Hi!
I am part of a solar racing team that has designed a Maximum Power Point Tracker (MPPT) using the SM72442 controller and SM72295 bridge driver. We based our design off of the SM3320-BAT-EV reference design.
Changes from the reference design:
- Changed input and output voltage dividers
- Create voltage rails off of external 12V rather than solar panel
- Modified power electronics for our specs (different voltages/currents)
- (accidental) Moved VOUT voltage sense to the output side of the current sense resistor (note: I tried flywiring it back to the other terminal of the sense resistor with, predictably, no change)
- Completely new microcontroller/non-Solar-Magic chips (these work)
We've gotten it to work in buck mode, but boost mode still doesn't work. We can successfully communicate using I2C to read voltages and set the duty cycle.
While debugging boost mode, I found that the output voltage divider is being messed with by an external force. It is being affected by the SM72442's AVOUT pin. The resistor divider is a 26.1k with a 2k at the bottom. The voltage while operating is wrong. With the output shorted, the resistor divider output is actually 0.4V, not 0V! When I remove the two resistors (leaving the AVOUT pin and a capacitor on that net), that net measures 1.4-1.6V. It looks like there is some sort of pullup going on inside the 72442. According to the datasheet this is not the case, so I am now stuck.
I tried the same procedure (removing other components, measuring voltage) on the A0 pin and it is 0V.
Is there something I am missing?
Here is the schematic PDF if you need it (copyright Purdue Solar Racing, please do not redistribute or reproduce, sorry). Note that the PM_OUT pin is pulled to GND on the board.
Edit: I forgot to mention that if I put the chip is reset, then the resistor divider works as expected.
Thanks in advance,
Brian
Purdue Solar Racing
