SSL2102 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
Product data sheet Rev. 1.1 — 2 December 2011 5 of 22
NXP Semiconductors
SSL2102
Dimmable Greenchip driver for LED lighting
8. Functional description
The SSL2102 is an 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 V
CC
passes the V
CC(startup)
level. The supply can be taken over
by the auxiliary winding of the transformer as soon as V
CC
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 V
CC
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 V
CC
drops below the V
CC(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 V
RC(max)
level and, starting
from a new primary stroke, it discharges to the V
RC(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 V
BRIGHTNESS
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.
RC
1
3.5
-------
1
f
osc
--------
t
ch earg
–
=
