ML4903
FUNCTIONAL DESCRIPTION
The ML4903 PWM controller permits the construction of a
simple yet sophisticated power supply for Intel’s Pentium
Pro and Pentium II microprocessor families. The ML4903
and its associated circuitry can be built either as a
Voltage Regulator Module (VRM) or as a dedicated supply
on the motherboard. The ML4903 controls a P-channel
MOSFET and an N-channel MOSFET 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 P DRV and N
DRV outputs are disabled.
VOLTAGE FEEDBACK LOOP
The ML4903 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 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 ML4903 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 P-
channel 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 P DRV
and N DRV 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.
POWER GOOD (PWR GOOD)
An open drain signal is provided by the ML4903 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 ML4903 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
Overcurrent sensing for the ML4903 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 ML4903’s internal
overcurrent comparator via the part’s I
SENSE
pin. The
overcurrent comparator has approximately 250ns of
leading-edge blanking. This blanking interval allows the
ML4903 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 I
SENSE
pin is more negative than
–80mV.
Each time the overcurrent comparator turns on, the
PROTECT pin of the ML4903 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 P DRV output to turn off. P DRV 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 ML4903 circuit will operate in a
“hiccup” mode to protect itself, the input supply, and the
output.
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
