Linear LT1937ES5, LT1937ESC6 Schematic [ru]

FEATURES

LT1937

White LED

Step-Up Converter in

SC70 and ThinSOT

U

DESCRIPTIO

■

Inherently Matched LED Current

■

High Efficiency: 84% Typical

■

Drives Up to Four LEDs from a 3.2V Supply

■

Drives Up to Six LEDs from a 5V Supply

■

36V Rugged Bipolar Switch

■

Fast 1.2MHz Switching Frequency

■

Uses Tiny 1mm Tall Inductors

■

Requires Only 0.22µF Output Capacitor

■

Low Profile SC70 and ThinSOTTM Packaging

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APPLICATIO S

■

Cellular Phones

■

PDAs, Handheld Computers

■

Digital Cameras

■

MP3 Players

■

GPS Receivers

U

TYPICAL APPLICATIO

The LT®1937 is a step-up DC/DC converter specifically
designed to drive white LEDs with a constant current. The
device can drive two, three or four LEDs in series from a
Li-Ion cell. Series connection of the LEDs provides iden­tical LED currents resulting in uniform brightness and
eliminating the need for ballast resistors. The LT1937
switches at 1.2MHz, allowing the use of tiny external
components. The output capacitor can be as small as

0.22µF, saving space and cost versus alternative solu-
tions. A low 95mV feedback voltage minimizes power loss
in the current setting resistor for better efficiency.

The LT1937 is available in low profile SC70 and ThinSOT
packages.

, LTC and LT are registered trademarks of Linear Technology Corporation.

ThinSOT is a trademark of Linear Technology Corporation.

V

3V TO 5V

OFF ON

L1

V

IN

SHDN

22µH

LT1937

GND

IN

C1
1µF

C1, C2: X5R OR X7R DIELECTRIC
D1: CENTRAL SEMICONDUCTOR CMDSH-3
L1: MURATA LQH3C-220 OR EQUIVALENT

SW

D1

LED 1

LED 2

LED 3

FB

15mA

R1

6.34Ω

Figure 1. Li-Ion Powered Driver for Three White LEDs

C2

0.22µF

1937 F01a

90

85

80

= 3V

V

IN

75

EFFICIENCY (%)

70

65

60

0

Conversion Efficiency

VIN = 3.6V

5101520

LED CURRENT (mA)

1937 TA01b

1937f

1

LT1937

WWWU

ABSOLUTE AXI U RATI GS

(Note 1)

Input Voltage (VIN) ................................................. 10V

SW Voltage ............................................................. 36V

FB Voltage .............................................................. 10V

SHDN Voltage ......................................................... 10V

UU

W

PACKAGE/ORDER I FOR ATIO

ORDER PART

TOP VIEW

SW 1

GND 2

FB 3

S5 PACKAGE

5-LEAD PLASTIC TSOT-23

T

= 125°C, θJA = 256°C/ W IN FREE AIR

JMAX

= 120°C ON BOARD OVER GROUND PLANE

θ

JA

Consult LTC Marketing for parts specified with wider operating temperature ranges.

5 V

IN

4 SHDN

NUMBER

LT1937ES5

S5 PART MARKING

LTYN

Extended Commercial
Operating Temperature Range (Note 2)... –40°C to 85°C

Maximum Junction Temperature ..........................125°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec).................. 300°C

ORDER PART

TOP VIEW

SW 1

GND 2

FB 3

SC6 PACKAGE

6-LEAD PLASTIC SC70

T

= 125°C, θJA = 256°C/ W IN FREE AIR

JMAX

= 150°C ON BOARD OVER GROUND PLANE

θ

JA

6 V

IN

5 GND
4 SHDN

NUMBER

LT1937ESC6

SC6 PART MARKING

LAAB

ELECTRICAL CHARACTERISTICS

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Operating Voltage 2.5 V
Maximum Operating Voltage 10 V
Feedback Voltage ISW = 100mA, Duty Cycle = 66% 86 95 104 mV
FB Pin Bias Current 10 45 100 nA
Supply Current 1.9 2.5 mA

SHDN = 0V 0.1 1.0 µA

Switching Frequency 0.8 1.2 1.6 MHz
Maximum Duty Cycle 85 90 %
Switch Current Limit 320 mA
Switch V

CESAT

Switch Leakage Current VSW = 5V 0.01 5 µA
SHDN Voltage High 1.5 V
SHDN Voltage Low 0.4 V
SHDN Pin Bias Current 65 µA

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

ISW = 250mA 350 mV

TA = 25°C, VIN = 3V, V

Note 2: The LT1937E is guaranteed to meet specifications from 0°C to

70°C. Specifications over the –40°C to 85°C operating temperature range
are assured by design, characterization and correlation with statistical
process controls.

= 3V, unless otherwise noted.

SHDN

2

1937f

UW

TEMPERATURE (°C)

–50

0.8

1.0

1.4

25 75

1937 G03

0.6

0.4

–25 0

50 100

0.2

0

1.2

SWITCHING FREQUENCY (MHz)

DUTY CYCLE (%)

0

350

300

250

200

150

100

50

0

60

1937 G06

20 40 80 100

CURRENT LIMIT (mA)

TYPICAL PERFOR A CE CHARACTERISTICS

LT1937

Quiescent Current SHDN Pin Bias Current

2.2

2.0

1.8

1.6

1.4

1.2

(mA)

1.0

Q

I

0.8

0.6

0.4

0.2
0

2468

0

Feedback Bias Current

60

50

40

30

20

25°C

VIN (V)

–50°C

100°C

10

1937 G01

400

350

300

250

200

150

100

SHDN PIN BIAS CURRENT (µA)

50

0

–50

SHDN = 3.6V SHDN = 3V

SHDN = 2.7V

–25 0 50

TEMPERATURE (°C)

Efficiency vs Temperature

85

= 3.6V

V

IN

3 LEDs

84

83

82

EFFICIENCY (%)

I

LED

I

LED

I

LED

25

= 20mA

= 15mA

= 10mA

Switching Frequency

SHDN = 10V

75 100

1937 G02

Current Limit vs Duty Cycle

FEEDBACK BIAS CURRENT (nA)

10

0

–50

–25 0

U

PI FU CTIO S

SW (Pin 1): Switch Pin. Connect inductor/diode here.
Minimize trace area at this pin to reduce EMI.

GND (Pin 2): Ground Pin. Connect directly to local ground
plane.

FB (Pin 3): Feedback Pin. Reference voltage is 95mV.
Connect cathode of lowest LED and resistor here. Calcu­late resistor value according to the formula:

RFB = 95mV/I

50 100

25 75

TEMPERATURE (°C)

UU

LED

1937 G04

81

80

–50

0

TEMPERATURE (°C)

50

100

1937 G05

SHDN (Pin 4): Shutdown Pin. Connect to 1.5V or higher to
enable device; 0.4V or less to disable device.

GND (Pin 5, SC70 Package): Ground Pin. Connect to Pin
2 and to local ground plane

VIN (Pin 5/Pin 6 SC70 Package): Input Supply Pin. Must
be locally bypassed.

1937f

3

LT1937

BLOCK DIAGRA

W

(PIN 6 FOR

SC70 PACKAGE)

V

IN

5

FB

3

–

V

REF

1.25V

SHDN

4

95mV

SHUTDOWN

A1

+

R

C

C

C

Figure 2. LT1937 Block Diagram

U

OPERATIO

The LT1937 uses a constant frequency, current mode
control scheme to provide excellent line and load regula­tion. Operation can be best understood by referring to the
block diagram in Figure 2. At the start of each oscillator
cycle, the SR latch is set, which turns on the power switch
Q1. A voltage proportional to the switch current is added
to a stabilizing ramp and the resulting sum is fed into the
positive terminal of the PWM comparator A2. When this
voltage exceeds the level at the negative input of A2, the SR
latch is reset turning off the power switch. The level at the
negative input of A2 is set by the error amplifier A1, and is
simply an amplified version of the difference between the
feedback voltage and the reference voltage of 95mV. In
this manner, the error amplifier sets the correct peak
current level to keep the output in regulation. If the error
amplifier’s output increases, more current is delivered to
the output; if it decreases, less current is delivered.

Minimum Output Current

The LT1937 can regulate three series LEDs connected at
low output currents, down to approximately 4mA from a

4.2V supply, without pulse skipping, using the same
external components as specified for 15mA operation. As
current is further reduced, the device will begin skipping

1

COMPARATOR

–

A2

+

RQ

S

DRIVER

SW

Q1

+

Σ

0.2Ω

–

RAMP

GENERATOR

1.2MHz

OSCILLATOR

(PINS 2 AND 5 FOR

SC70 PACKAGE)

2

GND

1937 BD1

pulses. This will result in some low frequency ripple,
although the LED current remains regulated on an average
basis down to zero. The photo in Figure 3 details circuit
operation driving three white LEDs at a 4mA load. Peak
inductor current is less than 50mA and the regulator
operates in discontinuous mode, meaning the inductor
current reaches zero during the discharge phase. After the
inductor current reaches zero, the switch pin exhibits
ringing due to the LC tank circuit formed by the inductor
in combination with switch and diode capacitance. This
ringing is not harmful; far less spectral energy is contained
in the ringing than in the switch transitions. The ringing
can be damped by application of a 300Ω resistor across
the inductor, although this will degrade efficiency.

V

SW

5V/DIV

I

L2

50mA/DIV

V

OUT

100mV/DIV

0.2µs/DIV

Figure 3. Switching Waveforms at I

LED

1937 F03

= 4mA, VIN = 3.6V

1937f

4