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I'm using a TLE5205 H-Bridge to drive a small DC motor. The setup works just fine with small load (an LED for example), however once I connect the motor, it won't spin, only flicks slightly each time the direction of the output current is changed.

I'm pretty sure this is caused by the integral overcurrent protection feature of the H-bridge, since the error flag pin state behaves according to this situation. The OC protection is set to trigger at I = 10A (As seen in the datasheet). When the motor spins continuously, this is more than enough space, since it draws a steady current of 0.7A at 7V, however, while stalled, the motor draws up to 21A which is a lot more than what the OCP tolerates.

The IC doesn't seem to have a current limiting feature aside from the OCP, which shuts it down completely. I could potentially connect a resistor in series with the motor to limit the current it draws, however this would also limit the overall power of the motor, which I don't want. Is there a way to solve this issue, preferably without having to redo the whole PCB which has already been manufactured?

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    \$\begingroup\$ (1) What is the DC resistance of the motor? (2) What is the supply voltage? (3) What current do you get when you divide (2) by (1)? (4) Have you run through the diagnosis table on page 8 of the datasheet? Put all the info into the question (along with a couple of paragraph breaks - 2 x <enter>). \$\endgroup\$ – Transistor Nov 4 '17 at 17:29
  • \$\begingroup\$ Soft start the motor and/or constant current limit. \$\endgroup\$ – winny Nov 4 '17 at 17:40
  • \$\begingroup\$ The resistor will limit the current and then the motor won't start...try the other ideas. \$\endgroup\$ – gbarry Nov 4 '17 at 17:47
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If it really is the overcurrent protection you could slowly ramp the motor speed up using pwm.

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  • \$\begingroup\$ This is the best solution IMO. It is also free, since it's done in software... \$\endgroup\$ – peufeu Nov 4 '17 at 20:43
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You can limit startup current by adding resistance in series with the motor.

If your motor draws 21A at 7V when stalled then its internal resistance is 7V/21A = 0.33Ω. The TLE5205's overcurrent shutdown point is 6A minimum, and total switch resistance is typically 0.45Ω at 3A. To limit startup current to 6A you need a total resistance of 7V/6A = 1.17Ω. Therefore the extra resistance required is 1.17Ω-(0.45Ω+0.33Ω) = 0.39Ω.

At 0.7A the added resistance will reduce motor voltage by 0.7A*0.39Ω = 0.27V. The motor will run 0.27V/7V = 4% slower than it would without the added resistance. If the load varies then speed regulation will be a bit worse because the total circuit resistance is now 1.17Ω/(0.45Ω+0.33Ω) = 1.5 times higher.

With current limited to 6A the motor should get 6A*0.33Ω = 2V at stall. If this is not enough to start up then you will need a more powerful H-bridge which can deliver more current (and preferably also has a lower switch resistance).

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  • \$\begingroup\$ Yes but why would you add additional losses if there are other ways of achieving your goal? \$\endgroup\$ – Jogitech Nov 4 '17 at 20:43
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    \$\begingroup\$ Because it's simple enough for a complete noob to add, and the additional losses are probably not prohibitive. " 'Best' is the enemy of good enough." \$\endgroup\$ – WhatRoughBeast Nov 4 '17 at 22:21
  • \$\begingroup\$ This method is probably best for me, since I don't need it to be perfect and as you showed, it doesn't affect the performance in a huge way, although I do need speed regulation using PWM on the motor and I haven't tested this extensively yet. On full power, the motor still spins faster than it would if I used a BJT H-bridge like L298 which incorporates current limiting, so there must be a way to control the speed, although it might relate differently to duty cycle than it would without any extra resistance. \$\endgroup\$ – DELTA12 Nov 8 '17 at 22:35

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