Ballast Demonstrator User Guide
-9
7629A–AVR–04/06
• P3.0/W0M0 Half Bridge low side drive
The Temperature monitor is a thermistor with a nominal 10K resistance at 25°C and
1.74K resistance at 80°C. It is mounted on the circuit board and so monitors ambient
temperature in the lamp housing.
Additional dedicated pins allow in-circuit programming of the flash memory using header
J2. Other pins provide connections for the oscillator and voltage reference components.
3.1.7
IXYS IXI859 Charge
Pump Regulator
The IXI859 charge pump regulator integrates three primary functions central to the PFC
stage of the ballast demonstrator. First it includes a linear regulated supply voltage out-
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 380V. 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, can't supply
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. There are a
couple of problems associated with this method. Namely, under normal operation as
previously mentioned, the internal zener diode clamps the input Vcc pin voltage and R
must dissipate power as long as the zener diode is clamped. Assuming that a rectified
sine wave is supplied at the Vcc means that the internal zener will be clamped and R will
be dissipating power as long as the input voltage is greater than the zener voltage.
Another problem is that when a universal range is used at the Vcc pin, 90-265V, 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 demonstrator, the Vcc pin is fed voltage by
way of a constant current source. 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 and once the microcontroller is running, shut off
current into the Vcc pin. The constant current source also has the ability to provide suffi-
cient power to run the microcontroller 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.
3.1.8
IXYS IXTP02N50D
Depletion Mode
MOSFET used as a
current source
The IXYS IXTP02N50D depletion mode MOSFET is used in this circuit to provide power
and a start-up voltage to the Vcc pin of the IXI859 charge pump regulator. The
IXTP02N50D acts as a current source and self regulates as the source voltage rises
above the 15V zener voltage and causes the gate to become more negative than the
