UM10501
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© NXP B.V. 2011. All rights reserved.
User manual
Rev. 2 — 16 November 2011
9 of 18
NXP Semiconductors
UM10501
SSL21083 reference board user manual
7. External OverTemperature Protection (OTP)
The SSL21083 supports external OTP by adding an external Negative Temperature
Coefficient (NTC) resistor. This feature is delivered by detecting a voltage on pin NTC. Pin
NTC has an integrated current source. The Resistance of the NTC resistor is decreased
as the temperature is raised. When the NTC temperature rises and the voltage on pin
NTC falls below 0.5 V, the SSL21083 lowers the threshold level for detecting peak current
in the inductor. Decreasing the peak current in the inductor causes the power current to
decrease. The output current is regulated to the point where a balance between
temperature and output current can be retained (the so called thermal management).
If the temperature on NTC increases continuously and the voltage on the pin drops below
0.3 V, the SSL21083 starts the NTC time-out timer. If the voltage on pin NTC pin does not
drop below 0.2 V within the time-out, the SSL21083 detects an abnormal condition and
stops switching. If the voltage reaches 0.2 V within the time-out period, a PWM signal is
assumed.
An NTC resistor can be directly connected to pin NTC. It is also possible to tune the
protection temperature by adding a resistor in parallel or in series with the NTC. One NTC
and one resistor are installed on the reference board. The values of these components
can be changed depending on the protection temperature requirement and component
availability.
The NTC should be mounted in thermal contact with the LED string.