ML4901
FUNCTIONAL DESCRIPTION
The ML4901 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 ML4901 controls a P-channel
and an N-channel MOSFET in a synchronous buck
regulator circuit, to convert a 12V 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 P DRV output is disabled, and the output voltage
is zero.
VOLTAGE FEEDBACK LOOP
The ML4901 contains two control loops to improve the
load transient response. The output voltage is directly
monitored via the V
FB
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 ML4901 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 ML4901 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 ML4901 contains a 3.535V, temperature
compensated, precision band-gap reference. The V
REF
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 (I
OUT
exceeds the current limit set point I
SET
),
the ML4901 will operate in a “hiccup” mode until the
overcurrent condition has been removed.
During an overcurrent condition, a current sink within the
ML4901 draws a small current (35µA) out of the PROTECT
pin for the time during which I
OUT
> I
SET
. 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 P DRV 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 ML4901 has undervoltage lockout protection circuits
for both the 12V (V
DD
) and 5V (PROTECT) supplies. The
hysteresis voltage is typically 450mV for each supply.
During an input undervoltage condition, the internal
reference and voltage monitor circuits remain in
operation, but P DRV and N DRV are disabled and the
PWR GOOD output will be false (low).
COMPENSATION
This pin connects to the output of the transconductance
amplifier which forms the gain block for the ML4901’s
proportional control loop. An RC network from this pin to
GND is used to compensate the amplifier.
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
