2.7

3.1

3.5

3.9

4.3

4.7

5.1

5.5

50

60

70

80

90

100

VIN(V)

TA = -40°C

TA = +25°C

TA = +85°C

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

60

64

68

72

76

80

84

88

92

96

100

ILED(A)

VIN =4.2V

VIN =3.9V

VIN =3.6V

VIN =3.3V

VIN =3.0V

VIN =2.7V

e

x

( )

C

25

R

=

T

R

°

298

1

273

+

C

°

T

1

E

-

¹

·

©

§

LM3556

www.ti.com

SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015

8.2.2.4 NTC Thermistor Selection

The TEMP pin is a comparator input for flash LED thermal sensing. NTC mode is intended to monitor an external

thermistor which monitors LED temperature and prevents LED overheating. An internal comparator checks the

voltage on the TEMP pin against the trip point programmed in the NTC Settings Register (0x02). The thermistor

is driven by an internally regulated current source, and the voltage on the TEMP pin is related to the source

current and the NTC resistance.

NTC thermistors have a temperature to resistance relationship of:

where

•

β is given in the thermistor datasheet

•

R25°C is the thermistor's value at 25°C.

(4)

8.2.3 Application Curves

VLED = 3.6 V

ILED = 1.5 A

VLED = 3.8 V

Figure 42. Flash LED Efficiency vs VIN

Figure 43. Flash LED Efficiency vs Flash LED Current

9 Power Supply Recommendations

The LM3556 is designed to operate from an input supply range of 2.5 V to 5.5 V. This input supply must be well

regulated and provide the peak current required by the LED configuration and inductor selected.

Copyright © 2011–2015, Texas Instruments Incorporated

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