VIPER53-E STMicroelectronics, VIPER53-E Datasheet - Page 22

• Manufacturer Part Number: VIPER53-E
• Description: Fixed frequency off line converter
• Manufacturer: STMicroelectronics

High voltage Start-up current source
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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
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
page
The external capacitor C
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
current I
down to I
rise. Device starts switching for V
energy to V
The charging current change at V
maintains a low restart duty cycle. This is especially useful for short circuits and overloads
conditions, as described in the following section.
DD
pin. As soon as the voltage on this pin reaches the high voltage threshold V
DD
17.
DDch1
pin through the auxiliary winding of the transformer, as shown on
DDch2
DD
at about 9 mA. When about V
capacitor after the start-up time tss.
which is about 0.6mA. This lower current leads to a slope change on the V
shows a typical start-up event. V
VDD
on the V
DD
DDoff
equal to V
DD
C
allows a fast complete start-up time t
VDD
pin must be sized according to the time needed by
DDoff
I
-------------------------- -
DD1
V
DDon
DDhyst
is reached, the charging current is reduced
, and the auxiliary winding delivers some
tss
DD
starts from 0V with a charging
Figure 19 on
SDU
, and
VIPer53 - E
DDon
of the
DD