VIPER20A STMicroelectronics, VIPER20A Datasheet - Page 13

• Manufacturer Part Number: VIPER20A
• Description: Switching Converters, Regulators & Controllers 700V 0.5A SMPS
• Manufacturer: STMicroelectronics

Specifications of VIPER20A

• Mounting Style: Through Hole
• Package / Case: Pentawatt HV
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Figure 15: Behavior of the high voltage current source at start-up
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
winding of the transformer, as shown on figure 15.
In case of abnormal condition where the auxiliary
winding is unable to provide the low voltage supply
current to the V
output of the converter), the external capacitor
discharges itself down to the low threshold voltage
V
to the inactive state where the internal circuits are
in standby mode and the start up current source is
activated. The converter enters an endless start
up cycle, with a start-up duty cycle defined by the
ratio of charging current towards discharging when
the VIPer20/20A tries to start. This ratio is fixed by
design from 2 to 15, which gives a 12% start up
duty cycle while the power dissipation at start up is
approximately 0.6 W, for a 230 Vrms input voltage.
This low value of start-up duty cycle prevents the
stress of the output rectifiers and of the
transformer when in short circuit.
The external capacitor C
be sized according to the time needed by the
converter to start up, when the device starts
switching. This time t
parameters, among which transformer design,
output
compensation network implemented on the COMP
pin. The following formula can be used for defining
the minimum capacitor needed:
DDoff
VDDoff
VDDon
of the UVLO logic, and the device gets back
capacitors,
VDD
C VDD
DD
DD
pin (i.e. short circuit on the
soft
pin through the auxiliary
------------------------- -
V
I
VDD
SS
DD
DDhyst
t
start
depends on many
on the V
SS
t
Auxiliary primary
feature
winding
DD
pin must
15 mA
C
2 mA
VDD
and
VDD
VIPer20
Start up duty cycle ~ 12%
where:
I
when switching. Refer to specified I
values.
t
device begins to switch. Worst case is generally at
full load.
V
logic. Refer to the minimum specified value.
Soft start feature can be implemented on the
COMP pin through a simple capacitor which will
also be used as the compensation network. In this
case, the regulation loop bandwidth is rather low,
because of the large value of this capacitor. In
case of a large regulation loop bandwidth is
mandatory, the schematics in figure 16 can be
used. It mixes a high performance compensation
network together with a separate high value soft
start capacitor. Both soft start time and regulation
loop bandwidth can be adjusted separately.
If the device is intentionally shut down by putting
the COMP pin to ground, the device is also
performing start-up cycles, and the V
oscillating between V
This voltage can be used for supplying external
functions, provided that their consumption doesn’t
exceed 0.5mA. Figure 17 shows a typical
application of this function, with a latched shut
down. Once the "Shutdown" signal has been
activated, the device remains in the off state until
the input voltage is removed.
DD
SS
1 mA
UNDERVOLTAGE
LOCK OUT LOGIC
DDhyst
Ref.
is the start up time of the converter when the
is the consumption current on the V
VIPer20/SP/DIP - VIPer20A/ASP/ADIP
15 mA
is the voltage hysteresis of the UVLO
3 mA
DDon
and V
DRAIN
SOURCE
DDoff
DD1
DD
.
FC00101A
voltage is
and I
DD
13/25
DD2
pin