TEA1751T_LT_2
?NXP B.V. 2009. All rights reserved.
Product data sheet
Rev. 02 23 December 2009
17 of 29
NXP Semiconductors
TEA1751T; TEA1751LT
GreenChip III SMPS control IC
7.3.6   Soft start-up (pin FBSENSE)
To prevent audible transformer noise during start-up, the transformer peak current, I
DM
is
slowly increased by the soft start function. This can be achieved by inserting a resistor and
a capacitor between pin 10, FBSENSE, and the current sense resistor.
An internal current source charges the capacitor to V = I
start(soft)fb
?R
SS2
, with a maximum
of approximately 0.5 V.
The start level and the time constant of the increasing primary current level can be
adjusted externally by changing the values of R
SS2
and C
SS2
.
The soft start current I
start(soft)fb
is switched on as soon as V
CC
reaches V
startup
. When the
voltage on pin FBSENSE has reached 0.5 V, the flyback converter starts switching.
The charging current I
start(soft)(PFC)
flows as long as the voltage on pin FBSENSE is below
approximately 0.5 V. If the voltage on pin FBSENSE exceeds 0.5 V, the soft start current
source starts limiting the current. After the flyback converter has started, the soft start
current source is switched off.
7.3.7   Maximum on-time
The flyback controller limits the on-time of the external MOSFET to 40 約 (typical). When
the on-time is longer than 40 約, the IC stops switching and enters the safe restart mode.
7.3.8   Overvoltage protection (FBAUX pin)
An output overvoltage protection is implemented in the GreenChip III series. This works
for the TEA1751(L)T by sensing the auxiliary voltage via the current flowing into
pin FBAUX during the secondary stroke. The auxiliary winding voltage is a well-defined
replica of the output voltage. Voltage spikes are averaged by an internal filter.
If the output voltage exceeds the OVP trip level, an internal counter starts counting
subsequent OVP events. The counter has been added to prevent incorrect OVP detection
which might occur during ESD or lightning events. If the output voltage exceeds the OVP
trip level a few times and not again in a subsequent cycle, the internal counter counts
down at twice the speed it uses when counting up. However, when typically eight cycles of
Fig 14.   Soft start-up of flyback.
?SPAN class="pst TEA1751T-N1-518_2409373_10">softstart    3   R
SS2
C
SS2
?/DIV>
?/DIV>
=
014aaa02
SOFT START
CONTROL
OCP
+
10
FBSENSE
0.5 V
I
start(soft)fb d 60 糀
S2
R
SS2
C
SS2
R
SENSE2
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