AN2934

Pins and their functions

Doc ID 15394 Rev 1

13/27

Both figures show a ride-through from 115 Vac at a12 V/100 mA constant current load.

Figure 4 shows that the bulk voltage (yellow trace) can reach 45 Vdc before a glitch

becomes visible on the output (green trace). With the brownout pin active, we can see a

definite pulse shutdown at ~70 Vdc. The purple trace is the AC line. Note that one

contradicts the other: the higher the difference of input voltage, the longer the hold-up or

ride-through.

4.2

CONT pin

The CONT pin has multiple purposes. One of them is to reduce the output current or power.

A resistor can be set from this pin to ground to reduce the pulse-by-pulse current limit

when lower power is needed and a smaller transformer is used as it prevents saturation of

the transformer. The results of this function can be seen in Figure 6. The function itself can

be activated by changing R3 on the VIPer17HN reference board. A second-level protection,

which latches the device if exceeded, ensures safety in the case of transformer or diode

shorts. If the VIPer17 detects a current pulse of 600 mA, it considers it to be a disturbance;

if it detects this disturbance two consecutive times, it interprets it as a hard short and shuts

down.

Another purpose of the CONT pin is overvoltage monitoring. A voltage over 3 V shuts down

the IC, reducing power consumption, which is useful for overvoltage sensing or if there is an

open component in the feedback loop caused by faulty soldering. This monitoring function

has been tested by paralleling a resistor with R9 to raise the output voltage. The unit went

into a "hiccup" mode at an output voltage of 18 V. The voltage limit can be adjusted as

needed by changing R14.

Figure 4.

165 ms ride-through

Figure 5.

128 ms ride-through

AM03421v1

AM03422v1

a.

Refer to the datasheet VIPER17: Off-line high voltage converters available on www.st.com.