Datasheet

VIPER27 Operation descriptions
Doc ID 15133 Rev 5 15/31
7.3 Power-up and soft-start up
If the input voltage rises up till the device start threshold, V
DRAIN_START
, the V
DD
voltage
begins to grow due to the I
DDch
current (see Table 7 on page 6 ) coming from the internal
high voltage start up circuit. If the V
DD
voltage reaches V
DDon
threshold (see Tab le 7 o n
page 6
) the power MOSFET starts switching and the HV current generator is turned OFF.
See
Figure 23 on page 16 .
The IC is powered by the energy stored in the capacitor on the VDD pin, C
VDD
, until when
the self-supply circuit (typically an auxiliary winding of the transformer and a steering diode)
develops a voltage high enough to sustain the operation.
C
VDD
capacitor must be sized enough to avoid fast discharge and keep the needed voltage
value higher than 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 applicator according to the output stage configurations (transformer, output
capacitances, etc.).
During the converter start up 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 start up converter or after a fault.
Figure 22. I
DD
current during start-up and burst mode
C
VDD
I
DDch
t
SSaux
×
V
DDon
V
DDoff
–
----------------------------------------=
BURST MODE
NORMAL MODE
START- UP
NORMAL M ODE
I
DDch
(-3 mA)
I
DD1
I
DD0
I
DD
V
FBbm
V
FB
V
DRAIN
V
FBbmhys
V
FBlin
V
FBolp
V
DD
V
DDoff
V
DDon
t
t
t
t