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High-side current sources for industrial applications, ADJ 2Q 2019 clarification

Nicola Fiorucci
Intellectual 300 points
Other Parts Discussed in Thread: CSD18541F5

Hi, in the table 3 the Iout_max is calculated as 21mA using the formula Pmax/Vds_max =0.5/23 = 21mA.

In the datassheet of the M1 mosfet used in Figure 2 (CSD18541F5) the Vds_max is 60V and not 23V as stated in the table.

So the Iout_max would be 0.5/60=8.3mA

Is this correct? 

Thanks

  • 0
    TI__Expert 3500 points

    Hello!

    Vds_max here is limited by the available supply, and compliance range of the current source, not by the maximum rating of the MOS transistor. in the given example Vcc=24V, as a quick evaluation I used 23V assuming only 1V drop over the Rset (which is not exact). this will lead to the number calculated (21mA)

    However, If we shorted the output we can get the exact numbers by solving those two equations simultaneously in Imax, and Vdsmax (providing Rset, and Vcc):

    1.) Imax*Rset+M1_Vds_max=Vcc  (assuming short circuit)

    2.) 0.5/M1_Vd_smax=Imax

    the solution of the resulting quadratic equation is Vds_max=21V, Imax=23.8mA

    Regards,

    Ahmed