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Consequence Going Beyond Max Voltage Drop Out

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itslopete

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Consequence Going Beyond Max Dropout Voltage

Hello All,

I will be using the LMS1585 or LMS1587 (they share the same datasheet) Low Dropout regulator. My input voltage is 5v and output voltage is 3.3v. Estimated output current is 1.74 Amps and I may go beyond this. If I go beyond the manfacturers spec of Max Dropout Voltage of 1.3v, will the LDO fail at this point? What happens during failure?

If not, is there calculations I can use to estimate the behavior of this LDO Regulator at the dropout voltage of 1.7v?

Thanks everyone!
 
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keith1200rs

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The dropout voltage is 1.3V maximum at 3A so you sound like you have plenty of spare.

In general I would think that if you run the input too low then the output will track the input by the dropout voltage (for a given current). So, with 4V in you might expect 2.7V out at 3A. As there aren't any performance graphs with such information the only way to find out is to measure what happens.

Keith
 

itslopete

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Attached is the link to the lms1858a datasheet: https://www.ti.com/lit/ds/symlink/lms1585a.pdf

Thanks for the reply Keith. I assumed that the datasheet stating a MAXIMUM dropout voltage (Vin - Vout) meant my Vin could not go beyond 3.3 + 1.3 = 4.6v. The behaviour you've described would refer to a MINIMUM dropout voltage non?

My dilemma is that my Vin is 5v which means Vin - Vout = 5v - 3.3v = 1.7v which is beyond the maximum dropout voltage of 1.3v. Please let me know if someone I've misinterpreted the datasheet in any way.
 

godfreyl

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Please let me know if someone I've misinterpreted the datasheet in any way.
Yes, It's like Keith said.
The input voltage has to be greater than the output voltage + the dropout voltage. So if Vout = 3.3V and Vdropout = 1.3V, then the minimum input voltage is 3.3 + 1.3 = 4.6V.

I think the confusion comes from the term "MAXIMUM dropout voltage". Dropout voltage is a property of the chip. If there's less than that amount of voltage between input and output, the chip won't work properly.

So if the maximum dropout voltage is 1.3V, then it's possible that some chips will have a dropout voltage of say 1.1V.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In other words:
"Maximum dropout voltage" is not the maximum voltage allowed between input and output.
It's the maximum possible value of the chip's dropout voltage, where the dropout voltage is the minimum voltage it needs between input and output.

- - - Updated - - -

I have to admit the way it's specified in the datasheet is a bit misleading, but consider this:
If you look at the top of page 5 of the datasheet (page 6 of the PDF), the current limits are specified with Vin - Vout = 5.5V.
Also, the "typical application circuit" at the top of page 1 shows Vin >= 4.75V for Vout = 3.3V.

At the top of page 4 it says "Maximum Input to Output Voltage
(VIN to GND) = 13V". -10 points to whoever wrote that, since it's self contradictory, but you get the idea.
 

itslopete

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Thanks guys for your great input. That explains a lot! Cheers!
 

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