ssl1750 NXP Semiconductors, ssl1750 Datasheet - Page 18

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

NXP Semiconductors
SSL1750_1
Product data sheet
7.3.7 Maximum on-time
7.3.8 Overvoltage protection (pin FBAUX)
V
The flyback controller limits the ‘on-time’ of the external MOSFET to 25 s (typ). When the
‘on-time’ is longer than 25 s, the IC stops switching and enters the safe restart mode.
An output overvoltage protection is implemented in this IC. This works for the SSL1750 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 ElectroStatic Discharge (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 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 13. Soft start-up of flyback
o ovp
=
s
is the number of secondary turns and N
----------- I
N
N
aux
s
FBAUX
ovp FBAUX
Rev. 01 — 15 September 2008
ovp(FBAUX)
can be adjusted to the turns ratio of the transformer, thus making an
ovp(FBAUX)
R SENSE2
S2
R SS2
C SS2
FBAUX
R
FBAUX
at which the OVP function trips, can be set by the
is internally trimmed.
. See
10
FBSENSE
I start(soft)fb
+
Equation
V
clamp FBAUX
60 A
+
0.5 V
5.
aux
SOFT START
CONTROL
is the number of auxiliary turns of the
OCP
SMPS control IC for LED drivers
014aaa020
SSL1750
© NXP B.V. 2008. All rights reserved.
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