SSL1750_1

© NXP B.V. 2008. All rights reserved.

Product data sheet

Rev. 01 — 15 September 2008

10 of 29

NXP Semiconductors

SSL1750

SMPS control IC for LED drivers

For applications with high transformer ringing frequencies (after the secondary stroke),

Pin PFCAUX should be connected via a capacitor and a resistor to the auxiliary winding.

A diode must then be placed from the ground connection to pin PFCAUX.

7.2.3 Frequency limitation

To optimize the transformer and minimize switching losses, the switching frequency is

the PFC MOSFET is only switched on at a minimum voltage across the switch (valley

switching).

7.2.4 Mains voltage compensation (pin VINSENSE)

The mathematical equation for the transfer function of a power factor corrector contains

the square of the mains input voltage. In a typical application this results in a low

bandwidth for low mains input voltages, while at high mains input voltages the Mains

Harmonic Reduction (MHR) requirements may be hard to meet.

To compensate for the mains input voltage influence, the SSL1750 contains a correction

circuit. Via pin VINSENSE the average input voltage is measured and the information is

fed to an internal compensation circuit. With this compensation it is possible to keep the

regulation loop bandwidth constant over the full mains input range, yielding a fast transient

response on load steps, while still complying with class-D MHR requirements.

In a typical application, the bandwidth of the regulation loop is set by a resistor and two

capacitors on pin PFCCOMP.

7.2.5 Soft start-up (pin PFCSENSE)

To prevent audible transformer noise at start-up or during hiccup, the transformer peak

The start level and the time constant of the increasing primary current level can be

(1)

below 0.5 V (typ). If the voltage on pin PFCSENSE exceeds 0.5 V, the soft-start current

τ

SoftStart

3R

SS1

×

C

SS1

×

=