LM3561 Datasheet, PDF (32/36 Pages) Texas Instruments – LM3561 Synchronous Boost Converter with 600mA High Side LED Driver and I 2 C-Compatible Interface

VLEDI/NTC vs Temp (V_BIAS = 1.8V, THERMISTOR = 10kΩ @+25C, β = 3380, R_BIAS =972Ω)
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FIGURE 16. Thermistor Resistive Divider Response vs Temperature
Another useful equation for the thermistor resistive divider is
developed by combining the equations for R_BIAS, and R(T)
and solving for temperature. This gives the following relation-
ship.
Using a spreadsheet such as Excel, different curves for the
temperature trip point T(°C) can be created vs R_BIAS, Beta, or
V_BIAS in order to help better choose the thermal components
for practical values of thermistors, series resistors (R3), or
reference voltages V_BIAS.
NTC THERMISTOR PLACEMENT
The termination of the thermistor must be done directly to the
cathode of the Flash LED in order to adequately couple the
heat from the LED into the thermistor. Consequentially, the
noisy environment generated from the switching of the
LM3561's boost converter can introduce noise from GND into
the thermistor sensing input. To filter out this noise it is nec-
essary to place a 0.1µF or larger ceramic capacitor close to
the LEDI/NTC pin. The filter capacitor's return must also con-
nect with a low-impedance trace, as close as possible to the
GND pin of the LM3561.
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