SSL21082T datasheet(9/24 Pages) PHILIPS

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SSL21081T_2T_3T_4T

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Preliminary data sheet

Rev. 2 — 6 December 2011

9 of 24

NXP Semiconductors

SSL21081T/2T/3T/4T

GreenChip drivers for LED lighting

to prevent the additional high-power losses, which would result if a regulator were used for

continuously powering the IC. Unlike an auxiliary supply, additional inductor windings are

not needed.

8.7 VCC regulator

During supply dips, the input voltage can drop too low to supply the required IC current

through the DVDT pin. Under these conditions, if the VCC voltage drops lower than

VCC(swon)reg level, another regulator with a current capability of up to IHVhigh(oper) is started.

The job of the regulator is to fill in the required supply current, which the DVDT supply

does not deliver, thus preventing the IC going into UVLO. When the VCC voltage is higher

than VCC(swon)reg level, the regulator is turned off.

8.8 NTC functionality and PWM dimming

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

the pin can be used as an input to disable/enable light output using a digital signal (PWM

dimming). The pin has an internal current source that generates the current of Ioffset(NTC).

An NTC resistor to monitor the LED temperature can be directly connected to the NTC

pin. Depending on the resistance value and the corresponding voltage on the NTC pin,

the converter reacts as shown in Figure 6.

When the voltage on the NTC pin is higher than 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, Figure 6 (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 6 (4). It is assumed that the reduction in peak current did not

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

Fig 6.

NTC control curve

001aan700

Peak Current

Ip / 2

Vth(ocp)SOURCE = 500 mV

Vth(ocp)SOURCE = 250 mV

VNTC

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