EVLB001 Datasheet, PDF (16/35 Pages) ATMEL Corporation – Automatic microcontroller dimmable ballast

Other features of the IXD611 driver include:
Wide supply voltage operation 10-35V
Matched propagation delay for both drivers
Undervoltage lockout protection
Latch up protected over entire operating range
+/- 50V/ns dV/dt immunity
Device Design & Application
5.4 IXYS IXI859
The IXI859 charge pump regulator integrates three primary functions central to the PFC
Charge Pump
stage of the ballast demonstrator. First it includes a linear regulated supply voltage out-
Regulator
put, and in this application the linear regulator provides 3.3V to run the microcontroller.
The second function is a gate drive buffer that switches an external power MOSFET
used to boost the PFC voltage to 400V. Once the microcontroller is booted up and run-
ning, it generates the input signal to drive the PFC MOSFET through the IXI859 gate
drive buffer. Finally, the third function provides two point regulated supply voltage for
operating external devices. As a safety feature, the IXI859 includes an internal Vcc
clamp to prevent damage to itself due to over-voltage conditions.
In general applications at start-up, an R-C combination is employed at the Vcc supply
pin that ramps up a trickle voltage to the Vcc pin from a high voltage offline source. The
value of R is large to protect the internal zener diode clamp and as a result, cannot sup-
ply enough current to power the microcontroller on it’s own. C provides energy to boot
the microcontroller. At a certain voltage level during the ramp up, the Under Voltage
Lock Out point is reached and the IXI859 enables itself. The internal voltage regulator
that supplies the microcontroller is also activated during this time. However, given the
trickle charge nature of the Vcc input voltage, the microcontroller must boot itself up and
enable PFC operation to provide charge pump power to itself. This means that the R-C
combination must be sized carefully so that the voltage present at the Vcc pin does not
collapse too quickly under load and causes the UVLO circuitry to disable device opera-
tion before the microcontroller can take over the charge pump operation. Also note that
there is an internal comparator that only releases charge pump operation when the Vcc
voltage drop below 12.85V. The charge pump is released and Vcc voltage is pumped up
to 13.15V at which time the internal comparator disables the charge pump. This results
in a tightly regulated charge pump voltage.
One problem with the R-C combination described above is that when a universal range
is used at the Vcc pin, 90-265VAC, R must dissipate nine times the power, current
squared function for power in R, over a three-fold increase of voltage from 90V at the
low end to 265V on the high end. As an alternative and as used in the ballast demon-
strator, the Vcc pin is fed voltage by way of a constant current source as previously
described in Section 5.2. This circuit brings several advantages over the regular R-C
usage. First we can reduce power consumed previously by R and replace it with a circuit
that can provide power at startup. It can also provide sufficient power to run the micro-
controller unlike the R-C combination. This would be an advantage in the case that a
standby mode is desired. Overall power consumption can be reduced by allowing the
microcontroller to enter a low power mode and shut down PFC operation without having
to reboot the microcontroller. Since the R-C combination cannot provide enough power
to sustain microcontroller operation, the microcontroller must stay active running the
PFC section to power itself.
ATAVRFBKIT / EVLB001 User Guide
5-14
7597B–AVR–10/07