SSL2102 datasheet(5/21 Pages) NXP

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SSL2102_1

Product data sheet

Rev. 01 — 29 June 2009

5 of 21

NXP Semiconductors

SSL2102

SMPS IC for dimmable LED lighting

Functional description

The SSL2102 is a LED driver IC that operates directly from the rectiﬁed mains.

The SSL2102 uses on-time mode control and frequency control to control the LED

brightness. The BRIGHTNESS and PWMLIMIT input of the IC can be used to control the

LED light output in combination with an external dimmer. The PWMLIMIT input can also

be used for Thermal Lumen Management (TLM) and for precision LED current control.

8.1 Start-up and UnderVoltage LockOut (UVLO)

Initially, the IC is self-supplying from the rectiﬁed mains voltage. The IC starts switching as

soon as the voltage on pin VCC passes the VCC(startup) level. The supply can be taken over

by the auxiliary winding of the transformer as soon as VCC is high enough and the supply

from the line is stopped for high efﬁciency operation. Alternatively the IC can be supplied

via a bleeder resistor connected to a high voltage.

Remark: The maximum VCC voltage rating of the IC.

If for some reason the auxiliary supply is not sufﬁcient, the high-voltage supply can also

supply the IC. As soon as the voltage on pin VCC drops below the VCC(UVLO) level, the IC

stops switching and will restart from the rectiﬁed mains voltage, if the internal current

delivered is sufﬁcient.

8.2 Oscillator

An internal oscillator inside the IC provides the timing for the switching converter logics.

The frequency of the oscillator is set by the external resistors and the capacitor on pin RC

and pin RC2. The external capacitor is charged rapidly to the VRC(max) level and, starting

from a new primary stroke, it discharges to the VRC(min) level. Because the discharge is

exponential, the relative sensitivity of the duty factor to the regulation voltage at low duty

factor is almost equal to the sensitivity at high duty factors. This results in a more constant

gain over the duty factor range compared to Pulse Width Modulated (PWM) systems with

a linear sawtooth oscillator. Stable operation at low duty factors is easily realized. The

frequency of the converter when VBRIGHTNESS is high can be estimated using Equation 1:

(1)

R equals the parallel resistance of both oscillator resistors. C is the capacitor connected at

the RC pin (pin 8).

The BRIGHTNESS input controls the frequency reduction mode. Figure 3 shows that the

oscillator switches over from an RC curve with R1//R2 to R1 only. A low BRIGHTNESS

voltage will reduce the switching frequency.

3.5

-------

osc

----------

arg

–

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