ETC LM2638

LM2638
Motherboard Power Supply Solution with a 5-Bit
Programmable Switching Controller and Two Linear
Regulator Controllers
General Description
Both linear controllers have under voltage latch-off.
The LM2638 provides a comprehensive embedded power
supply solution for motherboards hosting high performance
MPUs such as Pentium™ II, M II™, K6™-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.
Features
Switching Section — The switching regulator controller features an Intel-compatible, 5-bit programmable output voltage, 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 provides accurate over-current protection with the use of an external sense resistor. The other saves cost by taking advantage of the rDS_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 feature wide control bandwidth, N-FET and NPN transistor driving 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.
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
24-Lead SOIC
DS101034-1
Top View
Order Number LM2638M
See NS Package Number M24B
M II™ is a trademark of Cyrix Corporation a wholly owned subsidiary of National Semiconductor Corporation.
Pentium™ is a trademark of Intel Corporation.
K6™ is a trademark of Advanced Micro Devices, Inc.
© 1999 National Semiconductor Corporation
DS101034
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LM2638 Motherboard Power Supply Solution with a 5-Bit Programmable Switching Controller and
Two Linear Regulator Controllers
May 1999
Absolute Maximum Ratings (Note 1)
Storage Temperature
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
ESD Susceptibility
VCC
7V
VDD
17V
Junction Temperature
3 kV
Soldering Time, Temperature (10 sec.)
300˚C
Operating Ratings (Note 1)
4.75V to 5.25V
VCC
150˚C
Power Dissipation (Note 2)
−65˚C to +150˚C
Junction Temperature Range
0˚C to +125˚C
1.6W
Electrical Characteristics VCC = 5V, VDD = 12V unless otherwise specified. Typicals and limits appearing in
plain type apply for TA = TJ = +25˚C. Limits appearing in boldface type apply over the 0˚C to +70˚C range.
Symbol
Parameter
IEN
EN Pin Internal Pull-up Current
ICC
Operating VCC Current
IQ_VCC
VCC Shutdown Current
IQ_VDD
RDS_CP
Conditions
Min
Typ
Max
60
90
140
Units
µA
6
7.5
mA
1.5
3
mA
VDD Shutdown Current
EN = 5V, VID = 10111
EN = 0V, VID Pins Floating
EN = 0V, VID Pins Floating
4
µA
CP Pin Resistance
High Side FET
100
Low Side FET
10
Ω
SWITCHING SECTION
VDACOUT
5-Bit DAC Output Voltage
IVID
VID Pins Internal Pull-up
Current
fOSC
Oscillator Frequency
(Note 3)
RT = 100 kΩ
RT = 25 kΩ
N−1.5%
N
N+1.5%
V
60
90
140
µA
245
286
204
1000
kHz
DMAX
Maximum Duty Cycle
100
%
DMIN
Minimum Duty Cycle
0
%
RSNS1
SNS1 Pin Resistance to
Ground
RDS_SRC
Gate Driver Resistance When
Sourcing Current
6
Ω
RDS_SINK
Gate Driver Resistance When
Sinking Current
1.5
Ω
VCC_TH1
VCC Power-On-Reset
Threshold
4.0
8.5
10
13
4.3
V
VCC_TH2
VCC Shutdown Threshold
3.0
VDAC_IH
DAC Input High Voltage
3.5
VDAC_IL
DAC Input Low Voltage
GA
Error Amplifier DC Gain
76
dB
BWEA
Error Amplifier Unity Gain
Bandwidth
5
MHz
VRAMP_L
Ramp Signal Valley Voltage
1.25
V
VRAMP_H
Ramp Signal Peak Voltage
3.25
V
tSS
Soft Start Time
4096
Clock
Cycles
DSTEP_SS
Duty Cycle Step Change during
Soft Start
12.5
%
tPWGD
PWGD Response Time
SNS1 Rises from 0V to Rated
Output Voltage
2
8.4
15
µs
tPWBAD
PWGD Response Time
SNS1 Falls from Rated Output
Voltage to 0V
2
3.4
10
µs
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3.6
kΩ
V
V
1.3
2
V
Electrical Characteristics VCC = 5V, VDD = 12V unless otherwise specified. Typicals and limits appearing in
plain type apply for TA = TJ = +25˚C. Limits appearing in boldface type apply over the 0˚C to +70˚C range. (Continued)
Symbol
Parameter
Conditions
Min
Typ
Max
11.5
13
7
9
2.6
6
9.5
13
Units
SWITCHING SECTION
VPWGD_HI
PWGD High Trip Point
% Above Rated Output Voltage
When Output Voltage↑
% Above Rated Output Voltage
When Output Voltage ↓ (Note
4)
VPWGD_LO
PWGD Low Trip Point
% Below Rated Output Voltage
When Output Voltage↑
% Below Rated Output Voltage
When Output Voltage ↓ (Note
4)
VOVP_TRP
Over-Voltage Trip Point
ICS+
CS+ Pin Sink Current
VOCP
Over-Current Trip Point (CS+
and CS− Differential Voltage)
%
5
% SNS1 Above Rated Output
CS+ = 5V, CS− = 4.8V
CS+ = 2V, CS− Drops from 2V
%
6
15
25
35
%
126
185
244
µA
41
55
69
mV
1.5
1.538
1.5V LDO CONTROLLER SECTION
VDD = 12V, VCC = 4.75V to
5.25V, IG2 = 0 mA to 20 mA
VSNS2
SNS2 Voltage
ROUT2
Output Resistance
ISNS2
SNS2 Pin Bias Current
When Regulating
VPWGD_HI
PWGD High Trip Point
(Note 4)
VPWGD_LO
PWGD Low Trip Point
(Note 4)
0.44
V
1.463
V
200
Ω
21
µA
0.63
V
1.25V LDO CONTROLLER SECTION
VDD = 12V, VCC = 4.75V to
5.25V, IG3 = 0 mA to 20 mA
VSNS3
SNS3 Voltage
ROUT3
Output Resistance
ISNS3
SNS3 Pin Bias Current
When Regulating
VPWGD_HI
PWGD High Trip Point
VPWGD_LO
PWGD Low Trip Point
1.219
1.25
1.281
V
200
Ω
0
µA
(Note 4)
0.63
V
(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, TJMAX, the junction-to-ambient thermal resistance, θJA, and the
ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated using: PMAX = (TJMAX − TA)/θJA. The
junction-to-ambient thermal resistance, θJA, for LM2638 is 78˚C/W. For a TJMAX of 150˚C and TA of 25˚C, 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 switching section, the power good is a window. For the two linear sections, the power good is a threshold with some hysteresis.
3
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Electrical Characteristics VCC = 5V, VDD = 12V unless otherwise specified. Typicals and limits appearing in
plain type apply for TA = TJ = +25˚C. Limits appearing in boldface type apply over the 0˚C to +70˚C range. (Continued)
TABLE 1. 5-Bit DAC Output Voltage Table
(VCC = 5V, VDD = 12V ± 5%, TA = 25˚C, Test Mode)
Symbol
VDACOUT
Parameter
5-Bit DAC Output Voltages for Different VID Codes
Conditions
VID4:0 = 01111
VID4:0 = 01110
Typical
Units
1.30
V
VID4:0 = 01101
VID4:0 = 01100
1.40
VID4:0 = 01011
VID4:0 = 01010
VID4:0 = 01001
1.50
VID4:0 = 01000
VID4:0 = 00111
VID4:0 = 00110
1.55
1.60
1.65
1.70
1.75
1.80
VID4:0 = 00011
VID4:0 = 00010
VID4:0 = 00001
1.90
1.85
1.95
2.00
2.05
(shutdown)
2.1
VID4:0 = 11101
VID4:0 = 11100
2.2
VID4:0 = 11011
VID4:0 = 11010
VID4:0 = 11001
2.4
VID4:0 = 11000
VID4:0 = 10111
VID4:0 = 10110
4
1.45
VID4:0 = 00101
VID4:0 = 00100
VID4:0 = 00000
VID4:0 = 11111
VID4:0 = 11110
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1.35
2.3
2.5
2.6
2.7
2.8
2.9
VID4:0 = 10101
VID4:0 = 10100
VID4:0 = 10011
3.0
VID4:0 = 10010
VID4:0 = 10001
VID4:0 = 10000
3.3
3.1
3.2
3.4
3.5
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
VDD
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
VCC
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 VCC
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.
5
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Block Diagram
DS101034-3
Test Circuit
DS101034-4
FIGURE 1. LDO Controller Test Circuit
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6
FIGURE 2. Using LM2638 to Supply GTL+ Bus (VTT, 1.5V, 5.6A), 2.5V Standby (VCC2_5SBY, 2A Full Power and 180 mA Suspend) and
3.3V Standby (VCC3_3SBY, 1.5A Full Power, 0.5A Suspend)
DS101034-2
Typical Application
7
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LM2638 Motherboard Power Supply Solution with a 5-Bit Programmable Switching Controller and
Two Linear Regulator Controllers
Physical Dimensions
inches (millimeters) unless otherwise noted
24-Lead Small Outline Package
Order Number LM2638M
NS Package Number M24B
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