LM2638
Motherboard Power Supply Solution with a 5-Bit
Programmable Switching Controller and Two Linear
Regulator Controllers
General Description
The LM2638 provides a comprehensive embedded power
supply solution for motherboards hosting high performance
MPUs such as Pentium
TM
II, M II
TM
, K6
TM
-2 and other similar
high performance MPUs. The LM2638 incorporates a 5-bit
programmable, synchronous buck switching controller and
two high-speed linear regulator controllers in a 24-pin SO
package. In a typical application, the switching controller
supplies the MPU core, and the linear regulator controllers
supply the GTL+ bus and the clock or graphics chip core. A
charge pump pin helps provide the necessary voltage to
power the linear sections when 12V is shut off during system
standby such as STR mode.
Switching Section -- The switching regulator controller fea-
tures an Intel-compatible, 5-bit programmable output volt-
age, over-current and over-voltage protection, a power good
signal, and a logic-controlled output enable. There are two
user-selectable over-current protection methods. One pro-
vides accurate over-current protection with the use of an ex-
ternal sense resistor. The other saves cost by taking advan-
tage of the r
DS_ON
of the high-side FET. When there is an
over voltage, the controller turns off the high side FET and
turns on the low side.
Linear Section -- The two linear regulator controllers fea-
ture wide control bandwidth, N-FET and NPN transistor driv-
ing capability and an adjustable output. The wide control
bandwidth makes meeting the GTL+ bus transient response
requirement an easy job. In minimum configuration, the two
controllers default to 1.5V and 1.25V respectively.
Both linear controllers have under voltage latch-off.
Features
n
Provides 3 regulated voltages
n
Power Good flag and output enable
n
Charge pump pin
Switching Section
n
Synchronous rectification
n
5-bit DAC programmable down to 1.3V
n
Typical
1% DAC tolerance
n
Switching frequency: 50 kHz to 1 MHz
n
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 drive capability
n
Ultra fast response speed
n
Under voltage latch-off at 0.63V
n
Output voltages default to 1.5V and 2.5V yet adjustable
Applications
n
Embedded power supplies for 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
TM
is a trademark of Advanced Micro Devices, Inc.
24-Lead SOIC
DS101034-1
Top View
Order Number LM2638M
See NS Package Number M24B
May 1999
LM2638
Motherboard
Power
Supply
Solution
with
a
5-Bit
Programmable
Switching
Controller
and
T
w
o
Linear
Regulator
Controllers
1999 National Semiconductor Corporation
DS101034
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
3 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
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
I
Q_VDD
V
DD
Shutdown Current
EN = 0V, VID Pins Floating
4
A
R
DS_CP
CP Pin Resistance
High Side FET
100
Low Side FET
10
SWITCHING SECTION
V
DACOUT
5-Bit DAC Output Voltage
(Note 3)
N-1.5%
N
N+1.5%
V
I
VID
VID Pins Internal Pull-up
Current
60
90
140
A
f
OSC
Oscillator Frequency
RT = 100 k
204
245
286
kHz
RT = 25 k
1000
D
MAX
Maximum Duty Cycle
100
%
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
V
CC
Power-On-Reset
Threshold
4.0
4.3
V
V
CC_TH2
V
CC
Shutdown Threshold
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
GA
Error Amplifier DC Gain
76
dB
BW
EA
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 during
Soft Start
12.5
%
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
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2
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. (Continued)
Symbol
Parameter
Conditions
Min
Typ
Max
Units
SWITCHING SECTION
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
Over-Voltage Trip Point
% SNS1 Above Rated Output
15
25
35
%
I
CS+
CS+ Pin Sink Current
CS+ = 5V, CS- = 4.8V
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
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
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
1.25V LDO CONTROLLER SECTION
V
SNS3
SNS3 Voltage
V
DD
= 12V, V
CC
= 4.75V to
5.25V, I
G3
= 0 mA to 20 mA
1.219
1.25
1.281
V
R
OUT3
Output Resistance
200
I
SNS3
SNS3 Pin Bias Current
When Regulating
0
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 LM2638 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 1.25V section. For the switch-
ing section, the power good is a window. For the two linear sections, the power good is a threshold with some hysteresis.
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3
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. (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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4
Pin Description
Pin
Pin Name
Pin Function
1
LG
Low side N-FET gate driver output.
2
PGND
Ground for the two FET drivers of the switching section.
3
V
DD
Supply for the FET gate drivers. Usually tied to +12V.
4
SNS2
Feedback pin for the 1.5V linear regulator.
5
G2
Gate drive output for the external N-MOS of the fast 1.5V linear regulator.
6
SGND
Ground for internal signal circuitry and system ground reference.
7
V
CC
Supply voltage. Usually +5V.
8
SNS1
Output voltage monitor input for the switching regulator.
9
CS+
Switching regulator current sense input, positive node.
10
CS-
Switching regulator current sense input, negative node.
11
CP
Charge pump. Output is a square wave with 50% duty cycle. Amplitude is close to V
CC
voltage.
12
FREQ
Switching frequency adjustment pin. An external resistor is needed to set the desired
frequency.
13
EAO
Output of the error amplifier. Used for compensating the switching regulator.
14
FB
Inverting input of the error amplifier. Used for compensating the switching regulator.
15
PWGD
Open collector Power Good signal.
16
VID4
5-Bit DAC input, MSB.
17
VID3
5-Bit DAC input.
18
VID2
5-Bit DAC input.
19
VID1
5-Bit DAC input.
20
VID0
5-Bit DAC input, LSB.
21
G3
Gate drive pin for the external N-MOS of the 1.25V linear regulator.
22
SNS3
Feedback pin for the 1.25V linear regulator.
23
EN
Output Enable. A logic low shuts the whole chip down.
24
HG
High side N-FET gate driver output.
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5
Block Diagram
Test Circuit
DS101034-3
DS101034-4
FIGURE 1. LDO Controller Test Circuit
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6
Typical Application
DS101034-2
FIGURE
2.
Using
LM2638
to
Supply
GTL+
Bus
(VTT
,
1.5V
,
5.6A),
2.5V
Standby
(V
CC
2_5SBY
,
2
A
Full
Power
and
180
mA
Suspend)
and
3.3V
Standby
(V
CC
3_3SBY
,
1.5A
Full
Power
,
0.5A
Suspend)
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7
Physical Dimensions
inches (millimeters) unless otherwise noted
LIFE SUPPORT POLICY
NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
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Fax: 81-3-5639-7507
www.national.com
24-Lead Small Outline Package
Order Number LM2638M
NS Package Number M24B
LM2638
Motherboard
Power
Supply
Solution
with
a
5-Bit
Programmable
Switching
Controller
and
T
w
o
Linear
Regulator
Controllers
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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