LM4140: Question about Vout drop

Satoshi

Guru 19485 points

Part Number: LM4140
Other Parts Discussed in Thread: TL431

Although customer want to make 7.5Vin to 3.8Vout for LM4041CEM3X-ADJ, attached simulation results was less than 2Vout.

I think that current is enough (IRMIN(max.)<60uＡ), but actual measure was less than 2Vout, too.

If there find a cause or mistake point in attached schematic, please let me know.

WebenchReportsServlet.p3.8A.pdf

Best regards,

Satoshi

over 6 years ago

0 Diego Lewis over 6 years ago

TI__Intellectual 1540 points

Hi Satoshi,

The LM4041-adj is a common cathode shunt reference, not common anode like the TL431.

The output voltage equation can be seen in figure 19 below from the datasheet. Note that the ratio is R2/R1, not R1/R2.

You should flip Rfbb and Rfbt.

Regards, Diego

0 Satoshi over 6 years ago in reply to Diego Lewis

Guru 19485 points

Diego-san

Thank you for reply.

Sorry for addition,

Although attached report is simply schematic, but Vout was increased.

Is it normal operation? (I thought that Vout will 1.24V, but result was different)

WebenchReportsServlet_P1.2A.pdf

Best regards,

Satoshi

0 Diego Lewis over 6 years ago in reply to Satoshi

TI__Intellectual 1540 points

Satoshi-san,

As I mentioned above the LM4041 is a common cathode shunt reference, not common anode like the TL431.

If you look at the equation above you can see that R2 needs to be zero in order for the gain to be 1, not R1. Therefore, the REF pin needs to be shorted to the anode (ground) not the cathode.

Regards, Diego Lewis

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LM4140: Question about Vout drop