SSL21082T datasheet(10/24 Pages) PHILIPS

10 / 24 page

SSL21081T_2T_3T_4T

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Preliminary data sheet

Rev. 2 — 6 December 2011

10 of 24

NXP Semiconductors

SSL21081T/2T/3T/4T

GreenChip drivers for LED lighting

• If the low-level Vdeact(tmr)NTC is reached within the time tto(deact)NTC, 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 dimming can be implemented this

way. 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 6 (3), the convertor starts with half of the

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

Figure 6 (4) is reached.

8.9 Heat sink

For SSL2108X (SO12) applications, the copper of the PCB acts as the heat sink. The

SSL2108X (SO12) uses thermal leads (pins 2, 6, 10 and 11) for enhanced heat transfer

from die to the PCB copper heat sink. The thermal lead connection can drastically reduce

thermal resistance.

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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