ADP3338: Thermal and overcurrent protections

Document created by analog-archivist Employee on Feb 23, 2016
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I´m using the ADP3338 LDO, and I´d like to clarify a question regarding thermal
manegement.
From my point of view the bigger the termal cooling area on the PCB the better
(even in the LDO does not have to dissipate many power).
Imagine that there is a short at the output of the LDO, then the current
limiting protection will work limiting the output current to a maximum value,
but if the cooling PCB area provides a good termal relief  it will take a lot
of time for the thermal protection to shutdown the device.
Then, is there any risk for the device to provide the maximum current for a
"long" time? And in a practical way, is bad a very good thermal relief ?

 

The current & thermal protection schemes in LDOs are generally designed to work
hand in hand.
Let's consider the case where a hard short from the output to ground occurs.
At first the device will current limit, so that ~2A is conducted into the short.
This will cause a fast temperature rise in the junction (die), because the
thermal relief due to bond wires, package, and PCB copper area, will react
(relatively) slowly.
Therefore the thermal shutdown circuitry on the die will activate once the die
is greater than ~160 deg, and reduce/shut off the output current.
With reduced/no output current flowing, there will be no power dissipation, and
no more heating, therefore the die will begin to cool.
Once the die has cooled enough, the thermal protection circuitry will
de-activate.
If the short from output to ground still exists, the part will again conduct
~2A, and again the die will heat up, faster than the thermal relief than
respond.
As long as the short remains, you will have a continuous cycle of current
limit, die temp increase, thermal shutdown on, die cool, thermal shutdown off,
current limit.

To answer your questions:
1. Is there any risk for the device to provide the maximum current for a "long"
time ?
Depends what you mean by a "long" time.
As stated above, the current limit and thermal protection react quickly to
short circuit conditions.
The device's bond wires, metal layers etc. are designed to be able to deliver
the current limit level.
The device will survive short periods of short circuiting.
If left in a short circuit state for a pro-longed period of time, you will
shorten the life of the device for sure.

2. Is bad a very good thermal relief ?
No amount of thermal relief will be able to cope with the die heat-up resulting
from a short circuit.
That’s what thermal protection is for.
Thermal relief reacts slowly. Even if it responded quickly (very low theta ja &
jc numbers), there is only so much thermal relief it can provide.
Thermal relief benefits during normal operation.
It will allow you to use the device over a wider vin, vout, load, and ambient
temperature range.
Always use as much thermal relief as possible.

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