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isdischarged to V
UVLO
.The output voltage will decrease depending on the load.
z
t3: Charge ofV
CC
when gate drive is inactive Thethird interval starts when the UVLO is
reached. The internal currentsource charges the V
CC
capacitor(also the soft start
capacitor is recharged). Once the V
CC
capacitor is chargedto the start-up voltage, the
driver is activated and a new burstcycle is started.
Figure:
Protection Events
The SMPS IC 7520 has the following protectionfeatures:
Demagnetization sense
This feature guarantees discontinuous conductionmode operation in every situation. The
oscillator will not starta new primary stroke until the secondary stroke has ended. Thisis to
ensure that FET 7521 will not turn on until the demagnetization oftransformer 5520 is
complete.The function is an additional protectionfeature against:
z
saturationof the transformer,
z
damage of the components during initial start-up,
z
an overload of the output.
The demag(netization) sense is realized by an internalcircuit that guards the voltage (Vdemag)
at pin 4 that is connectedto V
CC
winding byresistor R
1
(R3522). The Figure below shows the
circuit and the idealized waveformsacross this winding.
Pa
g
e 22 of 32SPMS
7/8/2004