tea1753lt NXP Semiconductors, tea1753lt Datasheet - Page 18

• Manufacturer Part Number: tea1753lt
• Description: Tea1753lt Greenchip Iii Smps Control Ic
• Manufacturer: NXP Semiconductors

NXP Semiconductors
TEA1753LT
Objective data sheet
7.3.7 Maximum on-time
7.3.8 Overvoltage protection (FBAUX pin)
The charging current I
approximately 0.63 V. If the voltage on pin FBSENSE exceeds 0.63 V, the soft start
current source starts limiting the current. After the flyback converter has started, the soft
start current source is switched off.
The flyback controller limits the ‘on-time’ of the external MOSFET to 40 ms. When the
‘on-time’ is longer than 40 ms, the IC stops switching and enters the safe restart mode.
An output overvoltage protection is implemented in the GreenChip III series. In the
TEA1753LT this works 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, after 8 cycles of subsequent
OVP events are detected, the IC assumes a true OVP and the OVP circuit switches the
power MOSFET off. As the protection is latched, the converter only restarts after the
internal latch is reset. In a typical application the mains should be interrupted to reset the
internal latch.
The output voltage V
demagnetization resistor, R
where N
transformer. Current I
The value of R
accurate OVP detection possible.
Fig 14. Soft start-up of flyback.
V
o OVP

s

is the number of secondary turns and N
=
---------- - I
N
N
FBAUX
aux
All information provided in this document is subject to legal disclaimers.
s

ovp FBAUX
o(OVP)
can be adjusted to the turns ratio of the transformer, thus making an
ovp(FBAUX)
R SENSE2
start(soft)(FB)
S2

Rev. 01 — 4 March 2011
R SS2
C SS2
at which the OVP function trips, can be set by the
FBAUX


10
is internally trimmed.
FBSENSE
R
:
flows as long as the voltage on pin FBSENSE is below
FBAUX
3 μA
I
start(soft)fb
+
V
ocp level
clamp FBAUX
≤ 60 μA

aux
SOFT START
is the number of auxiliary turns of the
CONTROL

OCP

GreenChip III SMPS control IC
014aaa748
TEA1753LT
© NXP B.V. 2011. All rights reserved.
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