TEA1751T_LT_2
?NXP B.V. 2009. All rights reserved.
Product data sheet
Rev. 02 23 December 2009
9 of 29
NXP Semiconductors
TEA1751T; TEA1751LT
GreenChip III SMPS control IC
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 (VINSENSE pin)
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 TEA1751(L)T contains a
correction circuit. Via the VINSENSE pin 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 the PFCCOMP pin.
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
.
The charging current I
start(soft)PFC
flows 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 5
.
?SPAN class="pst TEA1751T-N1-518_2409373_9">softstart 3 R
SS1
C
SS1
?/DIV>
?/DIV>
=
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