Functional description VN5MB02-E
14/20 DocID023796 Rev 5
3 Functional description
3.1 Normal operation
When a nominal load is connected to the OUT pin, the device oscillates by charging the
C
EXT
capacitor up to the threshold voltage V
ch
quickly, and then slowly discharging C
EXT
to
the threshold voltage V
cl
by a constant current I
CEXT
.
The self oscillating frequency of the device is determined by the relation:
n is the internal digital counter (equal to 1066).
3.2 Low load condition
If the load current is lower than I
df
, the device will detect the low load at the end of the ON
phase, and will double the oscillating frequency.
3.3 Current protection and thermal limitation
The internal current shutdown latches the VN5MB02-E when a heavy overload occurs and,
if the overload even is removed, the device may be turned on at the following on time.
The thermal limitation reduces the stress on the device if the junction temperature rises (for
instance when a soft over current event, not triggering the current latch protection,
happens).
If the overload condition lasts more than a time t
fault
, the VN5MB02-E is latched; when the
overload condition is not anymore present, device can be turned on at the following on time.
f
OSC
I
freq
4nC
freq
× V
ch
V
cl
–()××
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