L6563H
Application information
6.8
High-voltage start-up generator
Figure 51 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 MΩ
resistor, with a temperature-compensated current generator connected to its source.
Figure 51. High-voltage start-up generator: internal schematic
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The HV generator is physically located on a separate chip, made with BCD off-line
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 52, 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's consumption, charges the bypass capacitor connected from pin
Vcc (16) to ground and makes its voltage rise almost linearly.
Doc ID 16047 Rev 2
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STMICROELECTRONICS [ ST ]
