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Confirmation on How Does TPS2511 CS Pin Work

Bohdan Dackow
Prodigy 20 points

Hi,

I am hoping someone is able to help me with a newbie question. I am using the nCS pin to compensate for voltage drop and have two questions:

1) Docs say nCS changes state when current is 50% of ilim_set value (rlim is 51k for 1A). I am observing a change at ~250ma which is 25%. Does the handshaking have any impact on when nCS changes state?

2) Based on my reading of the docs I think the way the nCS pin works is: When current is below 50% then the CS signal is low which means that the nCS pin is connected sinking the current putting the resistor (R4) into the voltage divider circuit used for FB. When current is > 50% then CS goes high which then means that nCS opens and there is no current flow through R4 taking it out of the voltage divider circuit. Am I reading this correctly? The docs say that you can calc the voltage change by selecting R4, but it appears to lower the voltage and not raise it by the calculated amount. My voltage divider calcs confirm this as does my testing. I am using R1=100k, R2=1.5M, R3=180k, R4=1.2M. I use Vfb=0.5v. The low current V=5.337V and the high current V=4.948.

What am I doing wrong?

Any help would be greatly appreciated.

Thanks - Bohdan

  • 0
    TI__Mastermind 42795 points

    For #1, the datasheet only says that CS will change from open drain to low when the channel current goes ~50% above what is programmed. So, no handshaking effects are discussed. There is hysteresis which comes into play after the current goes above 50% and then falls back down. Do you think the hysteresis effect is what you are observing on the bench?

    For the equations and calculations for choosing the resistors, are you following the guidelines on page 19 and are these EQ's helpful?