LM3561 Datasheet, PDF (31/36 Pages) Texas Instruments – LM3561 Synchronous Boost Converter with 600mA High Side LED Driver and I 2 C-Compatible Interface

ficient (NTC) thermistor and a series resistor which forms a
resistive divider (see Figure 15).
FIGURE 15. NTC Circuit
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The NTC thermistor senses the LEDs temperature via con-
ducting the LEDs heat into the NTC thermistor. Heat conduc-
tion is improved with a galvanic connection at GND (LED
cathode and NTC thermistor GND terminal) and by placing
the thermistor in very close proximity to the flash LED.
NTC thermistors have a temperature to resistance relation-
ship of:
where R(T)_TRIP is the thermistor's value at the temperature
trip point, V_BIAS is the bias voltage for the thermistor circuit,
and V_TRIP = 1V (typical). Choosing R_BIAS here gives a more
linear response around the temperature trip voltage. For ex-
ample with V_BIAS = 1.8V and a thermistor whose nominal
value at +25°C is 10kΩ and a β = 3380K, the trip point is cho-
sen to be +93°C. The value of R(T) at 93°C is:
where β is given in the thermistor datasheet and R_25C is the
thermistor's value at +25°C. R_BIAS is chosen so that it is equal
to:
Figure 16 shows the linearity of the thermistor resistive divider
of the previous example.
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