SSL21081AT datasheet(11/22 Pages) PHILIPS

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SSL21081AT

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Product data sheet

Rev. 6 — 3 October 2013

11 of 21

NXP Semiconductors

SSL21081AT

Compact non-dimmable LED driver IC

When the voltage on the NTC pin exceeds Vth(high)NTC (see Figure 6 (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 6 (3)). The peak current is

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

(see Figure 6 (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 (see Figure 6 (1)),

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

the HV pin voltage. The converter restarts from an NTC protection shutdown when the

voltage on the NTC pin exceeds Vth(high)NTC (see Figure 6 (4)). It is assumed that the

reduction in peak current does 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 (see Figure 6 (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. PWM regulation and consequently LED

output current regulation can be implemented this way. The output current rises again

when the voltage exceeds Vth(low)NTC.

8.7.1 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 (see Figure 6 (3)), the converter starts with half of

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

(see Figure 6 (4)) is reached.

8.8 Heat sink

The copper of the PCB acts as the heat sink for SSL21081AT applications.

Equation 1 shows the relation between the maximum allowable power dissipation P and

the thermal resistance from junction to ambient.

(1)

Where:

Rth(j-a) = thermal resistance from junction to ambient

Tj(max) = maximum junction temperature

Tamb = ambient temperature

P = power dissipation

th j

–



jmax



amb

–

 P



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