LM2720
5-Bit Programmable, High Frequency Multi-phase PWM
Controller
General Description
The LM2720 provides an attractive solution for power sup-
plies of high power microprocessors exhibiting ultra fast load
transients. Compared to a conventional single-phase supply,
an LM2720 based multi-phase supply distributes the thermal
and electrical loading among components in multiple phases
and greatly reduces the corresponding stress in each com-
ponent. The LM2720 can be programmed to control either a
3-phase converter or a 4-phase converter. Phase shift
among the phases is 120 in the case of three phase and 90
with four-phase. Because the power channels are out of
phase, there can be significant ripple cancellation for both
the input and output current, resulting in reduced input and
output capacitor size. Due to the nominal operating fre-
quency of 2 MHz per phase, the size of the output inductors
can be greatly reduced which results in a much faster load
transient response and a dramatically shrunk output capaci-
tor bank. Microprocessor power supplies with all surface
mount components can be easily built.
The internal high speed transconductance amplifier guaran-
tees good dynamic performance.
The internal master clock frequency of up to 8 MHz is set by
an external reference resistor. An external clock of 10 MHz
can also be used to drive the chip to achieve frequency con-
trol and multi-chip operation.
The LM2720 also provides input under-voltage lock-out with
hysteresis, input over-current protection and output voltage
power good detection.
Features
n
Ultra fast load transient response
n
Enables all surface-mount-design
n
Selectable 2, 3, 4 phase operation
n
Clock frequency from 40 kHz to 10 MHz
n
Precision load current sharing
n
5-bit programmable from 3.5V to 1.3V
n
VID code compatible to VRM 8.X specification
n
Output voltage is 2.0V for VID code 11111
n
Selectable internal or external clock
n
Digital 16-step soft start
n
Input under-voltage lock-out, over-current protection
n
Open-drain power good signal output
Applications
n
Servers and workstations
n
High current, ultra-fast transient microprocessors
Pin Configuration
24-Pin Plastic SOIC
DS101322-1
Top View
See NS Package Number M24B
February 2000
LM2720
5-Bit
Programmable,
High
Frequency
Multi-phase
PWM
Controller
2000 National Semiconductor Corporation
DS101322
www.national.com
Absolute Maximum Ratings
(Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
CC
5V
7V
PWRGD
(V
CC
5V + 0.2V)
V
CC
12V
20V
Junction Temperature
125C
Power Dissipation (Note 2)
1.6W
Storage Temperature
-65C to +150C
ESD Susceptibility (Note 8)
2 kV
Soldering Time, Temperature
10 sec., 300C
Operating Ratings
(Note 1)
V
CC
5V
4.5V to 5.5V
V
CC
12V
10V to 18V
Junction Temperature Range
0C to 70C
Electrical Characteristics
V
CC
5V = 5V, V
CC
12V = 12V unless otherwise specified. Typicals and limits appearing in plain type apply for T
A
= T
J
= +25C.
Limits appearing in boldface type apply over the entire operating temperature range.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
V
DACOUT
5-bit DAC Output Voltage
(Note 3)
N-1.5%
N
N+1.5%
V
I
CC
12V
Quiescent V
CC
12V Current
Enable = 5V, VID = 00001,
DRV Outputs Floating
1
2
mA
I
CC
5V
Operating V
CC
5V Current
V
OUT
= 2.00V
4.3
8
mA
V
REF
Rref Pin Voltage
1.225
V
V
INL
Vid0:4, Clksel, Divsel, and
Enable Pins Logic Threshold
Logic Low (Note 4)
1.8
1.5
V
V
INH
Logic High (Note 5)
3.5
2.8
V
I
INL
Vid0:4 and Enable Pins Internal
Pullup Current
The Corresponding Pin = 0V
60
100
140
A
Clksel, Divsel Pins Internal
Pullup Current
-10
0
10
Gate Driver Resistance When
Sinking Current
I
SINK
= 50 A, V
CC
12V = 14V
12
V
DRV
DRV0:3 Output Voltage
I
DRV
= 10 mA, V
CC
12V = 14V
V
CC
12V
- 1.5V
V
CC
12V
-1.0V
V
t
fall
DRV0:3 Fall Time
(Note 6)
7
ns
I
SRC
DRV0:3 Source Current
DRV0:3 = 0V, V
CC
12V = 14V
40
60
mA
I
SINK
DRV0:3 Sink Current
DRV0:3 = 5V, V
CC
12V = 14V
90
160
250
mA
VB
gOUT
B
gOUT
Voltage
Current Limit Not Activated
4
V
Current Limit Activated
0
IB
gOUT
B
gOUT
Sink Current
B
gOUT
= 1V
1.0
2.4
5
mA
I
FB
FB Pin Bias Current
FB = 2V
30
nA
F
OSC
Oscillator Frequency
8.02k
from Rref Pin to
Ground
7.0
8.0
8.7
MHz
D
DRV0:3 Duty Cycle Match
Duty Cycle = 50%
-1
+1
%
ph
DRV0:3 Phase Accuracy
Duty Cycle = 50%, F
clock
= 8
MHz
-1
+1
Deg
T
off
PWM Off time
Divide by 4
22
%
Divide by 3
22
V
PWRGD
Power Good Detection
Window,
V
OUT
1.3V
V
OUT
1.9V
100
mV
Hysteresis
50
Power Good Detection
Window,
V
OUT
2V
V
OUT
3.5V
150
Hysteresis
50
LM2720
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2
Electrical Characteristics
(Continued)
V
CC
5V = 5V, V
CC
12V = 12V unless otherwise specified. Typicals and limits appearing in plain type apply for T
A
= T
J
= +25C.
Limits appearing in boldface type apply over the entire operating temperature range.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
I
PWRGD
Power Good Sink Current
V
PWRGD
= 0.4V
0.8
1
mA
V
OCC_CM
Over-current Comparator
Common Mode Range
3
12
V
IB_OC+
OC+ Input Bias Current
V
IN
= 5V, OC+ = 5V, OC- =
4V
100
145
200
A
IB_OC-
OC- Input Bias Current
V
IN
= 5V, OC+ = 6V, OC- =
5V
85
120
165
A
V
OS_OCC
Over-current Comparator Input
Offset Voltage
V
IN
= 5V
2
16
42
mV
V
IN
= 12V
21
D
MAX
Maximun Duty Cycle
FB = 0V
78
%
gm
Error Amplifier
Transconductance
1.36
mmho
V
ramp
Ramp Signal Peak-to-Peak
Amplitude
2
V
I
comp
COMP Pin Source Current
250
360
550
A
I
comp
COMP Pin Sink Current
160
260
400
A
V
comp_hi
COMP Pin High Clamp
2.9
V
V
comp_lo
COMP Pin Low Clamp
0.19
V
V
POR
Power On Reset Trip Point
Vcc5V Pin Voltage Rising
4.0
V
Vcc5V Pin Voltage Falling
3.6
Vcc12V Minimum Working
Voltage
(Note 7)
3.8
V
t
SS
Soft Start Delay
F
OSC
= 8MHz
1.6
ms
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating ratings do not imply guaranteed performance limits.
Note 2: Maximum allowable power dissipation is a function of the maximum junction temperature, T
JMAX
, the junction-to-ambient thermal resistance,
JA
, and the
ambient temperature, T
A
. The maximum allowable power dissipation at any ambient temperature is calculated using:
P
MAX
= (T
JMAX
- T
A
)/
JA
.
The
junction-to-ambient thermal resistance,
JA
, for LM2720 is 78C/W. For a T
JMAX
of 150C and T
A
of 25C, the maximum allowable power dissipation is 1.6W.
Note 3: The letter
N
stands for the typical output voltages appearing in
italic boldface
type in
Table 1
.
Note 4: Max value of logic low means any voltage below this value is guaranteed to be taken as logic low whereas a voltage higher than this value is not guaranteed
to be taken as a logic low.
Note 5: Min value of logic high means any voltage above this value is guaranteed to be taken as logic high whereas a voltage lower than this value is not guaranteed
to be taken as a logic high.
Note 6: When driving bipolar FET drivers in the typical application circuit.
Note 7: When Vcc12V pin goes below this voltage, all DRV pins go to 0V.
Note 8: ESD ratings is based on the human body model, 100pF discharged through 1.5k
.
LM2720
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3
Electrical Characteristics
(Continued)
TABLE 1. 5-Bit DAC Output Voltage Table
Symbol
Parameter
Conditions
Typical
Units
V
DACOUT
5-Bit DAC Output Voltages for Different VID Codes
VID4:0 = 01111
1.30
V
VID4:0 = 01110
1.35
VID4:0 = 01101
1.40
VID4:0 = 01100
1.45
VID4:0 = 01011
1.50
VID4:0 = 01010
1.55
VID4:0 = 01001
1.60
VID4:0 = 01000
1.65
VID4:0 = 00111
1.70
VID4:0 = 00110
1.75
VID4:0 = 00101
1.80
VID4:0 = 00100
1.85
VID4:0 = 00011
1.90
VID4:0 = 00010
1.95
VID4:0 = 00001
2.00
VID4:0 = 00000
2.05
VID4:0 = 11111
2.0
VID4:0 = 11110
2.1
VID4:0 = 11101
2.2
VID4:0 = 11100
2.3
VID4:0 = 11011
2.4
VID4:0 = 11010
2.5
VID4:0 = 11001
2.6
VID4:0 = 11000
2.7
VID4:0 = 10111
2.8
VID4:0 = 10110
2.9
VID4:0 = 10101
3.0
VID4:0 = 10100
3.1
VID4:0 = 10011
3.2
VID4:0 = 10010
3.3
VID4:0 = 10001
3.4
VID4:0 = 10000
3.5
LM2720
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4
Pin Description
Pin
Pin Name
Pin Function
1
Vcc5V
Supply Voltage Input (5V nominal)
2
Divsel
Selects Phase Mode. Logic low selects 4 phase. Logic high selects 3 phase. 2 phase
operation is achieved by using 2 outputs in 4 phase mode.
3
Clksel
Clock Select: Logic high selects internal clock. Logic low selects external clock.
4
Extclk
External Clock Input. Output frequency = Clock Input / No. of Phases. Connect to Vcc5V to
select internal clock.
5
Rref
Connects to external reference resistor. Sets the operating frequency of the internal clock
and the ramp time for the PWM. Reference voltage at this pin is 1.26V.
6
Vid0
5-Bit DAC Input (LSB).
7
Vid1
5-Bit DAC Input.
8
Vid2
5-Bit DAC Input.
9
Vid3
5-Bit DAC Input.
10
Vid4
5-Bit DAC Input (MSB)
11
OC+
Over-current Comparator. Non-inverting input.
12
OC-
Over-current Comparator. Inverting input.
13
COMP
Compensation Pin. This is the output of the internal transconductance amplifier.
Compensation network should be connected between this pin and feedback ground FBG.
14
FB
Feedback Input. Normally Kelvin connected to supply output.
15
Bgout
Current Limit Flag. Goes to logic low when current limit is activated. When over-current
condition is removed, this pin is weakly pulled up to Vcc5V.
16
FBG
Feedback Ground. This pin should be connected to the ground at the supply output.
17
ENABLE
Output Enable Pin. Tie to logic high to enable and logic low to disable.
18
GND
Power Ground Pin.
19
DRV2
Phase 2 Output.
20
DRV0
Phase 0 Output.
21
Vcc12V
Supply Voltage for FET Drivers DRV0:3.
22
DRV1
Phase 1 Output.
23
DRV3
Phase 3 Output.
24
PWRGD
Power Good. This is an open-drain output that goes low (low impedance to ground) when
the output voltage travels out of the threshold window, and resume to open-drain state when
V
OUT
recovers into the 50mV hysteresis window. The power good window threshold, i.e.
100mV for 1.3V
V
OUT
1.9V or
150mV for 2V
V
OUT
3.5V, is automatically set
according to VID code. Connect a 1nF capacitor to ground to filter out the switching noise
coupling from DRV3 to this pin.
LM2720
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5
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