NSC LM2637MX, LM2637M Datasheet

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

™

, Pentium™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 fea­tures 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 pro­tection 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 lev­els 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 fea­ture wide control bandwidth, N-FET and NPN transistor driv­ing capability, and an adjustable output voltage. The wide
control bandwidth makes meeting fast load transient re­sponse 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

MII™is a trademark of CyrixCorporation a wholly owned subsidiary of National Semiconductor Corporation.
Pentium

™

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

Two 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 150˚C
Power Dissipation (Note 2) 1.6W

Storage Temperature −65˚C to +150˚C
ESD Susceptibility 2.5 kV
Soldering Time, Temperature (10 sec.) 300˚C

Operating Ratings (Note 1)

V

CC

4.75V to 5.25V

Junction Temperature Range 0˚C to +125˚C

Electrical Characteristics

V

CC

=

5V, V

DD

=

12V unless otherwise specified. Typicals and limits appearing in plain type apply for T

A

=

T

J

=

+25˚C. Limits

appearing in boldface type apply over the 0˚C to +70˚C range.

Symbol Parameter Conditions Min Typ Max Units

I

EN

EN Pin internal Pull-Up Current 60 90 140 µA

I

VID

VID Pins internal Pull-Up
Current

60 90 140 µA

I

CC

Operating VCCCurrent EN=5V, VID=10111 6 7.5 mA

I

Q_VCC

VCCShutdown Current EN=0V, VID Pins Floating 1.5 3 mA

SWITCHING SECTION

V

DACOUT

5-Bit DAC Output Voltage (Note 3)

N

−1.5

%

NN

+1.5

%

V

I

Q_VDD

VDDShutdown Current EN=0V, VID Pins Floating 4 µA

f

OSC

Oscillator Frequency RT=100 kΩ 204 245 286

kHz

RT=25 kΩ 1000

D

MAX

Maximum Duty Cycle 95

%

D

MIN

Minimum Duty Cycle 0

%

R

SNS1

SNS1 Pin Resistance to
Ground

8.5 10 13 kΩ

R

DS_SRC

Gate Driver Resistance When
Sourcing Current

6 Ω

R

DS_SINK

Gate Driver Resistance When
Sinking Current

1.5 Ω

V

CC_TH1

Rising VCCThreshold for
Power-On Reset

4 4.3 V

V

CC_TH2

Falling VCCThreshold 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 µs

t

PWBAD

PWGD Response Time SNS1 Falls from Rated Output

Voltage to 0V

2 3.4 10 µs

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 µA

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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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

=

+25˚C. Limits

appearing in boldface type apply over the 0˚C to +70˚C 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

=

0mAto20mA

(

Figure 1

)

1.463 1.5 1.538 V

R

OUT2

Output Resistance 200 Ω
I

SNS2

SNS2 Pin Bias Current When Regulating 21 µA
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

=

0mAto20mA

(

Figure 1

)

2.438 2.5 2.563 V

R

OUT3

Output Resistance 200 Ω
I

SNS3

SNS3 Pin Bias Current When Regulating 21 µA
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, θJA, and the

ambient temperature, T

A

. The maximum allowable power dissipation at any ambient temperature is calculated using:

P

MAX

=

(T

JMAX−TA

)/

θ

JA

.

The

junction-to-ambient thermal resistance, θ

JA

, for LM2637 is 78˚C/W. For a T

JMAX

of 150˚C and TAof 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 2.5V section.

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Electrical Characteristics (Continued)

TABLE 1. 5-Bit DAC Output Voltage Table

(V

CC

=

5V, V

DD

=

12V

±

5%,T

A

=

25˚C, 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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Typical Applications

DS100848-3

FIGURE 2. Motherboard Power Supply for Pentium II Processor Core (1.3V - 2.8V, 14.2A), GTL+ Bus (1.5V, 4A), and

Legacy I/O (2.5V, 0.3A). External sense resistor is used to provide both over-current limit and dynamic voltage

positioning.

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