DRV8701: SO sampling waveform

Annie Liu

Part Number: DRV8701

Hi Team,

The waveform collected by SO when the motor is short-circuited is as follows:

Examples of SO waveforms in the manual are as follows:

When a peak current occurs, it is observed in the manual that SO has a climbing process, but the actual measured waveform is directly at a high level.

1. Why does the collected SO not change with the current trend?

2. Why does the SO waveform not change with the current trend during the chopping process?

Regards,

Annie

7 months ago

0 Murugavel4637 7 months ago

TI__Mastermind 26595 points

Hi Annie,

I assume this post is related to the same system referred in this previous post you had, https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/1331555/drv8701-nfault-pin-remains-high-after-exceeding-hardware-current-limit/5083981?tisearch=e2e-sitesearch&keymatch=drv8701#5083981.

The SO is looking like it had saturated because the short circuit current peak was very high. What is the short circuit current waveform (purple trace) peak value shown in the scope capture?

What was the Rsense value used? Thanks.

Regards, Murugavel

0 Annie Liu 7 months ago in reply to Murugavel4637

TI__Genius 10295 points

Hi Murugavel,

Murugavel4637 said:
I assume this post is related to the same system referred in this previous post you had, https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/1331555/drv8701-nfault-pin-remains-high-after-exceeding-hardware-current-limit/5083981?tisearch=e2e-sitesearch&keymatch=drv8701#5083981.

Yes, it is a related question.

Rsense=20mΩ

Current peak=40A

This is the customer's schematic:

Regards,

Annie

0 Murugavel4637 7 months ago in reply to Annie Liu

TI__Mastermind 26595 points

Hi Annie,

Voltage output at SO = I*(AV*Rsense) + VOFF. In this setup, SO = 40*(20*0.02) + ).05 = 16.05V while current reaches 40A. Customer had clamped the SO output with a 3.3V Zener. The SO output voltage starts clipping at 8.125A. The di/dt is pretty fast with a short so 8.125A is reached pretty quick. This is why SO looks like a square wave. This is expected behavior.

If the customer wants to see the full 40A current excursion profile on the SO they must decrease the value of the Rsense significantly such that SO at 40A would be < 3.3V. This would mean Rsense would have to be ≤ 4mΩ. This is not a valid case because then current won't be regulated at 8.125A.

Short answer, based on the described test case the SO is showing expected behavior.

Regards, Murugavel

0 Annie Liu 7 months ago in reply to Murugavel4637

TI__Genius 10295 points

Hi Murugavel,

Customer also wants to know why their MCU cannot read the voltage when the SO sampling point has a high level within about 2.5us (corresponding to the time of tBlank)?

Regards,

Annie

0 Murugavel4637 7 months ago in reply to Annie Liu

TI__Mastermind 26595 points

Hi Annie,

Did the customer make sure the ADC in their MCU does a conversion exactly during the time window where SO is present? They should trigger the ADC conversion with the PWM edge for proper timing and capture. It is related to their MCU and ADC performance not the DRV8701 SO output. Thanks.

Regards, Murugavel

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DRV8701: SO sampling waveform