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This is my LDO. 3A part.

  • Input is 12V
  • Output is 5V.
  • Load current is 2A.

As per Figure 2-41 on page 15, at 85degC, I am limited to 0.2A of max load current.

How can I get the 2A load current with the Vin-Vout of 7V?

If I calculate the Tj of device at this condition, it is = (14*56)+85 = 869degC. That's not within the limits.

I'm actually using a TO-263 package, but I have taken values of 56 thermal resistance for the TO-252 package which is a close one.

Even if 2A load at Vin-Vout=7V is OK from the SOA graph, my Tj is very high. The device is not actually safe, right? What is wrong in my understanding?

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  • \$\begingroup\$ As you intended to ask about 0.2A load instead of 2A, edit the numbers in. \$\endgroup\$
    – Justme
    Mar 17, 2023 at 6:47
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    \$\begingroup\$ At 2 A, forget the LDO and get yourself a buck converter. \$\endgroup\$
    – winny
    Mar 17, 2023 at 7:19
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    \$\begingroup\$ Even if you boost the output current with an external transistor, 2 A at 7 V is 14 W, which is a huge amount of power to waste. Don't use a linear regulator for this. \$\endgroup\$
    – Hearth
    Mar 17, 2023 at 14:22

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If you look at the bottom axis of that chart you see that at a \$V_{in}-V_{out}\$ of 7 V and \$T_A\$ of \$85^\circ C\$ the current is around 1/4 A, so a 2 A load at that voltage is not okay by the SOA graph. Figure 2-42 shows pretty much the same thing. If you want to get 2 A out of it you'd need to reduce the input voltage or maybe heat sink it enough to keep it in the SOA, but that might not be possible. When you look at an SOA chart you want to stay at that horizontal part of the curve before it starts to drop. For high \$V_{in}-V_{out}\$ a switching regulator is generally better.

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  • \$\begingroup\$ Thank you for the answer. So, yes, my bad. It is 200mA only. Even with 0.2A, right, 7 * 0.2 = 1.4W. (1.4 * 56) + 85 also is high junction temperature, right? \$\endgroup\$
    – Freshman
    Mar 17, 2023 at 6:38
  • \$\begingroup\$ Hence, it is not even safe to drive 0.2A, with that differential voltage of 7V? \$\endgroup\$
    – Freshman
    Mar 17, 2023 at 6:38
  • \$\begingroup\$ @Freshman I think with 200mA you're okay, getting close though. Maybe higher with enough cooling, but you're better off reducing the input voltage, it's an LDO after all, it's designed to work with a low Vin - Vout. If you're stuck using 7V in you might consider a switching regulator. \$\endgroup\$
    – GodJihyo
    Mar 17, 2023 at 6:50

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