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Original question:

LM7480-Q1: Mosfet Fails in Pulse Test (Jumpstart)

LM7480-Q1: LM7480x-Q1 Questions about datasheet and MIL-STD1275E circuit

Thomas Leipfinger
Prodigy 80 points
Part Number: LM7480-Q1
Other Parts Discussed in Thread: LM5069, TIDA-010242, LM74800-Q1

Hello Team,

i'm working through the datasheet and the APP-Note TIDA-010242. If i look at the schematic of MIL_STD_1275E_LM5069_Clamping_Circuit, i wonder about diode D1.

Can you clarify the function of the D1 in this circuit? At the moment i can only see a variation (amount of current on top) of calculated inrush current, because of 1µF capacitor belonging to the VS-Zener diode is charging via a path through Q1 D to S, through D1 to 1µF capacitor.

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Another thing is capacitor named "Ca". There are different values in the data sheet examples and the MIL-schematic

Data sheet figure 10-25 Ca=1µF

MIL-schematic: Ca=C8=0.015µF.

Is there any rule to calculate this capacitor, or how can i get the right value? From what parameters does it depend?

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And last but not least, is it right that LM74800-Q1 can block reverse current and LM74801-Q1 not, or is the only difference the way how they do it?

74800 with the help of linear regulation and 74801 with comparator?

Thanks a lot and best regards

Thomas

  • +1
    TI__Guru** 103221 points

    Hi Thomas,

    The purpose of D2 is to supply voltage to the VS pin from A pin. The 10 kohms in series with VS pin (which is placed to limit the current into Zener) limits the current required by the charge pump during startup conditions. So, a diode from A to VS provides uninterrupted current into the VS pin. 

    The capacitance CA on the A pin acts as a decoupling capacitor. Without a cap on the A pin, the noise on A pi pin can cause the voltage across A and C to be less than V(AC_REV)  resulting in turn OFF of DGATE. Higher value for CA is better. Please use 1uF

    Both LM74800-Q1 and LM74801-Q1 blocks reverse current but the way they achieve it is different. Please refer Section "6.1 Linear Regulation Control Vs Hysteretic ON/OFF Control" in app note https://www.ti.com/lit/an/slvae57b/slvae57b.pdf for more details.

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 80 points

    Hi Rakesh,

    thanks for quick reply. This helps to understand the mode of operation. Solved!Thumbsup