LM5140-Q1: Low drop-out operation

tateo yamashiro

Part Number: LM5140-Q1

Hi,

We got a question from customer about LM5140-Q1.
Could you help us?

[Question]
If the off time of the high-side FET is beyond the minimum value(100ns max), what is the behavior of the IC? Our customer found the phenomenon that the IC reduce switching frequency. Is this phenomenon correct?

Vin=8.5V, Vout=8V, Iout=3A, setting fsw=2.2MHz

Best Regards,
tateo

over 8 years ago

0 Arief Hernadi over 8 years ago

TI__Mastermind 23845 points

Hi,

It seems that it is a correct behavior the IC to foldback frequency to maintain regulation.
I will ask the application engineer just to be sure

Thanks
-Arief

0 tateo yamashiro over 8 years ago in reply to Arief Hernadi

Guru 10890 points

Hi, Thank you for your reply. Please let me know any update on this.

Anyway, we got addtional question. Could you help us?

- They are considering separating VIN from power stage. Please see below figure. Is there any concerns in this usage?

Best Regards,
tateo

0 tateo yamashiro over 8 years ago in reply to Arief Hernadi

Guru 10890 points

Hi, Is there any update on that? I would appreciate it if you give us your feedback.

Best Regards,
tateo

0 Terry Allinder over 8 years ago

TI__Expert 3545 points

Tateo,

Yes the LM5140-Q1 will reduce it's osciilator to maintain output voltage regulation when Vin approaches Vout. For more information refer to section 7.3.3 (Oscillator) in the LM5140-Q1 data sheet.

Terry

0 tateo yamashiro over 7 years ago in reply to Terry Allinder

Guru 10890 points

Hi, Sorry for the delayed reply. Thank you for your support. If the input voltage is reduced to equal to the output voltage, how much is the output voltage reduced? We are thinking it is related to the minimum off time and the switching frequency. How much is the switching frequency reduced in their condition?

- Customer condition -
Vin=8V to 16V
Vout=8V
Iout=5.5A max
fsw=460kHz by external clock

Best Regards,
tateo

0 tateo yamashiro over 7 years ago in reply to tateo yamashiro

Guru 10890 points

Hi, Sorry for rush, but could you give us your feedback by end of this week? I would appreciate it if you give us your feedback.

Best Regards,

tateo

0 Terry Allinder over 7 years ago in reply to tateo yamashiro

TI__Expert 3545 points

Tateo,

With VIN=8V, and VOUT set to 8V, with a 5.5A load, Vout drops to 7.55V, and the oscillator is reduced to 55kHz.

As VIN is lowered and the LM5140 reaches it minimum off time, it will drop the oscillator frequency to keep the output in regulation. It will ignore the SYNCIN signal, for more information refer to section 7.3.3 in the data sheet.

Dropout_data_ 8V.pptx

0 tateo yamashiro over 7 years ago in reply to Terry Allinder

Guru 10890 points

Hi, Thank you for your support. We got the waveform of low dropout operation at the osillator frequency 2.2MHz. The minimum switching frequency is about 300kHz. Please refert to first post. Is the minimum switching frequency at low dropout operation different fsw=400kHz and fsw=2.2MHz?

Best Regards,
tateo

0 Terry Allinder over 7 years ago in reply to tateo yamashiro

TI__Expert 3545 points

Tateo,

The minimum frequency when the oscillator is set to 2.2MHz is 277kHz, when it is set to 440kHz the minimum frequency is 57kHz.

Terry

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LM5140-Q1: Low drop-out operation