LM3554TMXSLASHNOPB datasheet(32/47 Pages) TI1

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+ E

TRIP x

x 298

273

C x

298

° )

(

TRIP

BIAS

)

0.5

0.6

0.7

0.8

0.9

1.1

1.2

1.3

1.4

1.5

100 105 110

TEMPERATURE (°C)

VBIAS = 2.5V,

@ +25°C, B = 4500,

R3 = 9 k:

RTHERMISTOR = 100 k:

LM3554

SNVS549C – JUNE 2009 – REVISED FEBRUARY 2016

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Figure 42. Thermistor Resistive Divider Response vs Temperature

Another useful equation for the thermistor resistive divider is developed by combining the equations for R3, and

R(T) and solving for temperature. This is shown in Equation 7:

(7)

Using, for example, Excel® spreadsheet software, different curves for the temperature trip point T (°C) can be

created vs R3, Beta, or VBIAS in order to help better choose the thermal components for practical values of

thermistors, series resistors (R3), or reference voltages VBIAS.

Programming bit [3] of the Configuration Register with a 1 selects thermal comparator mode making the

LEDI/NTC pin a comparator input for flash LED thermal sensing. Figure 43 shows the internal block diagram of

the thermal sensing circuit which is OR’d with both the TX1 and ENVM/TX2 (TX2 mode) to force the LM3554

from flash to torch mode. This is intended to prevent LED overheating during flash pulses.

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