Datasheet

Operation description VIPER37
20/35 Doc ID 022218 Rev 1
8.3 Power-up and soft-start
If the input voltage rises up to the device start threshold V
DRAIN_START
, the V
DD
voltage
begins to grow due to the I
DDch
current (see Tabl e 7 ) coming from the internal high voltage
startup circuit. If the V
DD
voltage reaches the V
DDon
threshold (see Ta bl e 7 ), the Power
MOSFET starts switching and the HV current generator is turned off (see
Figure 25 ).
The IC is powered by the energy stored in the capacitor on the V
DD
pin, C
VDD
, until the self-
supply circuit (typically an auxiliary winding of the transformer and a steering diode)
develops a voltage high enough to sustain the operation.
The C
VDD
capacitor must be sized correctly in order to avoid fast discharge and keep the
needed voltage value higher than the V
DDoff
threshold. In fact, a too low capacitance value
could terminate the switching operation before the controller receives any energy from the
auxiliary winding.
The following formula can be used for the V
DD
capacitor calculation:
Equation 1
The t
SSaux
is the time needed for the steady-state of the auxiliary voltage. This time is
estimated by the applicator according to the output stage configurations (transformer, output
capacitances, etc.).
During the converter startup time, the drain current limitation is progressively increased to
the maximum value. In this way the stress on the secondary diode is considerably reduced.
It also helps to prevent transformer saturation. The soft-start time lasts 8.5 ms and the
feature is implemented for every attempt of the startup converter or after a fault.
C
VDD
I
DDch
t
SSaux
×
V
DDon
V
DDoff
–
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