LM3561TMX National Semiconductor Corporation, LM3561TMX Datasheet - Page 30

• Manufacturer Part Number: LM3561TMX
• Description: Synchronous Boost Converter With 600ma High Side Led Driver And I2c-compatible Interface
• Manufacturer: National Semiconductor Corporation

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NTC THERMISTOR SELECTION
Programming bit [4] of Configuration Register 1 with a (1) se-
lects Thermal Comparator mode, making the LEDI/NTC pin
a comparator input for flash LED thermal sensing. The ther-
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 β is given in the thermistor datasheet and R
thermistor's value at +25°C. R
to:
BIAS
is chosen so that it is equal
25C
FIGURE 15. NTC Circuit
is the
30
mal sensing circuit consists of a negative temperature coef-
ficient (NTC) thermistor and a series resistor which forms a
resistive divider (see
where R(T)
trip point, V
and V
linear response around the temperature trip voltage. For ex-
ample with V
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:
Figure 16
of the previous example.
TRIP
shows the linearity of the thermistor resistive divider
= 1V (typical). Choosing R
TRIP
BIAS
BIAS
is the thermistor's value at the temperature
is the bias voltage for the thermistor circuit,
= 1.8V and a thermistor whose nominal
Figure
15).
30113966
BIAS
here gives a more