TB6612FNG and protection

[claus]

Hi all,

I am building a small robot with the metal geared micro motors from pololu. I have measured them to draw 80mA when running free and 1.6A when stalled (at 6V). The driver I have is a TB6612FNG. I have learned that to make this more or less fool proof, with a motor driver generally one needs back EMF protection and some mechanismo to avoid that a stall blows the driver or motor. As far as I can tell this driver has already 4 diodes for each channel build in, so I got back-EMF if I understand that correctly:

http://www.sparkfun.com/datasheets/Robotics/TB6612FNG.pdf

However, the peak output rating is 3.2A only for a few ms, so if some kid holds the wheels of my robot - and they will do so - I would like a mechanism to avoid burning the driver/motor. I first thought about a resettable fuse, but they seem to be too slow (not in the ms range). Should one simply put a resistor R_L in series with the motor (say to limit current to 0.5A at 5V we need 10 Ohms with a rating of 2.5W, right?) or is there a more clever method to do this (something like detecting too much current by measuring the voltage drop on R_L and setting the driver to standby mode)? I would like this do be done by hardware, if possible.

Any suggestions? Or maybe another driver, that has such a protection built-in? Should be cheap ....

[waltr]

is there a more clever method to do this (something like detecting too much current by measuring the voltage drop on R_L and setting the driver to standby mode)? I would like this do be done by hardware, if possible.

This is possible and is used on other motor driver circuits.
The general method is:
Put a small value R in series with the TB6612FNG's motor ground connection. When the motor's current draw increases the voltage drop across this resistor increases. Then use a voltage comparator that 'trips' when the voltage drop across the resistor reaches a preset value.
Use the comparator's output to inhibit the TB6612FNG.
You probably need a latch on the comparator's output since the current will drop to zero when the TB6612FNG is shut-down which would turn the TB6612FNG back on and re-trip the over-current sense (the enable/disable oscillates). The latch can be re-set from the processor (have the comparator output feed to the processor so it knows when there is a over current condition) or re-set from a timer circuit (RC , 555 or other).
For some more details look up and read various motor driver ic data sheets and App Notes.

[claus]

Sounds like a good idea with the latch! Do you know of any motor driver where this is implemented so I can check the app notes?

[waltr]

I seem to remember the L298 H-bridge data sheet or App Note having a hard current limiting circuit.
Also look to App Notes from Allegro and Microchip for circuit examples/ideas.
Microchip does have a number of decent App Notes on motors and motor control and also covers a current limit circuit.

[claus]

If someone has the same question, the TI DRV8833 seems to fullfill the needs mentioned above, I will get a breakout to check. Thanx for pointing me in the right direction!