LM3560 Datasheet, PDF (37/42 Pages) Texas Instruments – LM3560 Synchronous Boost Flash Driver w/ Dual 1A High-Side Current Sources (2A Total Flash Current)

NTC THERMISTOR SELECTION
Programming bit [4] of Configuration Register 1 with a '1' se-
lects NTC mode and makes the LEDI/NTC pin a comparator
input for flash LED thermal sensing. Figure 21 shows the
LM3560 using the NTC thermistor circuit. The thermal sensor
resistor divider is composed of R3 and R(T), where R(T) is
the Negative Temperature Coefficient Thermistor, VBIAS is
the bias voltage for the resistive divider, and R3 is used to
linearize the NTC's response around the NTC comparators
trip point. C_BYP is used to filter noise at the NTC input.
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FIGURE 21. Typical Application Circuit with Thermistor
In designing the NTC circuit, we must choose values for
VBIAS, R(T) and R3. To begin with, NTC thermistors have a
non-linear relationship between temperature and resistance:
where R(T)_TRIP is the thermistor's value at the temperature
trip point and V_TRIP = 1V (typical). As an example, with V_BIAS
= 2.5V and a thermistor whose nominal value at +25°C is 100
kΩ and a β = 4500K, the trip point is chosen 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. R3 is chosen so that the temper-
ature to resistance relationship becomes more linear and can
be found by solving for R3 in the R(T) and R3 resistive divider:
Figure 22 shows the linearity of the thermistor resistive divider
of the previous example.
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