HTC Korea LM1938SF6 Schematics

White LED Step-Up Converter LM1938

FEATURES

z Inherently Matched LED Current
z Drives Up to 27 LEDs from a 5V Supply
z 36V Rugged Bipolar Switch
z Fast 1.2MHz Switching Frequency
z Vovp(MAX) = 29V
z Moisture Sensitivity Level 3

APPLICATION

z Cellular Phones
z PDAs, Handheld Computers
z Digital Cameras
z MP3 Players
z GPS Receivers

ORDERING INFORMATION

Device Package

LM1938SF6 SOT-23 6L

SOT-23 6L PKG

DESCRIPTION

The LM1938 is a step-up DC/DC converter specifically designed to drive white LEDs with a constant current.
The device can drive up to 27 LEDs from a 5V supply.
Additional feature include output voltage limiting when LEDs are disconnected.

Absolute Maximum Ratings

(Note 1)

CHARACTERISTIC SYMBOL RATINGS UNIT

Input Voltage VIN 10 V

SW Voltage VSW 36 V

FB Voltage VFB 10 V

SHDN Voltage V

Lead Temperature (Soldering, 10 sec) T

Storage Temperature Range T

Operating Temperature Range T

Maximum Junction Temperature T

Note 1. Absolute Maximum Ratings are those values beyond which the life of the device may be impaired.

10 V

SHDN

300 °C

SOL

-65 ~ 150 °C

STG

0 ~ 70 °C

OPR

125 °C

J(max)

Aug. 2011 – Pre - 1 - HTC

White LED Step-Up Converter LM1938

Ordering Information

Package Order No. Package Marking Supplied As Status

SOT-23 6L LM1938SF6 LM1938 Reel Active

PIN CONFIGURATION

PIN DESCRIPTION

SOT-23-6L

Pin No.

1 SW

2 GND

SW

GND

FB

1

2

34

SOT-23-6

Name Function

Switching Pin. This is the collector of the internal NPN power switch.
Connect to inductor and diode. Minimize the metal trace area
connected to this pin to reduce EMI.

Ground Pin. Connect directly to local ground plane.

6

5

Vin

OVP

/SHDN

3 FB

4 SHDN

5 OVP

6 Vin

Feedback Pin. Reference voltage is 95mV.
Connect LEDs and a resistor at this pin.

Shutdown Pin. Connect to 1.5V or higher to enable device; 0.4V or less
to disable device.

Over voltage protection, 29.

Input Supply Pin. Bypass this pin with a capacitor as close to the device
as possible.

Aug. 2011 – Pre - 2 - HTC

White LED Step-Up Converter LM1938

TYPICAL APPLICATION

4.75

4.75

4.75

4.75

4.75

4.75

4.75

4.75

4.75

Fig. 1 Li-Ion Powered Driver for 27 White LEDs

BLOCK DIAGRAM

IN

6

V

REF

1.25V

OVP

5

SHDN

4

SHUTDOWN

Aug. 2011 – Pre - 3 - HTC

FBV

3 1

COMPARATOR

-

A2

+

DRIVER

R Q

S

95mV

-

A1

+

R

C

C

C

Σ

Overvoltage

Protection

RAMP

GENERATOR

1.2MHz

OSCILLATOR

Fig2. Block Diagram

SW

Q2

+

-

0.3

2

Q1

GND