TPS54232 as a constant current power

Philippe CHAPUS

Other Parts Discussed in Thread: INA193, TPS54232, TPS62175, TPS54231

Hello,

I need to power a sensor (resistive Wheatstone bridge 2x R = 8 ohms) with a constant current I = 150mA.

I use a TPS54232 converter, with a current monitor INA193 : The INA193 measure the current, and its output is injected in the reference pin of the TPS54232.

The output is not stable (Ring Vpp = 0.5V).

I think I haven’t choose the right compensation component, because the feedback is an INA193, I was not sure the datasheet calculation for the compensation components apply. So I use the “standard” component values from the datasheet, and decided to manufacture the prototypes.

Texas WEBENCH simulator can’t perform the simulation, and the error message is “failed minimum Ton”. I don't understand this error.

Could you help me to fix this problem?

The characteristics of my design are :

Vin =24V +/-10%.

Iout = 150mA.

Vout = 2.4 to 5.4V (depending the sensor + cable resistance )

My schematic :

Webench simulation failed....

over 10 years ago

0 JohnTucker over 10 years ago

TI__Guru**** 191445 points

The compensations does look off to me. Webench cannot simulate it because your output voltage to input voltage ratio is too low. I violates the minimum on time constraint so you will be skipping pulses. Let me see if we can propose a better solution.

0 Chris Glaser over 10 years ago

TI__Guru**** 152430 points

With such a low output current, the TPS62175 is a much smaller solution.

0 JohnTucker over 10 years ago in reply to Chris Glaser

TI__Guru**** 191445 points

Will it easily support constant current operation?

0 Chris Glaser over 10 years ago in reply to JohnTucker

TI__Guru**** 152430 points

Sure. I don't see why it can't be used as the schematic above shows.

0 Philippe CHAPUS over 10 years ago in reply to Chris Glaser

Prodigy 160 points

John, Chris : Thank you very much for your answer.

Yes I agree, the TPS62175 is a bettersolution.

However :
The TPS52231 and TPS542333 are pin to pin compatible with TPS54232, and are lower frequency.
Since the board already exist, It is easyer to me to use them instead of TPS62175.
Do you think because they work at a lower frequency, Ton minimum time will be ok, and the outpout won't oscillate ?

If yes, which compensation component value should I use for my application (constant current 150mA, see schematic above).

Philippe

0 JohnTucker over 10 years ago in reply to Philippe CHAPUS

TI__Guru**** 191445 points

You may be able to try one of the lower frequency devices. One issue you may have with them is pulse skipping. At light loads these devices will enter eco-mode and pulse skip. It is normal operation, but may not be desirable for your application. Also, you cannot measure the loop response in skip mode. You should probably design your compensation for CCM operation, then you will still be stable at lighter loads.

0 Philippe CHAPUS over 10 years ago in reply to JohnTucker

Prodigy 160 points

How can I design the compensation for CCM operation with the TPS54232?
Could you propose me an AN?

0 JohnTucker over 10 years ago in reply to Philippe CHAPUS

TI__Guru**** 191445 points

Set the load current > 1/2 ILp-p = 1/2 * (Vout/Vin) * (Vin-Vout) / (Fsw*L). You should be able to let WB design the compensation for you.

0 Philippe CHAPUS over 10 years ago in reply to JohnTucker

Prodigy 160 points

This is already my design :
With Fsw=1MHz and L =10µH : 1/2 Ilpp = 108mA.
Iload = 150mAW > 108mA.

Even with higher Iload, Webench failed to simulate if Vout = 2.4V when Vin = 24V. It can simulate if Vout >=3.0V. (failed minimum Ton)

0 JohnTucker over 10 years ago in reply to Philippe CHAPUS

TI__Guru**** 191445 points

TPS54231 (570 kHz) is able to run 24 +/- 10% to 2.4 V in webench.

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TPS54232 as a constant current power