SSL2129AT datasheet(9/21 Pages) NXP

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SSL2129AT

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Product data sheet

Rev. 2 — 23 October 2012

9 of 21

NXP Semiconductors

SSL2129AT

Dimmable LED controller IC

8.8 NTC functionality

The NTC pin can be used as a control method for LED thermal protection.

When the voltage on the NTC pin is higher than Vth(high)NTC, see Figure 5 (4), the

converter delivers nominal output current. When the voltage is lower than this level, the

peak current is gradually reduced until Vth(low)NTC is reached, see Figure 5 (3). The peak

current is now half the peak current of nominal operation. When Vact(tmr)NTC is passed, see

Figure 5 (2), a timer starts to run to distinguish between the following situations:

• If the low-level Vdeact(tmr)NTC is not reached within time tto(deact)NTC Figure 5 (1), LED

overtemperature is detected. The IC stops switching and attempts to restart from the

HV pin voltage. Restart takes place when the voltage on NTC pin is higher than

Vth(high)NTC, see Figure 5 (4). It is assumed that the reduction in peak current did not

result in a lower NTC temperature and LED OTP is activated.

• If the low-level Vdeact(tmr)NTC is reached within the time tto(deact)NTC Figure 5 (1), it is

assumed that the pin is pulled down externally. The restart function is not triggered.

Instead, the output current is reduced to zero. The output current rises again when the

voltage is higher than Vdeact(tmr)NTC.

8.8.1 Soft-start function

The NTC pin can be used to make a soft start function. During switch-on, the level on the

NTC pin is low. By connecting a capacitor (in parallel with the NTC resistor), a time

constant can be defined. The time constant causes the level on the NTC pin to increase

slowly. When passing level Vth(low)NTC Figure 5 (3), the convertor starts with half of the

maximum current. The output current slowly increases to maximum when Vth(high)NTC

Figure 5 (4) is reached.

Fig 5.

NTC control curve

001aan700

Ipk

Peak current

Ipk / 2

Vth(ocp)SOURCE = 500 mV

Vth(ocp)SOURCE = 250 mV

VNTC

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