LM2637
Motherboard Power Supply Solution with a 5-Bit
Programmable Switching Controller and Two Linear
Regulator Controllers
General Description
The LM2637 provides a comprehensive embedded power
supply solution for motherboards hosting high performance
MPUs such as M II
TM
, Pentium
TM
II, K6-2 and other similar
high performance MPUs. The LM2637 incorporates a 5-bit
programmable, synchronous buck switching controller and
two high-speed linear regulator controllers in a 24-pin SO
package.
Switching Section -- The switching regulator controller fea-
tures
a
5-bit
programmable
DAC,
over-current
and
over-voltage protection, under-voltage latch-off, a power
good signal, and output enable. The 5-bit DAC has a typical
tolerance of 1%. There are two user-selectable over-current
protection methods. One provides accurate over-current pro-
tection with the use of an external sense resistor. The other
saves cost by taking advantage of the r
DS_ON
of the
high-side FET. The over voltage protection provides two lev-
els of protection. The first level keeps the high-side FET off
and the low-side FET on. The second provides a gate signal
that can be used to fire an external SCR.
Linear Section -- The two linear regulator controllers fea-
ture wide control bandwidth, N-FET and NPN transistor driv-
ing capability, and an adjustable output voltage. The wide
control bandwidth makes meeting fast load transient re-
sponse requirement such as that of the GTL+ bus an easy
job. In minimum configuration, the two controllers default to
1.5V and 2.5V respectively.
Both linear controllers have under voltage latch-off.
Features
n
Provides 3 regulated voltages
n
Power Good flag and output enable
n
Under-voltage latch-off
Switching Section
n
Synchronous rectification
n
5-bit DAC programmable from 3.5V to 1.3V
n
Typical 1% DAC tolerance
n
Switching frequency: 50 kHz to 1 MHz
n
Two levels of over-voltage protection
n
Two methods of over-current protection
n
Adaptive non-overlapping FET gate drives
n
Soft start without external capacitor
Linear Section
n
N-FET and NPN driving capability
n
Ultra fast response speed
n
Output voltages default to 1.5V and 2.5V yet adjustable
Applications
n
Embedded power supplies for PC motherboards
n
Triple DC/DC power supplies
n
Programmable high current DC/DC power supply
Pin Configuration
M II
TM
is a trademark of Cyrix Corporation a wholly owned subsidiary of National Semiconductor Corporation.
Pentium
TM
is a trademark of Intel Corporation.
K6 is trademark of Advanced Micro Devices, Inc.
24-Lead SOIC
DS100848-1
Top View
NS Package Number M24B
October 1998
LM2637
Motherboard
Power
Supply
Solution
with
a
5-Bit
Programmable
Switching
Controller
and
T
w
o
Linear
Regulator
Controllers
1999 National Semiconductor Corporation
DS100848
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
7V
V
DD
17V
Junction Temperature
150C
Power Dissipation (Note 2)
1.6W
Storage Temperature
-65C to +150C
ESD Susceptibility
2.5 kV
Soldering Time, Temperature (10 sec.)
300C
Operating Ratings
(Note 1)
V
CC
4.75V to 5.25V
Junction Temperature Range
0C to +125C
Electrical Characteristics
V
CC
= 5V, V
DD
= 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 0C to +70C range.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
I
EN
EN Pin internal Pull-Up Current
60
90
140
A
I
VID
VID Pins internal Pull-Up
Current
60
90
140
A
I
CC
Operating V
CC
Current
EN = 5V, VID = 10111
6
7.5
mA
I
Q_VCC
V
CC
Shutdown Current
EN = 0V, VID Pins Floating
1.5
3
mA
SWITCHING SECTION
V
DACOUT
5-Bit DAC Output Voltage
(Note 3)
N-1.5%
N
N+1.5%
V
I
Q_VDD
V
DD
Shutdown Current
EN = 0V, VID Pins Floating
4
A
f
OSC
Oscillator Frequency
RT = 100 k
204
245
286
kHz
RT = 25 k
1000
D
MAX
Maximum Duty Cycle
95
%
D
MIN
Minimum Duty Cycle
0
%
R
SNS1
SNS1 Pin Resistance to
Ground
8.5
10
13
k
R
DS_SRC
Gate Driver Resistance When
Sourcing Current
6
R
DS_SINK
Gate Driver Resistance When
Sinking Current
1.5
V
CC_TH1
Rising V
CC
Threshold for
Power-On Reset
4
4.3
V
V
CC_TH2
Falling V
CC
Threshold for
Power-On Reset
3.0
3.6
V
V
DAC_IH
DAC Input High Voltage
3.5
V
V
DAC_IL
DAC Input Low Voltage
1.3
V
t
PWGD
PWGD Response Time
SNS1 Rises from 0V to Rated
Output Voltage
2
8.4
15
s
t
PWBAD
PWGD Response Time
SNS1 Falls from Rated Output
Voltage to 0V
2
3.4
10
s
V
PWGD_HI
PWGD High Trip Point
% Above Rated Output Voltage
when output Voltage
11.5
13
%
% Above Rated Output Voltage
when output Voltage
(Note 4)
5
7
9
V
PWGD_LO
PWGD Low Trip Point
% Below Rated Output Voltage
when output Voltage
2.6
6
%
% Below Rated Output Voltage
when output Voltage
(Note 4)
6
9.5
13
V
OVP_TRP
OVP Pin Trip Point
% SNS1 Above Rated Output
15
18
21
%
I
CS+
CS+ Pin Sink Current
CS+ = 5V
126
185
244
A
V
OCP
Over-Current Trip Point (CS+
and CS- Differential Voltage)
CS+ = 2V, CS- Drops from 2V
41
55
69
mV
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2
Electrical Characteristics
(Continued)
V
CC
= 5V, V
DD
= 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 0C to +70C range.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
SWITCHING SECTION
I
OVP
OVP Pin Source Current
OVP = 3V
10
mA
GA
Error Amplifier DC Gain
76
dB
B
WEA
Error Amplifier Unity Gain
Bandwidth
5
MHz
V
RAMP_L
Ramp Signal Valley Voltage
1.25
V
V
RAMP_H
Ramp Signal Peak Voltage
3.25
V
t
SS
Soft Start Time
4096
Clock
Cycles
D
STEP_SS
Duty Cycle Step Change in
Soft Start
12.5
%
1.5V LDO CONTROLLER SECTION
V
SNS2
SNS2 Voltage
V
DD
= 12V, V
CC
= 4.75V to
5.25V, I
G2
= 0 mA to 20 mA
(
Figure 1)
1.463
1.5
1.538
V
R
OUT2
Output Resistance
200
I
SNS2
SNS2 Pin Bias Current
When Regulating
21
A
V
PWGD_HI
PWGD High Trip Point
(Note 4)
0.63
V
V
PWGD_LO
PWGD Low Trip Point
(Note 4)
0.44
V
2.5V LDO CONTROLLER SECTION
V
SNS3
SNS3 Voltage
V
DD
= 12V, V
CC
= 4.75V to
5.25V, I
G3
= 0 mA to 20 mA
(
Figure 1)
2.438
2.5
2.563
V
R
OUT3
Output Resistance
200
I
SNS3
SNS3 Pin Bias Current
When Regulating
21
A
V
PWGD_HI
PWGD High Trip Point
(Note 4)
0.63
V
V
PWGD_LO
PWGD Low Trip Point
(Note 4)
0.44
V
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating ratings are conditions under which the device operates
correctly. 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 LM2637 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: The output level of the PWGD pin is a logic AND of the power good function of the switching section, the 1.5V section and the 2.5V section.
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3
Electrical Characteristics
(Continued)
TABLE 1. 5-Bit DAC Output Voltage Table
(V
CC
= 5V, V
DD
= 12V
5%, T
A
= 25C, Test Mode)
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
(shutdown)
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
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Block Diagram
Test Circuit
DS100848-30
DS100848-2
FIGURE 1. LDO Controller Test Circuit
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