ssl1750 NXP Semiconductors, ssl1750 Datasheet - Page 10

• Manufacturer Part Number: ssl1750
• Description: Smps Control Ic For Led Drivers
• Manufacturer: NXP Semiconductors

NXP Semiconductors
SSL1750_1
Product data sheet
7.2.3 Frequency limitation
7.2.4 Mains voltage compensation (pin VINSENSE)
7.2.5 Soft start-up (pin PFCSENSE)
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.
To optimize the transformer and minimize switching losses, the switching frequency is
limited to f
f
the PFC MOSFET is only switched on at a minimum voltage across the switch (valley
switching).
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.
To prevent audible transformer noise at start-up or during hiccup, the transformer peak
current, I
inserting R
An internal current source charges the capacitor to V
voltage is limited to V
The start level and the time constant of the increasing primary current level can be
adjusted externally by changing the values of R
The charging current I
below 0.5 V (typ). If the voltage on pin PFCSENSE exceeds 0.5 V, the soft-start current
source starts limiting current I
I
sw(PFC)max
start(soft)PFC
SoftStart
=
DM
sw(PFC)max
limit, the system switches over to discontinuous conduction mode. Also here,
SS1
3
, is increased slowly by the soft-start function. This can be achieved by
current source is switched off, see
R
and C
SS1
. If the frequency for quasi-resonant operation is above the
SS1
Rev. 01 — 15 September 2008
C
start(soft)PFC
start(soft)PFC
SS1
between pin PFCSENSE and current sense resistor R
start(soft)PFC
.
flows as long as the voltage on pin PFCSENSE is
. As soon as the PFC starts switching, the
Figure
SS1
and C
PFCSENSE
6.
SMPS control IC for LED drivers
SS1
. See
= I
start(soft)PFC
Equation
SSL1750
© NXP B.V. 2008. All rights reserved.
1.
R
SENSE1
SS1
10 of 29
. The
.
(1)