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NXP Semiconductors
SSL152x
SMPS ICs for mains LED drivers
8. Functional description
The SSL152x family is the heart of a compact flyback converter, with the IC placed at the
primary side. The auxiliary winding of the transformer can be used for indirect feedback to
control the isolated output. This additional winding also powers the IC. A more accurate
control of the output voltage and/or current can be implemented with an additional
secondary sensing circuit and optocoupler feedback.
The SSL152x family uses voltage mode control. The switching frequency is determined by
the maximum transformer demagnetizing time and the frequency of the oscillator. In the
first case, the converter operates in the Self Oscillating Power Supply (SOPS) mode. In
the latter case, it operates at a constant frequency, which can be adjusted with external
components R RC and C RC . Furthermore, a primary stroke is started only in a valley of the
secondary ringing. This can use constant power or constant current mode to drive LEDs.
The valley switching principle minimizes capacitive switch-on losses.
8.1 Start-up and undervoltage lockout
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 is 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.
If the auxiliary supply is not sufficient, the high-voltage supply also supplies the IC. As
soon as the voltage on pin V CC drops below the V CC(stop) level, the IC stops switching and
restarts from the rectified mains voltage.
8.2 Oscillator
The frequency of the oscillator is set by the external resistor and capacitor on pin RC. 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 systems with a linear sawtooth oscillator. Stable operation
at low duty factors is easily realized. For high efficiency, the frequency is reduced as soon
as the duty factor drops below its low power threshold. This is accomplished by increasing
the oscillator charge time.
To ensure that the capacitor can be charged within the charge time, the value of the
oscillator capacitor should be limited to approximately 1 nF.
8.3 Duty factor control
The duty factor is controlled by the internal regulation voltage and the oscillator signal on
pin RC. The internal regulation voltage is equal to the external regulation voltage (minus
2.5 V) multiplied by the gain of the error amplifier (typically 20 dB).
SSL152X
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 3 — 17 September 2010
? NXP B.V. 2010. All rights reserved.
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