Datasheet

High voltage Start-up current source VIPer53 - E
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9 High voltage Start-up current source
An integrated high voltage current source provides a bias current from the DRAIN pin during
the start-up phase. This current is partially absorbed by internal control circuits in standby
mode with reduced consumption, and also supplies the external capacitor connected to the
V
DD
pin. As soon as the voltage on this pin reaches the high voltage threshold V
DDon
of the
UVLO logic, the device turns into active mode and starts switching. The start-up current
generator is switched off, and the converter should normally provide the needed current on
the V
DD
pin through the auxiliary winding of the transformer, as shown on Figure 19 on
page 17.
The external capacitor C
VDD
on the V
DD
pin must be sized according to the time needed by
the converter to start-up, when the device starts switching. This time tss depends on many
parameters, including transformer design, output capacitors, soft start feature, and
compensation network implemented on the COMP pin and possible secondary feedback
circuit.
The following formula can be used for defining the minimum capacitor needed:
Equation 5
Figure 21 on page 23 shows a typical start-up event. V
DD
starts from 0V with a charging
current I
DDch1
at about 9 mA. When about V
DDoff
is reached, the charging current is reduced
down to I
DDch2
which is about 0.6mA. This lower current leads to a slope change on the V
DD
rise. Device starts switching for V
DD
equal to V
DDon
, and the auxiliary winding delivers some
energy to V
DD
capacitor after the start-up time tss.
The charging current change at V
DDoff
allows a fast complete start-up time t
SDU
, and
maintains a low restart duty cycle. This is especially useful for short circuits and overloads
conditions, as described in the following section.
C
VDD
I
DD1
tss⋅
V
DDhyst
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