L6591 datasheet(13/32 Pages) STMICROELECTRONICS

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L6591

Application information

13/32

5.1

High-voltage start-up generator

Figure 5 shows the internal schematic of the high-voltage start-up generator (HV generator).

It is made up of a high-voltage N-channel FET, whose gate is biased by a 15 MW resistor,

with a temperature-compensated current generator connected to its source.

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, as it reaches about 80 V, the HV

generator is enabled to operate (HV_EN is pulled high) and draws about 1 mA. This current,

diminished by the IC consumption, charges the bypass capacitor connected between pin

Vcc (9) and ground and makes its voltage rise almost linearly.

As the Vcc voltage reaches the start-up threshold (13.5 V typ.) the IC 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 develops a voltage high

enough to sustain the operation. The residual consumption of this circuit is just the one on

the 15 MW 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 5.

High-voltage start-up generator: internal schematic

L6591

15 M

GN D

HVSTART

Vcc

Icharge

Vcc_OK

I HV

CONTROL

HV_EN

Ω

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