Datasheet

Application information L6591
22/41 Doc ID 14821 Rev 6
6.1 High-voltage startup generator
Figure 40 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
MW resistor, with a temperature-compensated current generator connected to its source.
With reference to the timing diagram of
Figure 41 , 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 startup 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 startup circuit made with an external dropping
resistor.
Figure 40. High-voltage startup generator: internal schematic
AM13256v1
L6591
15 M &
GN D
HVSTART
Vcc9
11
16
I
charge
Vcc_OK
I HV
CONT ROL
HV_EN
Ω