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I designed a PCB that contains, among other things, a switching regulator circuit and an H-bridge for a DC motor. The whole circuit is powered by a 15V Lithium battery. The switching regulator converts the battery voltage to 5V to power the digital logic on the board. The DC motor is driven directly from the battery through the H-bridge. The motor itself was torn out of a hand-held impact driver with a stall current around 15A.

Without the motor attached, I'm able to get full functionality of all the digital logic components, so the switching regulator is doing its job. When I attached the DC motor, however, the switching regulator went pop. It appears to be completely dead. No signs on life on any pin.

Here's the switching regulator circuit using an LT8609. I've intentionally cut off the input and output caps so the schematic will fit nicely on the screen. There's about 32uF of capacitance on Vin and about 88uF downstream of the inductor. VIN of the regulator is attached to the net named V_BATT: enter image description here

Here's the H-bridge using the MC33883 H-bridge driver. I apologize if the image is unreadable here. You may have to view it in a separate tab on your browser or save it and open it in an image editor:

enter image description here

The ACS711 in the upper-right hand corner is just a Hall-effect current sensor.

Here's the sequence that I performed that lead to the LT8609 failing:
1. Battery power was applied to the circuit.
2. The LT8609 powered on and was stable, providing 5V to the digital logic components.
3. The microcontroller (which uses the 5V) was programmed in debug mode.
4. Almost instantly upon running the code in debug mode, the LT8609 popped.

The only instructions that were programmed into the microcontroller (besides the usual oscillator and other initialization settings) were:
1. Enable the H-bridge (set pin G_EN high on the MC33883).
2. Read a voltage value from a potentiometer.
3. Set the duty cycle (based on the pot value) of the PWM signals to the H-bridge to turn the motor. At the time, the pot was turned to 0V, which would have translated to a near-zero duty cycle of the H-bridge.
4. Loop forever

Both the LT8609 regulator and the MC33883 H-bridge are powered from V_BATT. The worst thing I can think of is the H-bridge caused a shoot-through condition as the microcontroller that controlled it was coming online. But even so, I don't see how that would kill the LT8609.

I would appreciate if anyone can give me ideas as to what might have been the culprit to help me troubleshoot. Also, point out any fatal flaws in the circuit that I'm missing.

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  • \$\begingroup\$ 2ns diagram alternativeadded - delete one or other as desired (I don't mind if you dislke my version - just a suggestion). \$\endgroup\$ – Russell McMahon Apr 28 '15 at 7:41
  • \$\begingroup\$ @RussellMcMahon, thanks, much easier to read. What did you use to darken it, if you don't mind me asking. \$\endgroup\$ – Dan Laks Apr 28 '15 at 7:45
  • \$\begingroup\$ It was improved mainly by taking out horizontal space so that it was more compact so it occupies more screen area. I opened the image in Irfanview then: Cut right hand block just to left of ACS711 and slid it left to take up space between it and the 3 vertical wires. Then cut block from left of 3 vertical wires to far right and slide it left to take up empty space. Filter caps at far left - cut and paste names from left to above and right of caps. Slide lh cap right to take up space. Trim white just outside vertical edges. .... \$\endgroup\$ – Russell McMahon Apr 28 '15 at 8:26
  • \$\begingroup\$ ... Image is now maximally compact horizontally so will fill more area when pasted. NOW use Gamma/Saturation/Brightness/ Contrast for best visual effect. Post. ||| NB the fabulous free Irfanview from irfanview.com is fabulous and free and fabulous. (Most used free photo editor in the world, I'm told). \$\endgroup\$ – Russell McMahon Apr 28 '15 at 8:33
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Some things to think about:

  • The H-bridge was tried with Vb and micro on a lab supply, but without 5V switcher, right?

  • Has the source impedance (of Vb) been compared to load impedance of motor drive and switcher combo? Source impedance needs to be much lower than load impedance. If not, supply could oscillate, especially if kicked by a motor hard start.

  • Are all the PWM signals generated by timers in the micro, so that they maintain proper phasing and timing (i.e., not bit banged in code)?

  • Were all the PWM inputs to MC33883 correct before giving enable? No tri-stated output,or highs when should be low.

  • Are you sure the ADC readings from pot are OK? Noise here could cause trouble.

  • Is motor drive frequency compatible with switcher filter, both input and output? I.e., not on top of a filter resonance.

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I think the problem is on the inductive load (high flyback voltage from motor), have you tried a resistive load. If is working with resistive load you may need some separation on the supply of LT8609 or a snubber on the motor.

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Except what was already said (i strongly support overvoltage protection) i can tell, that this acs711 will give you nothing, put it on the motor.

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