NSC LM2638M, LM2638MX Datasheet

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 suchas Pentium

™

II,MII™,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 isshut off during system
standby such as STR mode.

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 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 fea­ture wide control bandwidth, N-FETand 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.

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

MII™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.

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

Two 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 NationalSemiconductor 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 3 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

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

I

Q_VDD

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

R

DS_CP

CP Pin Resistance High Side FET 100

Ω

Low Side FET 10

SWITCHING SECTION

V

DACOUT

5-Bit DAC Output Voltage (Note 3)

N

−1.5

%

NN

+1.5

%

V

I

VID

VID Pins Internal Pull-up
Current

60 90 140 µA

f

OSC

Oscillator Frequency RT=100 kΩ 204 245 286

kHz

RT=25 kΩ 1000

D

MAX

Maximum Duty Cycle 100

%

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

VCCPower-On-Reset
Threshold

4.0 4.3 V

V

CC_TH2

VCCShutdown 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 µs

t

PWBAD

PWGD Response Time SNS1 Falls from Rated Output

Voltage to 0V

2 3.4 10 µs

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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. (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 µA

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

=

0mAto20mA

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

1.25V LDO CONTROLLER SECTION

V

SNS3

SNS3 Voltage V

DD

=

12V, V

CC

=

4.75V to

5.25V, I

G3

=

0mAto20mA

1.219 1.25 1.281 V

R

OUT3

Output Resistance 200 Ω

I

SNS3

SNS3 Pin Bias Current When Regulating 0 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 beyondwhichdamageto the device may occur.Operatingratings 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 LM2638 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 1.25V section. Fortheswitch­ing 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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