Short-circuit and overload protection VIPer53 - E
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10 Short-circuit and overload protection
A V
COMPovl
threshold of about 4.35V has been implemented on the COMP pin. When
V
COMP
goes above this level, the capacitor connected on the TOVL pin begins to charge.
When reaching typically V
OVLth
(4V), the internal MOSFET driver is disabled and the device
stops switching. This state is latched because of to the regulation loop which maintains the
COMP pin voltage above the V
COMPovl
threshold. Since the V
DD
pin does not receive any
more energy from the auxiliary winding, its voltage drops down until it reaches V
DDoff
and
the device is reset, recharging the V
DD
capacitor for a new restart cycle. Note: If VCOMP
drops below the V
COMPovl
threshold for any reason during the VDD drop, the device
resumes switching immediately.
The device enters an endless restart sequence if the overload or short circuit condition is
maintained. The restart duty cycle D
RST
is defined as the time ratio for which the device tries
to restart, thus delivering its full power capability to the output. In order to keep the whole
converter in a safe state during this event, D
RST
must be kept as low as possible, without
compromising the real start-up of the converter. A typical value of about 10% is generally
sufficient. For this purpose, both V
DD
and TOVL capacitors can be used to satisfy the
following conditions:
Equation 6
Equation 7
Refer to the previous start-up section for the definition of tss, and C
VDD
must also be
checked against the limit given in this section. The maximum value of the two calculus will
be adopted.
All this behavior can be observed on Figure 8 on page 10. In Figure 10 on page 11 the value
of the drain current Id for V
COMP
= V
COMPovl
is shown. The corresponding parameter I
Dmax
is the drain current to take into account for design purposes. Since I
Dmax
represents the
maximum value for which the overload protection is not triggered, it defines the power
capability of the power supply.
C
OVL
12.5 10
6–
tss⋅⋅>
C
VDD
810
4
1
D
RST
-------------- 1–
⎝⎠
⎛⎞
C
OVL
I
DDch2
⋅
V
DDhyst
---------------------------------------
⋅⋅ ⋅>