AN2067 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'AN2067' PDF : 25 Pages View PDF
White LED power supply
AN2067
2.2.5
Voltage limitation
The output voltage limitation is built by R106, R105, R108, C114 and the corresponding op-
amp of the TSM101.
R106, R105 create a bridge divider of the output voltage, in order to compare the fraction
with the reference voltage of the TSM101. The voltage limit is set by the below formula:
Vout
=
Vref
⋅
R-----1----0--R-5----1-+--0---R5----1----0---6--
If the voltage limit is reached, the cable AND of the TSM101 allows the voltage limitation
circuit to force the whole loop and reduce the output power. The stability of the voltage
limitation loop is given by R108 and C114.
Against electrical shocks, the limit is set to 40 V, as advised by safety standards.
2.3
No-load operation
The design of the VIPer53-E refers to burst mode in case of low load output. The application
designer doesn’t enter hiccup mode (or bad burst mode) when no-load is connected.
Resistors R106 and R105 have been designed in the application to have the VIPer53-E in
burst mode when no-load is connected. The current is drawn into R106 and R105,
dissipating the power transferred to the secondary side of the transformer during the burst
mode and using the minimum turn-on time (see Figure 7, Figure 9 and Section 5.7,
Figure 17 for measurements).
2.4
Short-circuit operation
Due to current generator structure, in case of short-circuit any malfunction or damage can
occur. The current is limited and the output power is low in that condition (refer to
Section 5.7 and Figure 18 for measurements).
2.5
Low voltage load
In case of low voltage load (less than 4 V on output), some instability problems may occur,
(inductive or capacitive load).
Since poles and zeros change a lot, the whole transfer function is instable.
This instability may be associated to audible noise due to some low frequency oscillations.
The user must avoid loads with a voltage below than 4 V, in order to ensure a correct
operation of the power supply.
10/25
DocID10973 Rev 2
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