DRV8251 DC Brushed Motor Driver

[Tnixsw]

Hi all,

I was looking at using this device for a project that uses a 5V brushed DC motor for a printer mechanism.

The device data sheet mentions that PWM can be used for speed control and that switching between driving and braking typically works best. This method allows full access to the current sense feature, which is not available during all switches OFF - (motor coasting).

This seems contradictory to me as you are trying to drive the motor and then stop it in a single PWM period.

I tried this and the motor doesn't seem happy.

I would have thought the current sense might have been better utilized during coasting for speed control feedback by using the back EMF of the motor.

I thought if someone has already used this device, they could share some experience.

cheers

Tony

[H.O]

I have not used this device and I'm not 100% sure but...
The device uses a low side current sense resistor. This means that during the off period, if you use coast/all switches off, the current thru the sense resistor reverses which produces a negative voltage on the sense input, an event from which the sense amplifier in the chip then needs to recover - and this happens every PWM cycle.

When using slow decay/both low side switches on, the current never flows thru the sense resistor during the off period of the normal PWM cycle, hence no negative voltage on the sense pin. Do note, however, that the device itself uses coast/all switches off during the current regulation off time.

Hopefully (surely) someone will correct me if I'm wrong :-)

[Tnixsw]

Thanks for the reply.

There isn't a current sense resistor in this device. It has an internal current mirror associated with each low side FET.

It outputs a current proportional to the sensed motor current through each of the low side FETs, and with that you can use an external resistor to read a proportional voltage. Unfortunately, this only works when either of the low side FETs are on.

cheers

Tony

[H.O]

Ah, you're using the DRV8251A, not DRV8251 - those are different...
As it says in the datasheet, in order for the current to be properly sensed it has to flow from drain to source in the low side MOSFETs. With slow decay (both low side switches on) the current does flow from drain to source in one MOSFET hence it can be sensed. In fast decay (all switches off) the current flows in reverse, from source to drain, thru the diode so it can't be sensed.

In my original post I wrote that the DRV8251 uses fast decay (all switches off) for current regulation but I must have misread that.