Datasheet

Operation descriptions VIPER17
26/32 DocID14419 Rev 9
7.13 2
nd
level overcurrent protection and hiccup mode
The VIPER17 is protected against short circuit of the secondary rectifier, short circuit on the
secondary winding or a hard-saturation of fly-back transformer. Such as anomalous
condition is invoked when the drain current exceed the threshold I
DMAX
(see Table 8 on
page 7).
To distinguish a real malfunction from a disturbance (e.g. induced during ESD tests) a
“warning state” is entered after the first signal trip. If in the subsequent switching cycle the
signal is not tripped, a temporary disturbance is assumed and the protection logic will be
reset in its idle state; otherwise if the I
DMAX
threshold is exceeded for two consecutive
switching cycles a real malfunction is assumed and the power MOSFET is turned OFF.
The shutdown condition is latched as long as the device is supplied. While it is disabled, no
energy is transferred from the auxiliary winding; hence the voltage on the V
DD
capacitor
decays till the V
DD
under voltage threshold (V
DDoff
), which clears the latch.
The start up HV current generator is still off, until V
DD
voltage goes below its restart voltage,
V
DD(RESTART)
. After this condition the V
DD
capacitor is charged again by 600 μA current,
and the converter switching restarts if the V
DDon
occurs. If the fault condition is not removed
the device enters in auto-restart mode. This behavioral results in a low-frequency
intermittent operation (Hiccup-mode operation), with very low stress on the power circuit.
See the timing diagram of Figure 32.
Figure 32. Hiccup-mode OCP: timing diagram
Vcc
V
DRAIN
I
DRAIN
Secondary diode is shorted here
t
t
t
DMAX
on
off
(RESTART)
Secondary diode is shorted here
t
t
t
I
V
DD
V
DD
V
DD
V
DD