Like every other semiconductor switching device a TB6612 will drop some voltage depending on the current the motor is taking. The specification says 0.5V at 1A which is reasonable.Steve
Specifically the output upper and lower FETs total on-resistance is quoted as 0.5 ohms (typical) in the datasheet, and from the saturation spec its clear worst-case is 0.7 ohms.So for a motor pulling 0.5A at 7.4V supply, it should see about 7V worst case (drop of 0.35V).Do not use a regulator for Vmot, take the 7.4V straight from the battery pack. Power electronicsis all about switching, not linear regulation. You'll only create lots of unwanted heat and wastepower using a regulator (as well as need more components). Use lots of decoupling (220uF or more)on the Vmot if possible to reduce switching noise.
I am actually aiming to obtain a linear variation between the voltage supplied to the motors.
i will still need to provide a regulated voltage to the Vmot pin on the TB6612 motor driver.
To be clear, the only "vary" that driver can supply to your motors is On/Off.To supply the motors with variable voltage you would need to adjust the supply voltage going to the driver or add a variable resistance between the driver and the motor.
Doing so will waste considerable energy and gain you nothing.
1. You have some unusual misconceptions and a lot to learn about motors. Do continue this journey by taking the time to experiment with some.2. If the torque drops due to battery voltage drop, increase the PWM percentage to compensate, like everyone else does.Aside: you seem to have chosen an appropriate user name. This is all purely theoretical, right?
it's not possible to increase the PWM further...?