July 1999

ML4903

High Current Synchronous Buck Controller

GENERAL DESCRIPTION

The ML4903 high current synchronous buck controller
provides high efficiency DC/DC conversion to generate
V

CCP

for processors such as the Pentium

Pro and Pentium

II from Intel

The ML4903 controller, when combined with two external
MOSFETs, generates output voltages between 1.8V and
3.5V from a 12V supply. The output voltage is selected via
an internal 2 chord 4-bit DAC. In the upper range, the
output can be set between 2.1V and 3.5V in 100mV steps.
In the lower range, the output can be set between 1.8V
and 2.05V in 50mV steps. Output currents in excess of
20A can be attained at efficiencies greater than 80%.

The ML4903 can be enabled/disabled via the SHDN pin.
While disabled, the output of the regulator is completely
isolated from the circuit's input supply. The ML4903
employs fixed-frequency PWM control combined with a
sophisticated control loop enhancement circuit to provide
excellent load transient response.

FEATURES

Designed to meet Pentium Pro and Pentium II VRM
power supply requirements

DC regulation to +1% maximum

Proprietary circuitry provides transient response of ą5%
maximum over a 0A to 14A load range

Programmable output voltage (1.8V to 3.5V) is set by
an onboard 2 chord 4-bit DAC

Synchronous buck topology for maximum power
conversion efficiency

Fixed frequency operation for easier system integration

Integrated anti-shootthrough logic, short circuit
protection, shutdown, and UV lockout

BLOCK DIAGRAM

3.5V

REFERENCE

VREF

GND

P DRV

N DRV

ISENSE

PWR GOOD

UVLO

VDD

VCC

RANGE

VDAC

2 CHORD

4 BIT DAC

4.4V

10.5V

PROTECT

3.5V

SHDN

VFB

VDAC + 3%

VDAC - 3%

VFB

VDAC - 3%

VDAC - 10%

VDAC + 3%

VDAC + 10%

VFB

PWR GND

CONTROL

LOGIC

-107mV

VDAC

COMP

200kHz

30ľA

ML4903

PIN CONFIGURATION

PIN

NAME

FUNCTION

LSB input to the DAC which sets the
output voltage

Input to the DAC which sets the
output voltage

MSB input to the DAC which sets the
output voltage

RANGE

Range selection bit for the 2 chord 4-

bit DAC. Logic 1 sets the range at
2.1V to 3.5V with an LSB of 100mV.
Logic 0 sets the range at 1.8V to
2.05V with an LSB of 50mV.

SHDN

Grounding this pin shuts down the
regulator

PWR GOOD This open drain output goes low

whenever SHDN goes low or when
the output is not within +10% of its
nominal value

PIN

NAME

FUNCTION

REF

Bypass connection for the internal
3.5V reference

GND

Analog signal ground

Output voltage feedback pin

SENSE

Current sense input

COMP

Connection for the compensation and
optional soft-start delay network

PWR GND

Power ground

N DRV

Synchronous rectifier driver output

P DRV

Buck switch driver output

Connection point for monitoring the
5V supply to determine the proper
condition of PWR GOOD

12V power supply input

PROTECT

Connection for the integrating current
limit network

PIN DESCRIPTION

TOP VIEW

ML4903

20-Pin TSSOP (T20)

RANGE

SHDN

PWR GOOD

VREF

GND

PROTECT

VDD
VCC
P DRV

N DRV

PWR GND

COMP

ISENSE
VFB

ML4903

ABSOLUTE MAXIMUM RATINGS

Absolute maximum ratings are those values beyond which
the device could be permanently damaged. Absolute
maximum ratings are stress ratings only and functional
device operation is not implied.

DD ...........................................................................................

13.5V

CC ..................................................................................................

Peak Driver Output Current ....................................... ą2A
V

Voltage ...................................... GND - 0.3V to 5.5V

SENSE

Voltage .................................. GND - 0.5V to 5.5V

All Other Inputs .................... GND - 0.3V to V

+ 0.3V

SHDN Input Current .............................................. 100

Junction Temperature .............................................. 150°C
Storage Temperature Range ..................... 65°C to 150°C
Lead Temperature (Soldering, 10 sec) ..................... 260°C
Thermal Resistance (

) .................................... 100°C/W

OPERATING CONDITIONS

Temperature Range ....................................... 0°C to 70°C
V

Range .............................................. 11.4V to 12.6V

Range ............................................... 4.75V to 5.25V

ELECTRICAL CHARACTERISTICS

Unless otherwise specified, V

= 12V, V

= SHDN = 5V, T

= Operating Temperature Range (Note 1)

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

REFERENCE

REF

Output Voltage

3.51

3.535

3.56

Line Regulation

11V < V

< 13V

0.5

mV/V

UV LOCKOUT

Start-up Threshold

10.0

10.5

10.8

Hysteresis

300

450

600

Start-up Threshold

4.25

4.4

4.6

Hysteresis

300

400

500

SHUTDOWN

Input Low Voltage

0.8

Input High Voltage

2.0

Delay to Output

POWER GOOD COMPARATOR

Output Voltage in Regulation

W pull-up to 5V

4.8

Output Voltage out of Regulation

< 90% V

DAC

or >110% V

DAC

0.4

Output Voltage in Shutdown

SHDN = 0V, 5k

W pull-up to 5V

0.4

BUCK REGULATOR

Oscillator Frequency

160

200

230

kHz

Duty Cycle Ratio

RANGE = 1, V

= 0V,

DAC (D3-D0) Code = 0100

RANGE = 1, V

> 3.193V,

DAC (D3-D0) Code = 0100

DAC (RANGE, D3-D0) Input Low Voltage

0.8

DAC (RANGE, D3-D0) Input High Voltage

2.0

ML4903

ELECTRICAL CHARACTERISTICS

(Continued)

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

BUCK REGULATOR (Continued)

Threshold Voltage (Note 2)

RANGE = 0, (D3-D0) Code = 0000

2.050

2.071

2.092

RANGE = 0, (D3-D0) Code = 0001

2.000

2.020

2.04

RANGE = 0, (D3-D0) Code = 0010

1.950

1.970

1.989

RANGE = 0, (D3-D0) Code = 0011

1.900

1.919

1.938

RANGE = 0, (D3-D0) Code = 0100

1.850

1.869

1.887

RANGE = 0, (D3-D0) Code = 0101

1.800

1.818

1.836

RANGE = 1, (D3-D0) Code = 0000

3.500

3.535

3.570

RANGE = 1, (D3-D0) Code = 0001

3.400

3.434

3.468

RANGE = 1, (D3-D0) Code = 0010

3.300

3.333

3.366

RANGE = 1, (D3-D0) Code = 0011

3.200

3.232

3.264

RANGE = 1, (D3-D0) Code = 0100

3.100

3.131

3.162

RANGE = 1, (D3-D0) Code = 0101

3.000

3.030

3.060

RANGE = 1, (D3-D0) Code = 0110

2.900

2.929

2.958

RANGE = 1, (D3-D0) Code = 0111

2.800

2.828

2.856

RANGE = 1, (D3-D0) Code = 1000

2.700

2.727

2.754

RANGE = 1, (D3-D0) Code = 1001

2.600

2.626

2.652

RANGE = 1, (D3-D0) Code = 1010

2.500

2.525

2.550

RANGE = 1, (D3-D0) Code = 1011

2.400

2.424

2.448

RANGE = 1, (D3-D0) Code = 1100

2.300

2.323

2.346

RANGE = 1, (D3-D0) Code = 1101

2.200

2.222

2.244

RANGE = 1, (D3-D0) Code = 1110

2.100

2.121

2.142

SENSE

Threshold Voltage

-80

-117

SENSE

Hysteresis

PROTECT Threshold Voltage

3.2

3.5

3.8

PROTECT Hysteresis

1.8

2.2

PROTECT Charging Current

V(I

SENSE

) = -120mV

ľA

PROTECT Leakage Current

+100

Transition Time,

= 5000pF, 10-90%

P DRV and N DRV

SUPPLY

Current

SHDN = 0V

650

900

ľA

DAC (D3-D0) Code = 0000

SHDN = 5V, V

= 5V

SHDN = 5V, V

= 0V, C

= 5000pF

Current

ľA

Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.

Note 2: Codes 00110 to 01111, and 11111 are not valid; applying these codes to the DAC will shut off P DRV and N DRV.

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

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

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
(30ľA) 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.

Part Number ML4903