LM3556_15 Datasheet, PDF (29/39 Pages) Texas Instruments – Synchronous Boost LED Flash Driver With High-Side Current Source

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LM3556
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:
R(T)
= R_25°C
xe
E_©§T
1
°C+ 273
-
1_·
298_¹
where
• β is given in the thermistor datasheet
• R_25°C is the thermistor's value at 25°C.
(4)
8.2.3 Application Curves
100
TA = -40°C
90
TA = +25°C
TA = +85°C
80
70
60
50
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
VIN(V)
V_LED = 3.6 V
I_LED = 1.5 A
100
96
92
88
84
80
76
VIN =4.2V
VIN =3.9V
72
VIN =3.6V
VIN =3.3V
68
VIN =3.0V
VIN =2.7V
64
60
0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5
ILED(A)
V_LED = 3.8 V
Figure 42. Flash LED Efficiency vs V_IN
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.
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