DRV11873: Drv11873 internal 5V LDO has no voltage output

Hi Chris,

Thanks for posting your question in MD forum. From your schematic I see that you are loading the internal 5V LDO to generate PWM signal. I think this PWM circuit is drawing more than 20 mA during the power up and this triggers UVLO fault on V5.When you connect external 5V, the device comes out of UVLO fault and spins the motor.  I'm guessing V5 is overloaded by the PWM circuit (may be inrush current) during the powerup and then the load current settles down to below 20 mA during normal operation. 

Can you apply around 5.2V to the V5 pin using an external power supply and see how much current is drawn during the power up?

Also, do you see this issue when there is no load on V5 pin? 

Regards,

Vishnu.

Hi Vishnu:

I have tested the current of external 5.2V power supply and the sartup current is 30~40mA by Fluke(The accuracy of current probe is not enough ),after is about 3mArms.But,We have done the following applications(It has been in mass production for one year ) :Power supply is 6V,PWM pin external pullup 100K to 5V.There is no 5V LDO has no output phenomenon.

About UVLO,UVLO fault  will be cleared when the power supply restarts?  This phenomenon will not disappear with the power on again .

When use extarnal 5.2V supply ,all current will go to the external power supply? By the way ,when use external 5.2V power supply,the Motor start-up jitter will be greater than internal 5V. I'm afraid this will affect the start-up current of V5.

If it is really like what you analyzed, how to solve the rush current at the moment of power on.  Also my my system needs PWM to control the speed .Is there a reference application circuit ?

Best Regards!

Chris.

Hi Vishnu:

I  update the information， I just received a machine with this problem, but the phenomenon is different from the previous machine. Also there is no 5V output ,and connect the V5 to external 5V, the motor doesn't work.Restart the power,remove the external 5V,5V LDO has output,but motor doesn't work either.

I check the volatge of U\V\W\COM.Vu=Vv=Vw=about 3V (just DC Voltage test by Oscilloscope),Vcom=0.2V DC. Normal Vu=Vv=Vw=Vcom=6Vrms,when power supply is 12V PWM input 50% .

Hope to help analyze the problem 

Best Regards!

Chris.

Hi Chris,

As expected, the PWM circuit is drawing above 20 mA and that's triggering UVLO fault on V5. UVLO fault gets cleared when the voltage rises above 4.3V (typical). At 5.2V (external power supply), load will be completely on the external power supply. To reduce inrush current,  you may try increasing the input impedance of the PWM circuit. Please refer to the DRV11873EVM design files for details on designing the PWM circuit. 

Regards,

Vishnu

If you do not see 5V on the V5 pin then the device is mostly damaged. 

Hi Vishnu:

I check the DRV11873EVM SCH design. PWM input pin use 10K pull-up,Series resistance is 0R.My application use 10k Pull-up,series resistance is 10K. So i don't think there is a problem here.

Maybe the external capacitance of V5 is a problem.Demo design is 2.2uF ,my application is 10uF . A bigger capacitance produces a larger rush current.I test the power-up sencence of V5.If there is a UVLO fault on V5, the voltage of V5 must will be under 4.3V. Following is the 5V power-up wave of 2.2uf and 10uF.

There is no collapse of the 5V waveform here(Motor successful starts) and when the voltage reached 4.3V the capacitance of 2.2uf needs 1.28ms, while that of 10uF needs 1.56ms .How long does it take from 5V power-up to UVLO detection ? By the way ,the logci of DRV11873 is also uses the V5 power supply ,if the V5 is not normally, logic circuit  will crash and  burn the MOS-FET?

Best Regards!

Chris.