DRV8703-Q1: How do I get PWM

Hi Huan,

Thank you for posting in this forum. In order for the device to power up in PWM input H-bridge mode the MODE pins must be HiZ while nSLEEP = 0 and transitions to 1. Then, when nSLEEP = 1 the device wakes up and latches the MODE status (input HiZ) to PWM input H-bridge mode. In this mode of bridge control as shown in Table 7-5, one of the inputs must be HIGH (IN1) and the other input (IN2) must be input with a PWM input with variable duty cycle to vary the speed of the BDC motor at the output. When IN2 is HIGH and IN1 is input with PWM the motor will spin in the opposite direction. What is the input PWM frequency and duty cycle provided to the input during the tests?

When PWM input = 0 the motor will be driven. When PWM input = 1, both IN1 and IN2 will be HIGH resulting in low side FET slow decay of the current in the motor winding. This process happens for every PWM on/off period. The motor can be driven with 100 % speed without using a PWM input and instead making one of the inputs LOW while the other input is HIGH.  

Was current regulation enabled and used during the tests? What were the settings for current regulation used and the sense resistor used? What was the supply voltage used? What is the type of load connected to the output? 

VDS and GDF Status Register = 0x04 means L2_VDS low side FET has VDS error. The default VDS setting is 0.96 V unless you changed it to a lower value. If the FET's you were using had 2 mΩ Rdson, 0.96 V would mean 0.96/0.002 = 480 A flowed via the LS2 FET which is abnormally high current - not really a possibility. What is the Rdson of the FET used. Were you able to measure the current spike through the load using a current probe? It will be helpful if you could share the current waveform taken while the VDS overcurrent is reported by triggering the capture with the nFAULT pin (requires pullup). If we could see the schematic around the driver and the output FETs it will help us with the debug. Thanks. 

Regards, Murugavel 

Hi,Murugavel

You said " one of the inputs must be HIGH (IN1) and the other input (IN2) must be input with a PWM input with variable duty cycle to vary the speed of the BDC motor at the output.". I can use SPI communication to make IN1 HIGH, but I don't know how I input a PWM input with variable duty cycle in IN2.Maybe I need another chip to generate PWM,then PWM is connected to the IN2 as input? I thought DRV8703 could generate PWM on its own.It seems that I misunderstood.

The supply voltage is 12V, and the type of load connected to the output is a DC speed reduction motor.

I made some changes to the hardware.Although the motor was successfully turned,FAULT Status Register = 0x88(VM undervoltage).The 12V voltage first becomes 6V and finally reaches 11V, and the current also reaches the maximum limit of 0.2A.I'm going to configure a PWM output and see what happens

BR,

Huan Yin

Hi Huan,

Yes the PWM is usually generated by a microcontroller. This is required only if you'd want to have a PWM speed control. Applications that don't need speed control can be controlled by SPI IN1 and IN2. The DRV8703 does not have internal PWM to drive the motor. 

Our EVM for this device https://www.ti.com/tool/DRV8703-Q1EVM?keyMatch=DRV8703-Q1EVM uses a TI microcontroller to generate PWM for controlling the motor speed. The hardware files including the EVM schematic can be downloaded from https://www.ti.com/lit/zip/slvc675. The EVM firmware source code can be downloaded from  https://dr-download.ti.com/secure/software-development/support-software/MD-KF25Q3LqZz/01.00.00.0A/slvc673a.zip. 

You said "I made some changes to the hardware. Although the motor was successfully turned, FAULT Status Register = 0x88(VM undervoltage).The 12V voltage first becomes 6V and finally reaches 11V, and the current also reaches the maximum limit of 0.2A.". Did the power supply have a current limit set? BDC motors have a large inrush current during startup which can reach 10's to 30's of amps until the motor reaches speed and generate enough back EMF (BEMF) to oppose the current flow until it reaches a equilibrium steady state current when the motor runs at a fixed speed. A current limited power supply will not able to sustain output voltage at these high currents and the driver will report a VM undervoltage. A VMUV will also disable the output drive during the UV situation so the motor coil will not reach the desired current to startup and run.

"I'm going to configure a PWM output and see what happens". Having a PWM input and ramping up duty cycle from 0 to 100 % for a soft start of the motor will mitigate this inrush current which in turn will reduce the peak available torque for the application during startup of the motor. So how you configure the start up depends on the application. This inrush current mitigation can also be done using the internal current regulation by configuring it to the desired target current. This does not require external PWM. Instead the driver generate tON and tOFF for the chopper current regulation internally. 

Regards, Murugavel    

Hi,Murugavel

I configured PWM and now made the motor run better.But there is still one error：FAULT Status Register = 0x88 (VM undervoltage lockout).What does that mean? You ask me "Did the power supply have a current limit set?",the answer is yes.I set a current limit for the 12V power ---- the current limit is 0.2A.Maybe the limit is that what caused the error?

You said "Applications that don't need speed control can be controlled by SPI IN1 and IN2.".I didn't understand the sentence, could you explain it again?How I use DRV8703 to control the motor only by SPI IN1 and IN2?Isn't external PWM necessary？

I'm going to increase the current limit and see if it gets better.

BR,

Huan Yin

Hi Huan,

Current limit of 0.2 A will definitely cause VMUV error. You may want to increase the current limit. Ideally the current limit should be able to support startup inrush current of the motor.

You can use the below highlighted bits in the Main Control Register to run the motor at full on or full off. Because there will be no PWM or soft start you should configure current regulation to limit the startup inrush current if need be.

This section of the datasheet discusses current regulation - see below.

Regards, Murugavel

Hi,Murugavel

Now I increase the current limit to 2A,FAULT Status Register is true(0x00). Thank you for your answer!

Maybe I seem to understand:If I want to configure IN1 and IN2, I can externally pull IN1 and IN2 up and down, or I also can use SPI communication to write Main Control Register. Both of these methods can achieve motor drive.(Correct me if I'm wrong)

Finally,I don't know what PH or EN mean(IN1/PH,IN2/EN).Maybe EN means "enable"?

BR,

Huan Yin

Hi,Murugavel

Thank you for your answer. It helps me a lot！

BR,

Huan Yin