AN4077

High-voltage startup generator

Doc ID 022997 Rev 1

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High-voltage startup generator

Figure 32 shows the internal schematic of the high-voltage startup generator (HV

generator). It is made up of a high-voltage N-channel FET, whose gate is biased by a 15 M

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

Figure 32.

High-voltage startup generator: internal schematic

The HV generator is physically located on a separate chip, made with BCD™ offline

technology able to withstand 700 V, controlled by a low-voltage chip, where all of the control

functions reside.

With reference to the timing diagram of

Figure 33, when power is first applied to the

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

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

current, minus the device consumption, charges the bypass capacitor connected from pin

Figure 33.

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

AM11463v1

15MW

GND

HVS

Vcc

14

12

8

Vcc_OK

CONTROL

HV_EN

AM11464v1

Vcc

(pin 14)

GD

(pin 13)

HV_EN

t

t

t

t

VHV

0.85 mA

t

t

Vcc_OK

Power-on

Power-off

Normal

operation

DC - DC loses regulation here

Input source is removed here

Bulk cap voltage

HV connected to

rectified input voltage

Rectified input voltage

HV connected to bulk cap