normal

operation. If the fault causing the overcurrent condition

has not been cleared, the overcurrent protection cycle will

repeat.

UNDERVOLTAGE LOCKOUT

The ML4900 has undervoltage lockout protection circuits

for both the 12V (VDD) and 5V (PROTECT) supplies. The

hysteresis voltage is typically 400mV for each supply.

During an input undervoltage condition, the internal

reference and voltage monitor circuits remain in

operation, but N DRV H and N DRV L are disabled and

the PWR GOOD output will be false (low).

COMPENSATION/SOFT-START

This pin connects to the output of the transconductance

amplifier which forms the gain block for the ML4900’s

proportional control loop. An RC network from this pin to

GND is used to compensate the amplifier.

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