April 1997
PRELIMINARY
ML4901
High Current Synchronous Buck Controller
1
GENERAL DESCRIPTION
The ML4901 high current synchronous buck controller has
been designed to provide high efficiency DC/DC
conversion for next generation processors such as the
Pentium
Pro from Intel
.
The ML4901 controller, when combined with 2 external
MOSFETs, generates output voltages between 2.1V and
3.5V from a 12V supply. The output voltage is selected via
an internal 4-bit DAC. Output currents in excess of 14A
can be attained at efficiencies greater than 90%.
The ML4901 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 ML4901
employs fixed-frequency PWM control combined with a
dual mode control loop to provide excellent load transient
response.
BLOCK DIAGRAM
(Pin Configuration Shown for 16-Pin SOIC Version)
FEATURES
s
Designed to meet Pentium
Pro power supply
requirements
s
DC regulation to +1% maximum
s
Proprietary circuitry provides transient response of +5%
maximum over 300mA to 14A load range
s
Programmable output voltage (2.1V to 3.5V) is set by an
onboard 4-bit DAC
s
Synchronous buck topology for maximum power
conversion efficiency
s
Fixed frequency operation for easier system integration
s
Integrated antishoot-through logic, short circuit
protection, and UV lockout
s
Shutdown control provides load isolation
3.5V
REFERENCE
7
VREF
GND
P DRV
N DRV
ISENSE
PWR GOOD
15
UVLO
13
10
14
VDD
+
16
PROTECT
+
VDAC
1
D0
2
D1
3
D2
4
D3
4 BIT DAC
4.4V
10.4V
8
5
SHDN
+
+
VFB
VDAC + 3%
VDAC - 3%
VFB
9
PWR GND
12
CONTROL
LOGIC
+
-95mV
+
VDAC
+
COMP
11
200kHz
35A
+
+
VFB
VDAC - 3%
VDAC - 10%
VDAC + 3%
VDAC + 10%
6
ML4901
2
PIN CONFIGURATION
PIN DESCRIPTION
(Pin Number in Parentheses is for TSSOP Version)
PIN#
NAME
FUNCTION
1 (1)
D0
LSB input to the DAC which sets
the output voltage
2 (2)
D1
Input to the DAC which sets the
output voltage
3 (3)
D2
Input to the DAC which sets the
output voltage
4 (4)
D3
MSB input to the DAC which sets
the output voltage
5 (6)
SHDN
Grounding this pin shuts down the
regulator
6 (8)
PWR GOOD
This open drain output goes low
whenever SHDN goes low or
when the output is not within
+10% of its nominal value
7 (9)
V
REF
Bypass connection for the internal
3.5V reference
PIN#
NAME
FUNCTION
8 (10)
GND
Analog signal ground
9 (11)
V
FB
Output voltage feedback pin
10 (12) I
SENSE
Current sense input
11 (13) COMP
Connection for the compensation
network
12 (15) PWR GND
Power ground
13 (16) N DRV
Synchronous rectifier driver output
14 (17) P DRV
Buck switch driver output
15 (19) V
DD
12V power supply input
16 (20) PROTECT
Connection for the integrating
current limit network and the
UVLO monitor for the 5V supply
D0
D1
D2
D3
NC
SHDN
NC
PWR GOOD
VREF
GND
PROTECT
VDD
NC
P DRV
N DRV
PWR GND
NC
COMP
ISENSE
VFB
TOP VIEW
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
ML4901
20-Pin TSSOP (T20)
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
TOP VIEW
ML4901
16-Pin Narrow SOIC (S16N)
D0
D1
D2
D3
SHDN
PWR GOOD
VREF
GND
PROTECT
VDD
P DRV
N DRV
PWR GND
COMP
ISENSE
VFB
ML4901
3
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, V
DD
= 12V, PROTECT = SHDN = 5V, T
A
= Operating Temperature Range (Note 1)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
REFERENCE
V
REF
Output Voltage
3.51
3.535
3.56
V
Line Regulation
11V < V
DD
< 13V
0.5
mV/V
UV LOCKOUT
V
DD
Start-up Threshold
10.0
10.4
10.8
V
V
DD
Hysteresis
300
450
600
mV
PROTECT (5V) Start-up Threshold
4.25
4.4
4.55
V
PROTECT (5V) Hysteresis
400
450
500
mV
SHUTDOWN
Input Low Voltage
0.8
V
Input High Voltage
2.0
V
Delay to Output
50
ns
POWER GOOD COMPARATOR
Output Voltage in Regulation
5k
pull-up to 5V
4.8
V
Output Voltage out of Regulation
V
FB
< 90% V
DAC
or >110% V
DAC
0.4
V
Output Voltage in Shutdown
SHDN = 0V, 5k
pull-up to 5V
0.4
V
BUCK REGULATOR
Oscillator Frequency
160
200
230
kHz
Duty Cycle Ratio
DAC (D3-D0) Code = 0100,
80
90
%
V
FB
= 0V
DAC (D3-D0) Code = 0100,
0
%
V
FB
> 3.193V
DAC (D3-D0) Input Low Voltage
0.8
V
DAC (D3-D0) Input High Voltage
2.0
V
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.
V
DD ..........................................................................................
13.5V
Peak Driver Output Current ....................................... 2A
V
FB
Voltage ....................................... GND - 0.3V to 5.5V
I
SENSE
Voltage ................................... GND - 0.5V to 5.5V
All Other Analog Inputs .......... GND - 0.3V to V
DD
+ 0.3V
SHDN Input Current .............................................. 100
A
Junction Temperature .............................................. 150C
Storage Temperature Range ...................... 65C to 150C
Lead Temperature (Soldering, 10 sec) ...................... 260C
Thermal Resistance (
JA
)
16-Pin Narrow SOIC ...................................... 100C/W
20-Pin TSSOP ................................................. 143C/W
OPERATING CONDITIONS
Temperature Range ........................................ 0C to 70C
V
DD
Range ............................................... 11.4V to 12.6V
PROTECT (5V Supply) Range .................... 4.75V to 5.25V
ML4901
4
ELECTRICAL CHARACTERISTICS
(Continued)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
BUCK REGULATOR (continued)
V
FB
Threshold Voltage
DAC (D3-D0) Code = 0000
3.500
3.535
3.570
V
DAC (D3-D0) Code = 0001
3.400
3.434
3.468
V
DAC (D3-D0) Code = 0010
3.300
3.333
3.366
V
DAC (D3-D0) Code = 0011
3.200
3.232
3.264
V
DAC (D3-D0) Code = 0100
3.100
3.131
3.162
V
DAC (D3-D0) Code = 0101
3.000
3.03
3.060
V
DAC (D3-D0) Code = 0110
2.900
2.929
2.958
V
DAC (D3-D0) Code = 0111
2.800
2.828
2.856
V
DAC (D3-D0) Code = 1000
2.700
2.727
2.754
V
DAC (D3-D0) Code = 1001
2.600
2.626
2.652
V
DAC (D3-D0) Code = 1010
2.500
2.525
2.550
V
DAC (D3-D0) Code = 1011
2.400
2.424
2.448
V
DAC (D3-D0) Code = 1100
2.299
2.323
2.347
V
DAC (D3-D0) Code = 1101
2.198
2.222
2.246
V
DAC (D3-D0) Code = 1110
2.097
2.121
2.145
V
DAC (D3-D0) Code = 1111
0.8
V
I
SENSE
Threshold Voltage
-85
-95
-105
mV
I
SENSE
Hysteresis
3
mV
PROTECT Discharge Current
V(I
SENSE
) = -120mV
35
A
PROTECT Leakage Current
+100
nA
Transition Time, N DRV and P DRV
C
L
= 5000pF, 10-90%
40
ns
SUPPLY
V
DD
Current
SHDN = 0V
300
450
A
DAC (D3-D0) Code = 0000
SHDN = 5V, V
FB
= 5V
1
2
mA
SHDN = 5V, V
FB
= 0V, C
L
= 5000pF
30
mA
Note 1:
Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
ML4901
5
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 (35A) 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.
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