SSL152X_2

© NXP B.V. 2009. All rights reserved.

Product data sheet

Rev. 02 — 10 March 2009

5 of 17

NXP Semiconductors

SSL152x

SMPS ICs for low power systems

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 opto coupler 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

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

from the line is stopped for high efficiency operation.

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

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

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).