SSL21084T/1,118 데이터 시트보기 (PDF) - NXP Semiconductors.

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SSL21084T/1,118

GreenChip drivers for LED lighting

NXP Semiconductors. 

NXP Semiconductors

Ip

Ip

Ip / 2

SSL2108X

Drivers for LED lighting

(1)

(2)

1

23

4

5 VNTC

001aan700

(1) Vth(ocp)SOURCE = 500 mV

(2) Vth(ocp)SOURCE = 250 mV

Fig 6. NTC control curve

When the voltage on the NTC pin is higher than Vth(high)NTC see Figure 6 (4), the converter

delivers nominal output current. Lower than this voltage 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:

• If the low-level Vdeact(tmr)NTC is not reached within time tto(deact)NTC, Figure 6 (1) LED

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

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

Vth(high)NTC, see Figure 6 (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 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.

8.9.1 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.

8.10 Heat sink

For SSL2108X (SO12) applications a copper heat sink forms part of PCB. The SSL2108X

(SO12) uses thermal leads (pins 2, 6, 10 and 11) for heat transfer from die to the PCB

copper heat sink. Enhanced thermal lead connection can drastically reduce thermal

resistance.

SSL2108X

Preliminary data sheet

All information provided in this document is subject to legal disclaimers.

Rev. 1 — 9 September 2011

© NXP B.V. 2011. All rights reserved.

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