HV9911 output current regulation not working?
I hope this is the right place to ask questions about the Microchip (formerly Supertex) HV9911, which I am attempting to use in a boost configuration. (I've successfully used the HV9910C buck LED driver with splendid results, zero complaints.)
The project is for an off-grid solar system, nominal "48v", design range of at least 40-60vDC. I am attempting to drive an off-the-shelf 4-foot 18W LED "tube, which runs at approximately 72vDC, thus the need for a boost converter.
I have gotten PCBs made, and they do function...sort of. While the HV9911 does work in boost fashion, output current regulation does not seem to function. (When testing, the LED tube lights up with 71vDC measured, from 12v input @ 800mA [current limited.])
I used Supertex AN-H55 to "plug in" values into the circuit, though I did make a few substitutions along the way. My schematic is as follows (the PWM controller off the left side, but it is not currently installed, and thus not applicable to this):
According to the datasheet, REF (pin 10) should be ~1.25v, which it is. If I manually adjust the pot to 0.05v (= 333mA LED current), the HV9911 does not oscillate (LED does not light, 1-2mA input current @ 40vDC). Setting it to 0.1v (= 666mA LED current)...still no dice. Twisting it up to 0.238v (=1.5A LED current, way past the project spec), the HV9911 starts, though it hits the power supply's manually specified current limit, resulting in 12v @ 0.8A input, 71v output.
Yes, I'm aware that running a current limit on non-resistive loads will send the voltage to the "threshold" point. To see if it would regulate properly, I tried setting the power supply to 50v @ 0.8A, turned it on, then jumpered PWMD to VDD (required to activate the chip), only to get a "flash" from the LED tube, and a blown LED driver board (43mA no load current, won't start, HV9911 gets hot...though REF is still 1.25v. FET is not shorted, though I can't prove that it is still functional.)
Basically, my plan was to set the HV9911's LED current limit to a very low value, and prove that current regulation was working (BEFORE trying full power). But the HV9911 doesn't seem to start if the IREF (datasheet is FDBK) pin is <0.2v.
I have a suspicion that there is an undocumented "minimum voltage" on IREF (FDBK), as the AN-H55 calculation recommends a 0.6-ohm current sense resistor--which at least to me is a significant power loss. I used a 0.15-ohm resistor simply because that was also the size I needed for MOSFET current feedback (which evidently did not work either on my test).
However, the HV9911 datasheet specifies the following regarding IREF (FDBK):
Note that this equation assumes a current limit at 120%
of the maximum input current. Also, if V CLIM is greater
than 450 mV, the saturation of the internal opamp will
determine the limit on the input current rather than the
CLIM pin. In such a case, the sense resistor R CS
should be reduced until V CLIM reduces below 450 mV.
It is recommended that no capacitor be connected
between CLIM and GND.
(DS20005580A-page 12.) So it can't be higher than 450mV, and doesn't work under 200mV?? Oh, and no capacitor between CLIM and GND? I was actually just trying that, as if I touch the pot (with a meter probe or a screwdriver), the HV9911 immediately goes into fault shutdown.
I will include a schematic and the AN-H55 calculations if I can figure out how to get past the bugs in Microchip's forum. If I write everything, I get the following message upon post submission:
You don't have permission to access "http://www.microchip.com/forums/post.aspx?" on this server.