TL1963A: Vout seems proportional to Vin when SHDN pin pulled down.

Sukwon Yoo

Part Number: TL1963A

This is a similar post to https://e2e.ti.com/support/power_management/linear_regulators/f/321/t/591923. But the three possible answers on this post do not apply to my design.

I have TL1963AKTTR in my design and, by default, the SHDN pin is pulled down to the ground with 10k resistor. I have 470uF output capacitor on Vout rail but it is not connected to the load.

When Vin is 8V, Vout is about 0.2V and when Vin is 18V, Vout is ~3.6V. One more test note is that while SHDN pin is being pulled down and Vin is set to 18V, I shorted the output cap with a tweezer monitoring Vout with a scope. Vout slowly (in sec) increased to ~3.6V.

I expected 0V on Vout rail when SHDN is pulled down. Please let me know if there is something that I missed.

Thanks

over 7 years ago

0 Jason Song over 7 years ago

TI__Mastermind 23361 points

Hi Sukwon,

Will you help to clarify the following questions?

1. Since you are using adjustable version of this device, what are the values of the resistor divider?
2. "I have 470uF output capacitor on Vout rail but it is not connected to the load." What do you mean by saying an output cap not connected?
3. Can you provide the part number for your output cap?
4. Do you have any load on the output of the LDO?

Regards,
Jason Song

0 Sukwon Yoo over 7 years ago in reply to Jason Song

Prodigy 70 points

Hello Jason-

1. Vout to Sense is 30.9k and Sense to GND is 3.01k.
2. Load is off when SHND pin is low. 470uF is on Vout rail.
3. EEH-ZK1E471P
4. Output load is off. (No load)

Thanks

0 kai klaas69 over 7 years ago in reply to Sukwon Yoo

Guru 140200 points

Hi Sukwon,

what happens if you connect the /SHDN pin directly to GND?

Kai

0 Jason Song over 7 years ago in reply to Sukwon Yoo

TI__Mastermind 23361 points

Hi Sukwon,

It seems to be the Vout voltage is due to leakage current from Vout pin during shutdown through SHND. When you have 34kOhms resistance load on your output, when a leakage of 100uA, it will push the vout pin voltage to a few volts.

I would suggest to decrease the size of the resistor. For example, change the 30.9K to 3.09K and 3.01K to 301.

Regards,
Jason Song

0 Sukwon Yoo over 7 years ago in reply to Jason Song

Prodigy 70 points

Hello Jason-
SHND is active-low, I don't know how can it get leakage current from SHND (=GND) pin. Again, when the LDO is in shutdown mode, 17V at Vin, current consumption is about 250uA in my setup. I will test again with recommanded resistor value.

Thanks

0 kai klaas69 over 7 years ago in reply to Sukwon Yoo

Guru 140200 points

Hi Sukwon,

in your first post you wrote: "...the SHDN pin is pulled down to the ground with 10k resistor."

What does this mean? Is the 10k resistor a pull-up resistor? Or do you have a 10k resistor mounted from the SHDN pin to GND?

Kai

0 Sukwon Yoo over 7 years ago in reply to kai klaas69

Prodigy 70 points

Hello Kai-

I have a 10k resistor from the SHDN pin to GND.

Thanks

0 kai klaas69 over 7 years ago in reply to Sukwon Yoo

Guru 140200 points

Hi Sukwon,

connect the SHDN pin directly to GND and check again.

Kai

0 Jason Song over 7 years ago in reply to kai klaas69

TI__Mastermind 23361 points

Hi Sukwon,

When device is in shutdown mode, internal pass element is OFF, OFF does not mean there is zero current out from the Vout pin. The leakage from the Vout pin may find a path to ground which will pull your Vout to a voltage slightly higher than ground. If it is due to the leakage, reducing the resistors divider's value should make Vout reading smaller at shutdown. We are trying to rule out all the possibilities one by one.

Regards,
Jason Song

Power management

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TL1963A: Vout seems proportional to Vin when SHDN pin pulled down.