TPS82084: device minimum off-time ?

Don Foong

Part Number: TPS82084
Other Parts Discussed in Thread: TPS62135, TPS82085

Hi Team,

Understand from the datasheet of TPS82084, is a synchronous step down converter with both high and low side switch using N-Ch MOSFET and is capable of 100% duty cycle operation. But...

1. For the high side N-MOS to be turn on, the gate needs to be higher than the source
2. To generate higher voltage for the gate, you will need some kind of pump charge, and for the pump charge to work, some switching is needed somewhere.
3. If some kind of switching is needed, how can the high side MOSFET be turn on 100%?

Is there a minimum-off-time require for the device ?

Pls help to clarify how 100% duty cycle is possible with TPS82084 ?

Looking forward to your feedback. Thanks in advance for the help.

over 6 years ago

0 Chris Glaser over 6 years ago

TI__Guru**** 150750 points

Hi Don,

Please have a look on the product folder and find this app note at the very top: www.ti.com/.../slyt747.pdf

0 Don Foong over 6 years ago in reply to Chris Glaser

TI__Expert 4100 points

Hi Chris,

Thanks for the reply and sharing of the document on 'Understanding 100% mode in low-power dc/dc converters'.

Hope you can help to clarify a little,

I do no see the document mention any TPS82040. Do you mean the TPS82084 has internal charge pump circuit similar to that of TPS62135 ?

For TPS62135, when the minimum off-time (typ 80nS) is reached, it scale down its switching frequency while it approaches 100% mode. Does this happened to TPS82040 ?

Thanks again.

0 Chris Glaser over 6 years ago in reply to Don Foong

TI__Guru**** 150750 points

Hi Don,

What does your customer need for their application? Are they debugging something around the TPS82084 or are they looking for a device recommendation? We can discuss these details of 100% mode, but I am not sure if it matters for your application.

Yes, the app note says it applies to all TPS62 devices. TPS82 is just the module version of these. Competitors do this too (NFET + charge pump), so it is not new and not unique. Why does the customer doubt this operation?

Yes, the off time is a characteristic of most devices with DCS-Control.

0 Don Foong over 6 years ago in reply to Chris Glaser

TI__Expert 4100 points

Hi Chris,

You have answered my questions. Thanks for supporting this enquiry.

0 Don Foong over 6 years ago in reply to Don Foong

TI__Expert 4100 points

Hi Chris,

Our customer is considering to use TPS82084 in a noise sensitive circuit and like to know the operating frequency of the internal charge pump circuit when the device run in 100% mode. Can you advise, thanks.

0 Chris Glaser over 6 years ago in reply to Don Foong

TI__Guru**** 150750 points

Hi Don,

Let me see if I can find something that I can share on E2E. Otherwise, I will email you.

I will say that I have never seen a customer issue from this. This or the higher current version of this device, TPS82085, are used on the highest frequency RF sampling data converter EVMs, like www.ti.com/.../tsw40rf80evm. In this design, it is used without LDOs to power an extremely sensitive analog load: www.ti.com/.../slaa758.pdf

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TPS82084: device minimum off-time ?