TB6612FNG DC Motor Driver Low Current Problem

Question

I have a TB6612FNG pololu DC motor Driver (datasheet), and pololu 488:1 6V DC motor (motorPage). It is clearly stated that motor has 2.9A stall current at 6V and driver has 1.2 continuous current and 3.2A peak current. When i properly connect my system via an Arduino Uno, with two DC motor, I observed that driver supplies only 0.8A when one motor is on stall, and the rpm of the other motor decreases significantly. At stall, voltage of the motors decreases significantly.

When I separate one of the motors and directly supply 6V, it sinks 1.6A at stall.

The power cabling of the system is such that, 6V from power supply connected to breadboard so that Arduino and motor driver is fed from here. Grounds are common. Logic 5V supply of the driver is taken from Arduino 5V pin. Which becomes 4.5V when I measure on the driver. It is okay for the driver.

1- Why the driver does not supply the voltage as stated in datasheet.

2- Why the motor does not sink 2.9A at 6V stall.

Answer 1

The datasheet you linked to is the datasheet of the component TB6612FNG made by Toshiba. You are writing about a TB6612FNG pololu DC motor Driver, which probably is a PCB on which the component TB6612FNG is used.

The first hit I got on "TB6612FNG pololu DC motor Driver" was this site (not sure if OP used this) which states 1 A continuous current, which is less than 1.2A. Moreover, a tiny PCB is is nice and small, but useless if they can't loss their (dissipated) heat.

From linked site:

Real-world power dissipation considerations

The TB6612 motor driver used on the carrier board has a peak current rating of 3 A per channel. The peak ratings are for quick transients (e.g. when a motor is first turned on), and the continuous rating of 1 A is dependent on various conditions, such as the ambient temperature. The actual current you can deliver will depend on how well you can keep the motor driver cool. The carrier’s printed circuit board is designed to draw heat out of the motor driver chip, but performance can be improved by adding a heat sink.

 
Reason why actual current is lower if not cooled well enough
When the IC becomes hotter, the drain resistance of the p-mosfets become larger, giving a bigger voltage drop, resulting in an increase of the "Driver Ron" Sunnyskyguy is already pointing out.

UPDATE in response to comments
If the current lowers gradually, it is due to the increasing \$R_{DS(ON)}\$ of the p-mosfets.

If the current alternates between a certain value and nothing, the TB6612 is triggering its thermal shut down circuit repeatedly.
This thermal shut down circuit disables the outputs when the IC becomes too hot (175°), until the IC has cooled down (by about 20° = thermal hysteresis). But since the motor is still being driven al the time, it will run hot again and enter the thermal protection again very fast, resulting in a hicup mode.

Try cooling the IC with a fan, e.g. an old computer fan. Even a tiny airflow will make significant difference compared to no airflow at all.

Answer 2

I expect “peak current” from driver is limited by either power dissipation in the driver and/or the duration of ESR*Cout used by the driver and/or the current limit in your supply or output impedance.

This may be shorter than the L/DCR current rise time of the motor, which is further slowed down by mechanical load.

Your shared supply current to board must be able to supply 2 stall currents at rated voltage to ensure stable operation.

For at least one or more of these reasons, your motor is not getting the rated current due to a drop in driver voltage.

Driver Ron = 0.5Ω @ 5V. (Typ) not worst case. @25’C

Motor DCR= 6V/2.9A=2.07 Ω using stall current.

supply ESR= TBD or active current limit = TBD

With more details on your measurements at all nodes, a solution can be suggested. like a heatsink for IC to prevent rise in Ron and thermal shutdown.