SSL2129AT All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 4 — 3 October 2013 5 of 19
NXP Semiconductors
SSL2129AT
Dimmable LED driver IC
8. Functional description
8.1 Introduction
The SSL2129AT is a driver IC solution for small form factor mains phase-cut dimmable
LED lamps in isolated and non-isolated applications.
8.2 Converter operation
The converter in the SSL2129AT is a Boundary Conduction Mode (BCM), peak current
controlled system. See Figure
3 for the basic application diagram. See Figure 4 for the
waveforms.
This converter type operates at the boundary between continuous and discontinuous
mode
. Energy is stored in inductor L each period that the switch is on. The inductor
current I
L
is zero when the MOSFET is switched on. The amplitude of the current build-up
in L is proportional to the voltage drop over the inductor and the time that the MOSFET
switch is on. When the MOSFET is switched off, the energy in the inductor is released
towards the output. The current then falls at a rate proportional to the value of V
OUT
. The
LED current I
LED
depends on the peak current through the inductor (SSL2129AT
controlled) and on the dimmer angle while it is optimized for a high-power factor. A new
cycle is started once the inductor current I
L
is zero. This quasi-resonant operation results
in higher efficiency.
8.3 Driver pin
The SSL2129AT is equipped with a driver output for controlling an external switch. The
voltage on the driver output pin is increased towards V
o(DRIVER)max
to open the switch
during the first cycle (t
0
to t
1
). The voltage on the driver output pin is pulled down towards
a low level from the start of the secondary stroke until the next cycle starts (t
0
to t
00
).
During transition from low to high and back, there is a controlled switching slope
steepness. This controlled condition limits the high-frequency radiation from the circuit to
the surrounding area. The switching slope can be controlled further using an external
resistor between IC and gate.
At the lowest VCC voltage (V
CC(stop)
), the voltage of the driver is V
O(DRIVER)min
.
Fig 3. SSL2129AT basic application diagram
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