13/23

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

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 15M

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

OUT

HV_EN

Vcc ON

Vcc OFF

Vcc restart

t

t

t

t

Vin

V HVstart

IHV

1 mA

t

t

Vcc_OK

Power-up

Power-down

Normal

operation

regulation is lost here

Vcc

OUT

HV_EN

Vcc ON

Vcc OFF

Vcc restart

t

t

t

t

Vin

V HVstart

IHV

1 mA

t

t

Vcc_OK

Power-up

Power-down

Normal

operation

regulation is lost here

Vcc

OUT

Vcc_OK

1 mA

Short circuit occurs here

t

t

t

t

Vcc

OUT

Vcc_OK

1 mA

Short circuit occurs here

t

t

t

t