DRV8702D-Q1: Operation details

Hello Joe,

Thank you for posting in this forum. I assume you were looking for only unidirectional drive and won't need forward and reverse movement hence interested in using this half-bridge gate driver. 

Joe Miller1 said:

Q1. Do I use MODE =0 or MODE=1

This device supports either MODE = 1 or MODE = Hi-Z. While MODE = Hi-Z internal current regulation is active which is typically used for mitigating startup inrush current of the motor or peak current encountered when the motor is stalled. If you use MODE = 1 then you'd need nSLEEP control to configure the output to Hi-Z. See below control table for these two modes.

Joe Miller1 said:

Q2. Can I drive IN1 and IN2 with static DC values or do they need to be PWM controlled?

IN1 and IN2 can support static DC input (100% duty cycle) as well as PWM control. They can be driven with GPIO for static input control as per the control logic table.

Joe Miller1 said:

The following is the sequence I believe.
1. tri-state (motor is off) IN1=0 (high side off), IN2=0 (low side off) 
2. turn on motor IN1=1 (high side on), IN2=0 (low side off).
3. wait until instructed to turn off.
4. turn off motor IN1=1 (high side on), IN2=1 (low side on) brake
5. tri-state (motor is off) IN1=0 (high side off), IN2=0 (low side off) 

The above sequence with tri-state IN1 =0 and IN2 = 0 holds good for MODE = Hi-Z.

Joe Miller1 said:

How long do you need to wait at step 4 before going to step 5.

This depends on the mechanical inertia of the system which you should characterized. Probably in the range of 100's ms.

Joe Miller1 said:

Q3. If voltage is induced into motor winding do I turn off lower and upper FETs?
I understand that I will need to block current flowing into upper FET and also protect SH pin.

If you render the bridge Hi-Z while the motor is running (motor coasting) the motor will continue to run and the back EMF will conduct via the FET body diode to VM. This could lead to VM voltage pumping and depending on energy stored in the system inertia could exceed the supply by a significant amount. In this case whether the system can withstand this increased voltage must be evaluated. Using current recirculation via LS-FET(brake) would be the preferred approach to dissipate this energy without pumping it back to VM. The thermals also would be minimum because the conducting FET low Rdson will dissipate less heat.

Joe Miller1 said:

The following is the sequence I believe.
1. tri-state (motor is off) IN1=0 (high side off), IN2=0 (low side off) 

Like I mentioned this is true with MODE = Hi-Z. If MODE = 1 is used then nSLEEP = 0 for bridge Hi-Z.

I hope this helps. Thank you.

Regards, Murugavel

Dear Murugavel,
Many thanks for the reply. Much appreciated.

The motor is unidirectional and has two windings for low and high speed.

I am currently using a LM74810-Q1 for each speed along with an active load to discharge the respective speed so it can be parked.
This solution does not provide any output short circuit protection or current limit for safety. 
I spent considerable working on the active load to ensure NMOS SOA.

Just to confirm that if no speed control is required then PWM control of IN1 and IN2 is not required.
Simply use regular GPIO pins to drive IN1, IN2 high or low.
MODE=hi-z
nSLEEP tied high.

1. tri-state (motor is off) IN1=0 (high side off), IN2=0 (low side off) 
2. turn on motor IN1=1 (high side on), IN2=0 (low side off).
3. wait until instructed to turn off.
4. turn off motor IN1=1 (high side off), IN2=1 (low side on) brake
5. tri-state (motor is off) IN1=0 (high side off), IN2=0 (low side off) 

Is there any issue to worry about for the steps mentioned above.
I am concerned about step 4 and current flow through the low side NMOS.
Does the LM74810-Q1 control the low side NMOS to avoid large current flow.

Regards Joe