SSL1750_1

© NXP B.V. 2008. All rights reserved.

Product data sheet

Rev. 01 — 15 September 2008

13 of 29

NXP Semiconductors

SSL1750

SMPS control IC for LED drivers

7.2.11 Driver (pin PFCDRIVER)

The driver circuit to the gate of the power MOSFET has a current sourcing capability of

typically 500 mA and a current sink capability of typically 1.2 A. This permits fast turn-on

and turn-off of the power MOSFET for efficient operation.

7.3 Flyback controller

The SSL1750 includes a controller for a flyback converter. The flyback converter operates

in quasi-resonant or discontinuous conduction mode with valley switching. The auxiliary

winding of the flyback transformer provides demagnetization detection and powers the IC

after start-up.

7.3.1 Multimode operation

The SSL1750 flyback controller can operate in various modes, see Figure 8.

At high output power the converter switches to quasi-resonant mode. The next converter

stroke is started after demagnetization of the transformer current. In quasi-resonant mode

switching losses are minimized as the converter only switches on when the voltage across

the external MOSFET is at its minimum (valley switching, see also Section 7.3.2).

To prevent high frequency operation at lower loads, the quasi-resonant operation changes

to discontinuous mode operation with valley skipping in which the switching frequency is

on when the voltage across the MOSFET is at its minimum.

At very low power and standby levels the frequency is controlled down by a Voltage

Controlled Oscillator (VCO). The minimum frequency can be reduced to zero. During

frequency reduction mode, the primary peak current is kept at a minimal level of Ipkmax/4

to maintain a high efficiency. (Ipkmax is the maximum primary peak current set by the

sense resistor and the maximum sense voltage.) As the primary peak current is low in

frequency reduction mode operation (Ipk = Ipkmax/4), no audible noise is noticeable at

switching frequencies in the audible range. Valley switching is also active in this mode.

Fig 8.

Multimode operation flyback

discontinuous

with valley

switching

quasi resonant

PFC burst mode

frequency

reduction

output power

switching frequency

014aaa025

PFC on