L6668 데이터 시트보기 (PDF) - STMicroelectronics
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L6668 Datasheet PDF : 23 Pages
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L6668
With reference to the timing diagram of figure 36, when power is first applied to the converter the voltage
on the bulk capacitor (Vin) builds up and, as it reaches about 80V, the HV generator is enabled to oper-
ate (HV_EN is pulled high) so that it draws about 1 mA. This current, diminished by the IC consumption,
charges the bypass capacitor connected between pin Vcc (5) and ground and makes its voltage rise al-
most linearly.
Figure 36. Timing diagram: normal power-up and power-down sequences
Vin
V HVstart
Vcc
Vcc ON
Vcc OFF
Vcc restart
OUT
regulation is lost here
t
t
t
HV_EN
t
Vcc_OK
IHV
t
1 mA
Power-up
Normal
operation
t
Power-down
As the Vcc voltage reaches the start-up threshold (13.5V typ.) the low-voltage chip starts operating and
the HV generator is cut off by the Vcc_OK signal asserted high. The IC is powered by the energy stored
in the Vcc capacitor until the self-supply circuit (typically an auxiliary winding of the transformer and a
steering diode) develops a voltage high enough to sustain the operation.
The residual consumption of this circuit is just the one on the 15MΩ resistor (≈10 mW at 400 Vdc), typically
50-70 times lower, under the same conditions, as compared to a standard start-up circuit made with an
external dropping resistor.
Figure 37. Timing diagram showing short-circuit behavior
Vcc
VccON
VccOFF
Vcc rest
Short circuit occurs here
OUT
t
Vcc_OK
t
IHV
t
1 mA
t
13/23
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