White LED power supply

AN2067

10/25

DocID10973 Rev 2

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:

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.

V

out

V

ref

R105

R106

+

R105

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