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Hi everyone!
I'm trying to drive an RC circuit (resistor in parallel with a capacitor) with the DRV8212 H-Bridge controller. The resistor is for now a substitute for a Peltier module which will be driven both to heat and cool down, hence the need for an H-Bridge. The reason for the parallel capacitor is that I want to use a very low voltage Peltier module (max 0.5V) and it's the most efficient way I can think of for a battery-powered device to do this. The voltage at the VM pin is 1.2V, and so I want to use duty cycles low enough that the RC circuit will filter the voltage to below the 0.5Vmax for the Peltier.
My problem comes when I use high frequencies (50kHz) and low duty cycles (<25%). When this is the case, i.e. pulse widths below 5µs, the H-Bridge seems to randomly stop working. I can only assume that, with pulse widths below 5µs, the DRV8212 isn't interpreting this pulse as wide enough to keep itself working and automatically shuts down. I've ran the tests at different frequencies and it seems that the breaking point are the 5µs that I've mentioned of PWM pulse width, i.e. I can run at 50kHz 25%Duty, or 10kHz 5%Duty, but below those duty cycles for these frequencies the system just stops working.
Can someone corroborate that this behavior is to be expected? I've tested that the input signals to the H-Bridge have the proper shape with an oscilloscope, so the fault is not on the microcontroller side driving the inputs (the edges look very vertical and smooth).
On the attached picture, the CH1 and CH2 traces are the voltage on either side of the load resistor for a PWM frequency of 10kHz at 5% duty cycle (5µs pulse width). In white, the difference between the voltages of the resistor, that proves the H-Bridge to be working at this frequency. Note that the Y positions on the CH1/CH2 has been offset to lower values for better visualization, as has been done in the opposite direction for the substracted waveform.
Thank you for your time
Gabriel
Hello Pablo, and thank you for the swift response.
The issue shouldn't be with the VM=1.2V, since I'm using the DSG-package variant of the chip, which should support down to VM=0V according to the datasheet:
I can provide a few screenshots of the schematic, yes, but it don't think it would clarify much since I'm using a microcontroller to do the job:
The pin MODE would therefore be connected to the pin 5.6 of my µC, the pin IN1/PH to pin 0.4, and the pin IN2/EN to pin 6.2. The ground signals have a different name because they're separate PCBs and hence also separate schematics, but of course they're connected together.
I have also attached a picture of the signal at the input of the IN1 pin of the DRV8212 generated by the microcontroller, where it can be seen that for a pulse of 5µs, the edges are very vertical and the voltage level is adequate at 3.3V:
So, based on what I'm seeing, I really don't suspect the input driving to be the issue here.
I've conducted a bit more of testing. I've created a routine in the microcontroller that will keep one of the control pins high for 200µs (double the time specified on the datasheet to wake up the chip, t_WAKE) in an attempt to wake it up and then keep it running as a workaround. It works for a few cycles of the PWM but then the device turns off for duty cycles below 4% at 10kHz, seemingly after (in the case of the screenshot) around 13 periods. At 10kHz, 13 periods amount to 1.3ms, which is consistent with the t_AUTOSLEEP of 0.9-2.6ms specified in the datasheet. We can observe this behavior in the picture below:
In yellow (CH1), the voltage on one side of the load resistor at the output of the H-Bridge. In magenta (CH2), the voltage at one of the input pins of the DRV8212. In the magenta line we can first see the wide 200µs pulse that wakes up the IC, and then the series of 4% duty cycle pulses. In yellow, we can see how the H-Bridge turns on shortly after the turn-on pulse at the beginning, then it works for the following ~13 periods, and then simply stops letting current flow (as seen by the decay of current through the resistor). Remember that the sawtooth waveform is given by the capacitor in parallel with the resistor.
It really seems to me as though the device is entering sleep mode based on the testing above. Maybe I'm missing something, so thank you for reading this in detail 
Best regards
Gabriel Rodríguez
Hi again Pablo.
Thank you very much! If you find something out please let me know
Best regards
Gabriel Rodríguez
Hello Brian. Indeed I'm running the IC on PWM mode and not on Enable/Phase mode. The reason for this is that, by default, the PH/EN mode's "off" status is actually a Break status, where the "motor" (in my case the Peltier element with the capacitor in parallel) is short circuited through the low side. Since my purpose is to lower the voltage using a PWM signal with the capacitor/peltier acting as a Low-Pass filter, this discharges my capacitor through the low-side of the H-Bridge instead of through the Peltier, which isn't the desired outcome:
On the other hand, the PWM mode has a "coast" status where, instead of short-circuited, the "motor" can be left floating, which is the desired status for my Low-Pass Filter:
That's why I can't really use the PH/EN mode for my application. That said, thank you very much for your time and your help :) I wish I could have just used the other mode!
Thank you again and have a great day
Gabriel
