Datasheet

VIPER37 Operation description
Doc ID 022218 Rev 1 31/35
In order to enable the brownout function, the BR pin voltage must be higher than the
maximum V
DIS
threshold (150 mV), see Tabl e 8 .
8.13 2
nd
level overcurrent protection and hiccup mode
The device is protected against short-circuit of the secondary rectifier, short-circuit on the
secondary winding, or a hard-saturation of the flyback transformer. Such an anomalous
condition is invoked when the drain current exceeds the threshold I
DMAX
(see Tabl e 8 ).
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 is 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 to the V
DD
undervoltage threshold (V
DDoff
), which clears the latch.
The startup HV current generator is still off, until the V
DD
voltage goes below its restart
voltage, V
DD(RESTART)
. After this condition the V
DD
capacitor is charged again by a 600 µA
current, and the converter switching restarts if the V
DDon
occurs. If the fault condition is not
removed the device enters auto-restart mode. This behavior results in a low-frequency
intermittent operation (hiccup-mode operation), with very low stress on the power circuit.
See the timing diagram of
Figure 34 .
Figure 34. Timing diagram: hiccup-mode OCP
V
DRAIN
V
DD
I
DRAIN
V
DDon
time
V
DDof f
V
DD(RESTART)
time
time
Hiccup-mode
Secondary diode
short circuit
Normal operation
I
DMAX