SSL21084T datasheet(6/24 Pages) PHILIPS

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SSL21081T_2T_3T_4T

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Preliminary data sheet

Rev. 2 — 6 December 2011

6 of 24

NXP Semiconductors

SSL21081T/2T/3T/4T

GreenChip drivers for LED lighting

8.2 Conversion frequency

The conversion frequency must be limited to below 200 kHz. Therefore, select an

inductance value so that the conversion frequency is always within limits, given the supply

voltage, LED voltage and component spread.

8.3 Valley detection

A new cycle is started when the primary switch is switched on (see Figure 5). Following

time t1, when the peak current is detected on the SOURCE pin, the switch is turned off

and the secondary stroke starts (3). When the secondary stroke is complete and the coil

current at t3 equals zero, the drain voltage starts to oscillate around the VIN  VOUT level.

The amplitude equals VOUT. A special feature, called valley detection is an integrated part

of the SSL2108X circuitry. Dedicated built-in circuitry connected to the DRAIN pin, senses

when the voltage on the drain of the switch has reached its lowest value. The next cycle is

then started and as a result the capacitive switching losses are reduced. A valley is

detected and accepted if both the frequency of the oscillations and the voltage swing are

within the range specified (fring and ∆Vvrec(min)) for detection. If a valid valley is not

detected, the secondary stroke is continued until the maximum off-time (toff(high)) is

reached, then the next cycle is started.

8.4 Protections

The IC has the following protections:

• UnderVoltage LockOut (UVLO)

• Leading-Edge Blanking (LEB)

• OverCurrent Protection (OCP)

Fig 5.

Buck waveforms and valley detection

001aan699

demagnetization

VIN

valley

internal MOSFET switch

VGATE

VOUT

magnetization

t00

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