L6566B datasheet(19/51 Pages) STMICROELECTRONICS

19 / 51 page

L6566B

Application information

19/51

With reference to the timing diagram of Figure 6, when power is first applied to the converter

the voltage on the bulk capacitor (Vin) builds up and, at about 80V, the HV generator is

enabled to operate (HV_EN is pulled high) so that it draws about 1 mA. This current, minus

the device’s consumption, charges the bypass capacitor connected from pin Vcc (5) to

ground and makes its voltage rise almost linearly.

Figure 6.

Timing diagram: normal power-up and power-down sequences

As the Vcc voltage reaches the turn-on threshold (14V typ.) the device starts operating and

the HV generator is cut off by the Vcc_OK signal asserted high. The device 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 15 M

Ω 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 external dropping resistors.

At converter power-down the system will lose regulation as soon as the input voltage is so

low that either peak current or maximum duty cycle limitation is tripped. Vcc will then drop

and stop IC activity as it falls below the UVLO threshold (10V typ.). The Vcc_OK signal is

de-asserted as the Vcc voltage goes below a threshold VCCrest located at about 5V. The HV

generator can now restart. However, if Vin < Vinstart, as illustrated in Figure 6, HV_EN is de-

asserted too and the HV generator is disabled. This prevents converter’s restart attempts

and ensures monotonic output voltage decay at power-down in systems where brownout

protection (see the relevant section) is not used.

The low restart threshold VCCrest ensures that, during short circuits, the restart attempts of

the device will have a very low repetition rate, as shown in the timing diagram of Figure 7 on

page 20

, and that the converter will work safely with extremely low power throughput.

Vcc

(pin 5)

(pin 4)

HV_EN

VccON

VccOFF

Vccrestart

Vin

VHVstart

Icharge

0.85 mA

Vcc_OK

Power-on

Power-off

Normal

operation

regulation is lost here

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