Voltage loss on LT3508 UVLO pin

Thread Starter

Dukel192

Joined Feb 21, 2018
16
I designed a PCB with the LT3508 Step-down regulator. To enable the IC, a voltage exceeding 2.63[V] should be applied to the SHDN (UVLO) pin. Therefore I connected a voltage divider to this pin (36K6 and 21K resistor), which should give me 4[V] over the 21K when applying 11.1[V]. However when I connected the power supply I got 1.2[V] over the 21K resistor and on the SHDN pin (which doesn't exceed the 2.63[V] threshold)

The datasheet for the LT3508 can be found in the link below.
https://www.analog.com/media/en/technical-documentation/data-sheets/3508fd.pdf

Best regards,
 

ronsimpson

Joined Oct 7, 2019
684
It takes 6 to 10uA to pull the pin high. (at 2.7v)
If you think of 2.7V and 10uA that is like a resistor to ground. (you do the math) Now add that to your voltage divider.
Shut down pin can go as high as 40V.
You could use only a pull up resistor that will give you 20uA and no resistor to ground.
 

Thread Starter

Dukel192

Joined Feb 21, 2018
16
It takes 6 to 10uA to pull the pin high. (at 2.7v)
If you think of 2.7V and 10uA that is like a resistor to ground. (you do the math) Now add that to your voltage divider.
Shut down pin can go as high as 40V.
You could use only a pull up resistor that will give you 20uA and no resistor to ground.

Hi Ronsimpson,

I'm not sure what you mean with 2.7V and 10uA is like a resistor to ground and adding this to the voltage divider. I continued my search and removed both resistors. I connected the power supply via a 31k6 resistor to the pin. Now the voltage becomes 0.14V at the pin side. I don't know what to make of this. There seems to be no short circuits as far as I can see.
 

Thread Starter

Dukel192

Joined Feb 21, 2018
16
I designed a PCB with the LT3508 Step-down regulator. To enable the IC, a voltage exceeding 2.63[V] should be applied to the SHDN (UVLO) pin. Therefore I connected a voltage divider to this pin (36K6 and 21K resistor), which should give me 4[V] over the 21K when applying 11.1[V]. However when I connected the power supply I got 1.2[V] over the 21K resistor and on the SHDN pin (which doesn't exceed the 2.63[V] threshold)

The datasheet for the LT3508 can be found in the link below.
https://www.analog.com/media/en/technical-documentation/data-sheets/3508fd.pdf

Best regards,
Okay I'm stupid. The chip was upside soldered upside down
 
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