SSL1750_1

© NXP B.V. 2008. All rights reserved.

Product data sheet

Rev. 01 — 15 September 2008

9 of 29

NXP Semiconductors

SSL1750

SMPS control IC for LED drivers

7.1.4 Fast latch reset

In a typical application, the mains can be interrupted briefly to reset the latched protection.

reset.

level. When the latched protection is set, the clamping circuit of the VINSENSE circuit is

disabled (see also Section 7.2.8). As soon as the VINSENSE voltage drops below

750 mV (typ) and is then raised to 870 mV (typ), the latched protection is reset.

pin HV.

7.1.5 Overtemperature protection (OTP)

An accurate internal temperature protection is provided in the circuit. When the junction

temperature exceeds the thermal shutdown temperature, the IC only stops switching. As

on pin HV or by the fast latch reset function, see Section 7.1.4.

7.2 Power factor correction circuit

The power factor correction circuit operates in quasi-resonant or discontinuous conduction

mode with valley switching. The next primary stroke is only started when the previous

secondary stroke has ended and the voltage across the PFC MOSFET has reached a

minimum value. The voltage on pin PFCAUX is used to detect transformer

demagnetization and the minimum voltage across the external PFC MOSFET switch.

reduction of a typical application is well within the class-D requirements.

7.2.2 Valley switching and demagnetization (pin PFCAUX)

The PFC MOSFET is switched on after the transformer is demagnetized. Internal circuitry

connected to pin PFCAUX detects the end of the secondary stroke. It also detects the

voltage across the PFC MOSFET. The next stroke is started if the voltage across the PFC

MOSFET is at its minimum in order to reduce switching losses and ElectroMagnetic

Interference (EMI) (valley switching).

If no demagnetization signal is detected on pin PFCAUX, the controller generates a Zero

Current Signal (ZCS), 50

µs (typ) after the last PFC gate signal.

If no valley signal is detected on pin PFCAUX, the controller generates a valley signal

4

µs (typ) after demagnetization was detected.

To protect the internal circuitry, for example during lightning events, it is advisable to add a

5k

Ω series resistor to this pin. To prevent incorrect switching due to external disturbance,

the resistor should be placed close to the IC on the printed-circuit board.