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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
limited to f sw(PFC)max . If the frequency for quasi-resonant operation is above the
f sw(PFC)max limit, the system switches over to discontinuous conduction mode. Also here,
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 in?uence, 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
current, I DM , is increased slowly by the soft-start function. This can be achieved by
inserting R SS1 and C SS1 between pin PFCSENSE and current sense resistor R SENSE1 .
An internal current source charges the capacitor to V PFCSENSE = I start(soft)PFC × R SS1 . The
voltage is limited to V start(soft)PFC .
The start level and the time constant of the increasing primary current level can be
adjusted externally by changing the values of R SS1 and C SS1 . See Equation 1 .
τ SoftStart = 3 × R SS1 × C SS1
(1)
The charging current I start(soft)PFC ?ows as long as the voltage on pin PFCSENSE is
below 0.5 V (typ). If the voltage on pin PFCSENSE exceeds 0.5 V, the soft-start current
source starts limiting current I start(soft)PFC . As soon as the PFC starts switching, the
I start(soft)PFC current source is switched off, see Figure 6 .
SSL1750_1
? NXP B.V. 2008. All rights reserved.
Product data sheet
Rev. 01 — 15 September 2008
10 of 29
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