LM3405A question of the Rzi calculation

Ryuji

Guru 20090 points

Other Parts Discussed in Thread: LM3405A

Hello,

I have the question of LM3405A about RZI calculation.
Please see the attached file.

LM3405A_RZI.pdf

I simulated the LM3405A with some Vin value( Vin=9V, Vin=18V, Vin=9V~18V)
As a result the Rzi values as the following.

Vin=9V , Rzi=634ohm
Vin=18V, Rzi=2.05kohm
Vin=9V~18V, Rzi=2.05kohm

I could find the calculation of Rzi from the datasheet P.10 and figure 17.
I think that we should choose 634ohm for Vin=9V~18V.
The Rzi is used for charge current for zener and boost cap.
Thus, the Vin(min) is worst case of charge current.
Is my understanding correct?

Thus , if we use E24 resistor, should we use 620ohm of Rzi for Vin=9V~18V, Vout=3.4V, Iled=700mA application.

Best Regards,
Ryuji Asaka

over 9 years ago

0 Clinton Jensen over 9 years ago

TI__Mastermind 25500 points

Your understanding is correct, use the value for the lowest input voltage to ensure you have enough current. Just also be sure that at the maximum input voltage that the zener you choose can handle the extra power.

0 Ryuji over 9 years ago in reply to Clinton Jensen

Guru 20090 points

Hello Clinton san,

Thank you for the reply.
I attached file in previous post is the webench simulation result.
So, is the webench simulation wrong?
If yes, please revise the simulaiton.

Best Regards,
Ryuji Asaka

0 Clinton Jensen over 9 years ago in reply to Ryuji

TI__Mastermind 25500 points

Webench designs are generally good, but they are not always perfect. At times there is very little interaction between the developers and the product line. It's always best to refer to the datasheet before finalizing a design from webench. I can forward this information to the webench team and they will look into it. Thanks.

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LM3405A question of the Rzi calculation