TPS53314: A few design questions

Roy Hsu

Part Number: TPS53314
Other Parts Discussed in Thread: TPS53318, TPS53119

Hi there,

Below are a few design questions I have on TPS53314:

1. From equation (5), Rds(on) is used to calculate IOCP. I think it refers to the low-side MOSFET but I couldn't find the Rds(on) in the datasheet.

What's the Rds(on) here?

2. What's the enable threshold voltage? It's shown as "typically 1.2V" in P.18 of the datasheet while shown as 1.8V (min.) in P.5.

Thank you.

Best regards,

Roy Hsu

over 6 years ago

0 JohnTucker over 6 years ago

TI__Guru**** 191445 points

Roy,

We are looking into this. I can answer question 2 about EN though
While it does look confusing, it is correct, just counter-intuitive. That 1.8 V is the minimum voltage below which the TPS53318 is guaranteed to be enabled. But more simply there is an enable threshold for the device somewhere in between 0.6 V and 1.8 V. Below 0.6 V, the IC is guaranteed to be disabled. Above 1.8 V the IC is guaranteed to be enabled. The actual threshold (with hysteresis) is in between, typically 1.2 V.

0 JohnTucker over 6 years ago in reply to JohnTucker

TI__Guru**** 191445 points

Roy,

Please use 6.5 mohm for the low side FET RDSon.

0 Roy Hsu over 6 years ago in reply to JohnTucker

TI__Genius 9320 points

Hi John,

Thanks for the comments!

However, I think something in your description should be flipped:

" Below 0.6 V, the IC is guaranteed to be disabled. Above 1.8 V the IC is guaranteed to be enabled."

That said, can I simply use 1.2V as the threshold when designing the resistor divider?

Best regards,

Roy Hsu

0 JohnTucker over 6 years ago in reply to Roy Hsu

TI__Guru**** 191445 points

Roy,

I fixed that, thanks. The EN signal is specified like a digital pin. I doubt there is a tight tolerance in the enable threshold. Are you trying to implement a precision external UVLO? Otherwise, I would design so that the EN pin voltage is above 1.8 V when you want to operate and below 0.6 V when you want to disable.

0 Roy Hsu over 6 years ago in reply to JohnTucker

TI__Genius 9320 points

Hi John,

I will confirm with my customer on that.

By the way, what's the recommended input capacitance at the VIN pins?

Thank you!

Best regards,

Roy Hsu

0 Roy Hsu over 6 years ago in reply to Roy Hsu

TI__Genius 9320 points

Hi John,

Any update on the question above?

Another question is that Rds,on of 7.5 ohm appears in the first page of the datasheet while you suggested me use 6.5 ohm to calculate the OCP threshold.

Which one is the actual Rds,on of the low-side FET?

Thanks!

Best regards,

Roy Hsu

0 JohnTucker over 6 years ago in reply to Roy Hsu

TI__Guru**** 191445 points

Roy,
Let me see what I can find out. I'll try to get back to you later today.

0 JohnTucker over 6 years ago in reply to JohnTucker

TI__Guru**** 191445 points

Roy,

Actually it may be a little longer. The person who supports TPS53314 is currently out of the office.

0 Roy Hsu over 6 years ago in reply to JohnTucker

TI__Genius 9320 points

Hi John,

Actually, I have the same question on TPS53318/9. Please also help check the values of Rds,on of the two devices.

Thank you!

Best regards,
Roy Hsu

0 JohnTucker over 6 years ago in reply to Roy Hsu

TI__Guru**** 191445 points

For the VIN capacitors, the input capacitance needed is based on the amount of the input RMS current the device will see. Once the maximum DC OCL set point is determined, the input RMS current can be calculated to determine the input capacitors needed. As a rule of thumb, we usually spec ~2A RMS per capacitor.

For RDS on typical values are:
TPS53314, HS = 15 mohm, LS = 6.5 mohm
TPS53118, HS = 9 mohm,LS = 4.5 mohm
TPS53119, HS = 9 mohm,LS = 4.5 mohm

0 Roy Hsu over 6 years ago in reply to JohnTucker

TI__Genius 9320 points

Hi John,

Thanks a lot for the clarification.

In that case, you might want the values in the datasheet to be corrected to prevent more questions like this from coming in.

Thank you.

Best regards,

Roy Hsu

0 JohnTucker over 6 years ago in reply to Roy Hsu

TI__Guru**** 191445 points

Roy,

Both sets of values are within the range of possible values. In any case they are "typical" and not guaranteed as the spec is only in the front page marketing information and not in the Electrical Characteristics table.

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TPS53314: A few design questions