LM5170-Q1: About 10kohm of SYNCOUT when circuit breaker function disabled

Toshihiro Watanabe

Part Number: LM5170-Q1
Other Parts Discussed in Thread: LM5170

Hello,

Is SYNCOUT pull down resister value needed 10kohm exactly?

When circuit breaker function is disabled, we need 10kohm pull down at SYNCOUT to GND. When four phase operation, SYNCOUT is connected SYNCIN of another LM5170-Q1. SYCIN pin has 100kohm internal resister. In this case, SYNCOUT pull down resister value will become around 9kohm.

Is there possibility circuit breaker function enabled when pull down resister value is 9kohm?

Best regards,

Toshihiro Watanabe

over 5 years ago

0 Youhao Xi over 5 years ago

TI__Guru* 78060 points

Hi Watanabe-san,

Thank you for posting. When the datasheet recommends 10k, the effects of the internal resistor have been take into accounts. In short, yes it works with the equivalent resistor of 9kOhm.

Thanks,

Youhao Xi, Applications Engineering

0 Toshihiro Watanabe over 5 years ago in reply to Youhao Xi

TI__Genius 10235 points

Hello Xi-san,

I understood SYNC_IN pin internal pull down resister is considered. Thank you.

Please let me ask one more question.

nFault goes low when UVLO is changed as high -> low -> high. The first UVLO high is because 24V power up and no issue.

UVLO goes low then high, nFault is low. SYNCOUT is pull down so fault detection will be disabled.

Is there any reason nFault becomes low when UVLO goes low to high?

Best regards,

Toshihiro Watanabe

0 Richard Fu22 over 5 years ago in reply to Toshihiro Watanabe

TI__Mastermind 30955 points

Hi Watanabe-san,

The engineer responsible for this part is currently out of office. He will be back by the week of the 28th.

Thanks,

Richard

0 Youhao Xi over 5 years ago in reply to Richard Fu22

TI__Guru* 78060 points

Hi Watanabe-san,

First, please be advised that I am currently on a biz trip. I will try to respond ASAP. However, my reply may be delayed.

nFAULT should not behave like that, if I understand your description correctly. Can you show me the waveforms of UVLO and nFAULT on the same scope picture?

Thanks,

Youhao

0 Youhao Xi over 5 years ago in reply to Youhao Xi

TI__Guru* 78060 points

My I assume the issue is resolved? Let me close it here but you can re-open it by adding a new post.

Thanks,

Youhao

0 Toshihiro Watanabe over 5 years ago in reply to Youhao Xi

TI__Genius 10235 points

Hello Youhao-san,

I missed your post. I am sorry. Let me check the waveform.

Best regards,

Toshihiro Watanabe

0 Toshihiro Watanabe over 5 years ago in reply to Toshihiro Watanabe

TI__Genius 10235 points

Hello Youhao-san,

I am sorry my reply late. Please let me share the waveform offline.

Best regards,

Toshihiro Watanabe

0 Youhao Xi over 5 years ago in reply to Toshihiro Watanabe

TI__Guru* 78060 points

Hi Watanabe-san,

Can we continue the discussion through email and close the thread here? As I mentioned earlier in the email, the UVLO pin is a high impedance, and passive pin before the circuit is enabled, so it will not actively clamp the voltage. Once the circuit is enabled (UVLO>2.5V), there is an internal 25uA current source being turned on to produce hysteresis. In your test, the holding-low of the UVLO pin at 1.4V is not caused by the LM5170 but by your external circuit. The 1.4V is just the result.

Thanks,

Youhao

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LM5170-Q1: About 10kohm of SYNCOUT when circuit breaker function disabled