TL783: Output drop

Hirotsugu Kobayashi

Mastermind 8985 points

Part Number: TL783

Hi team,

We have trouble about TL783 circuit.

When the power is turned on, it regulates to 24V, but after few minute the output gradually drops to about 17V.

-We use Cadj, could this be the cause?

-We are using 1kΩ for R1, but if the load current is 15mA or more, will it work even if it is not 82Ω?

-A large value of R1 increases the effect of leakage from the ADJ pin. Could this be the cause?

Best Regards,

over 1 year ago

0 Juliette over 1 year ago

TI__Mastermind 18466 points

Hey Hirotsugu,

Please allow me 1-2 business days to look into this.

Best,

Juliette

0 Hirotsugu Kobayashi over 1 year ago in reply to Juliette

Mastermind 8985 points

Hi Juliette,

Could you please respond to my question?

0 Juliette over 1 year ago in reply to Hirotsugu Kobayashi

TI__Mastermind 18466 points

Hey Hirotsugu,

I'm sorry I must have missed this E2E question.

Cadj could be affecting the output of your LDO. Does this problem persist if the capacitor is removed?

Because the output of this device drops after "minutes" I'm lead to think this may be a thermal issue.

Junction temperature cannot exceed 125C

The part needs a minimum of 15mA and a low R1 helps ensure that. However, if your load is going to be greater than 15mA, then you should be ok.

Best,

Juliette

0 Hirotsugu Kobayashi over 1 year ago in reply to Juliette

Mastermind 8985 points

Hi Juliette,

The datasheet specifies the input voltage slew rate, but what happens when a fast slew rate is applied?

Best Regards,

0 Juliette over 1 year ago in reply to Hirotsugu Kobayashi

TI__Mastermind 18466 points

Hey Hirotsugu,

When a faster slew rate is used to turn on the part, the output of the part can overshoot or cause the part to ring/become unstable if the adequate capacitors are not used.

Overshoot can trigger overvoltage protection, which can cause the system to fail to power up, cause it to restart constantly, or, even worse, result in damage to the next stage or load

Best,
Juliette

0 Hirotsugu Kobayashi over 1 year ago in reply to Juliette

Mastermind 8985 points

Hi Juliette,

Regarding the output drop problem, there is no IC temperature rise, and the output current is only 44mA.
After turning the power on and off several times, the problem disappears.
What could be the problem with such a phenomenon?

Best Regards,

0 Juliette over 1 year ago in reply to Hirotsugu Kobayashi

TI__Mastermind 18466 points

Hi Hirotsugu,

Would you happen to have scope shots that have a probe connected to VIN, VOUT, and IOUT.

I think that power cycling the device could be resetting something that the LDO is powering , making the LDO come out of current limit or thermal shutdown.

After power cycling, it doesn't go back into this lower voltage state?

Best,

Juliette

0 Hirotsugu Kobayashi over 1 year ago in reply to Juliette

Mastermind 8985 points

Hi Juliette,

This phenomenon occurs within 2 or 3 power cycles, but disappears after repeating the power cycle many times.

Could the protection circuit malfunction?

Best Regards,

0 Juliette over 1 year ago in reply to Hirotsugu Kobayashi

TI__Mastermind 18466 points

Hey Hirotsugu,

Honestly, this seems peculiar. I am going to message our product engineering team to see if we have seen an issue with this part in the past.

Best,

Juliette

+1 Juliette over 1 year ago in reply to Juliette

TI__Mastermind 18466 points

Hello Hirotsugu,

This part may have SOA protection meaning that it reduces the output if there is too much power being dissipated through it - similar to how thermal protection works on our newer parts.

A way of testing this is by lowering the input voltage to see if this happens still.

This part was released in 1981 so we have very limited information on this. We do not have SOA limits on new devices, so if this test works we would definitely recommend switching into a newer part.

Best,

Juliette

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TL783: Output drop