TPS81256 input capacitor

c_ 64

Other Parts Discussed in Thread: TPS81256, TPS81256EVM-121, TPS60400

Hi,

The spec sheet for the TPS81256 states that in most applications an external input capacitor is not needed (since there is already a built-in one). However, the TINATI transient model for this chip includes a 150uF input capacitor. This would imply a large component (1210 or something similar), really too big for my available area. Any recommendations? My specs are these: I'm making two voltage rails, a +5V with a current draw between 18 and 30mA, and a -5V rail with a current draw between 1 and 11 mA (depending on what sensor I will be using). For the -5V rail I will be using the LTC1751-5 from Linear, but will be cascaded after the TPS81256, so the current draw for the -5V rail will go through this IC). The source is a 3 V (nominal) Lithium battery.

Thanks!

over 12 years ago

0 Chris Glaser over 12 years ago

TI__Guru**** 150610 points

The TINA model just copies the EVM design: http://www.ti.com/tool/tps81256evm-121. The EVM includes the input cap as we have no idea what people will connect on the input. With your battery, you don't need the large input cap.

You could use our TPS60400 family of charge pumps and save a lot of money vs. the LT device.

0 Richard Elmquist over 12 years ago

TI__Mastermind 45250 points

c_64,

I ran simulations with the transient model using your load and voltage parameters and it looks like with your loads that the internal capacitance should be fine.

Here are the plots with no Cin:

And with 150uF Cin:

They response is almost identical.

Please let me know if you have any further questions.

Richard Elmquist

0 c_ 64 over 12 years ago in reply to Richard Elmquist

Intellectual 630 points

Excellent, thank you for the response : ) And thank you (Chris) for the suggested TI series for inverting charge pumps.

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TPS81256 input capacitor