LNK306 - Expecting 5.0V output... getting 10.4V output instead

I also wanted to add that as an experiment, I swapped out that 3.84K resistor with another 2K resistor, and nothing changed... still getting 10.4 volts. I'm pretty confused.

So I figured out what the initial problem was... I put a 3.3K load resistor on the output and it fixed it... so apparently my circuit was not pulling enough current?

I am getting a perfect 6.00 volts instead of 5.00 volts though, even though according to the calculations it should be a 2k resistor, and a 3.84k resistor.

According to what I have been experiencing (e.g. 2k & 3.84K = 6 volts, 2k & 2k = 4.48 volts), am I to assume that I need a 2k & 2.63k to get the desired 5 volts? 

Hello Mahonroy,

Thank you for considering using PI productor. 

In high-side feedback designs where the minimum load is <3 mA, a pre-load resistor is required to maintain output regulation. This ensures sufficient inductor energy to pull the inductor side of the feedback capacitor to input return. The value of pre-load should be selected to give a minimum output load of 3 mA.

From your schematic, i found you have little load at output. you can try with 1.65K resistor. this should give you the regulation. 

Best Regards

Jia

Thanks for the response! Yeah I realized I had selected the 3.3K off the 12V schematic by mistake. I swapped in a 1K instead since I have these on hand... however I am getting 4.5 volts output using a 2K and 3.84K feedback resistor. 

Is this normal? Do I just need to try different resistors until I get the desired 5.0v? According to Pi Expert Suite, the 2K and 3.84K should provide the 5.0v output.

Hello Mahonroy,

because the current throughD1 and D3 are not exactly the same, the voltage drop between thses two diode are not exactly the same. sometimes you need to adjust the resistor value to give you exactly 5V.

what kind of resistor were you using for the feedback divided R1 and R2. Are they 1% tolerance? 

The output load regulation will also give you small change aboutoutput voltage when you changing the load, but not too much. if possible, can you connect some e-load at the output and sweep the load from 4mA to 100mA to see how the output voltage will change. it seems to be a big change for me.

By the way, can you give me the part number of the inductor and how much is the ESR of your output capacitor C8 

Hello Mahonroy,

because the current throughD1 and D3 are not exactly the same, the voltage drop between thses two diode are not exactly the same. sometimes you need to adjust the resistor value to give you exactly 5V.

what kind of resistor were you using for the feedback divided R1 and R2. Are they 1% tolerance? 

The output load regulation will also give you small change aboutoutput voltage when you changing the load, but not too much. if possible, can you connect some e-load at the output and sweep the load from 4mA to 100mA to see how the output voltage will change. it seems to be a big change for me.

By the way, can you give me the part number of the inductor and how much is the ESR of your output capacitor C8 

Hello,

I am using the same diode for D1 and D3. Yes all resistors are 1% tolerance. I have all of the part #'s listed in my original thread up top, but the part # for the inductor is "46105C"

The impedance of the output capacitor C8 is 94 mOhm (the datasheet lists impedance instead of ESR), part # "35ZLH100MEFC6.3X11"

Hello Mahonroy,

can you capture the drain current or inductor current to see if the indutor is already saturated,. the datasheet didn't show any saturation current spec. the RMS current rating it show is only 0.27A. I am wondering if the inductor is saturated already.

Best Regards

I'm not entirely sure how to take that measurement. Are you suggesting I remove the inductor, connect one side of the inductor to the S pad like it was originally, and take a current reading between the other terminal and the 5V rail?

I'm looking at the datasheet for the inductor, can't we use the "Inductance vs. Current" graph? According to the graph the inductance doesn't drop off till after 0.3 amps or so.

Also, wouldn't I have to be pulling more than this 0.27 - 0.30 amps from the power supply in order to saturate the inductor?

Hello,

the peak current go through the inductor will be 482mA, the current limit. i am wondering if this will saturate your inductor or not. you can cut the trace and put a loop there or you can lift your indcutor and put a loop there and use an oscillascope to monitor the current.

normal probe is enough or any special probe is require. the similar design i am using. but i don't have D4 diode. how can i measure using oscilloscope. if i probe its blasting. if inductor saturATE it will make any ripple in output ?

Hello rajakumar.r,

Thank you for interested in using Power Integrations product.

Could I ask which probe and where do you probe when it blasted?

If you want to check the inductor current, normal probe is OK and you can probe in the same place as I mentioned. you can cut the trace and put a loop there or you can lift your indcutor and put a loop there and use an oscillascope to monitor the current.

Best Regards

thanks for your support. i don't have current probe. i want to measure the output voltage. so i checked with TBS1104 DSO and 10Mohm/12pf CAT 2 type voltage probe (TPP0201). in my case AC neutral and DC ground is common. there is no diode on the neutral side.
after this power integrator output i have 7805 regulator. so i just probed 12V output with respect to ground and 5 V output with respect to ground. is it require any differential probe to measure this voltage or single end probe is fine. if require what kind of probe we want to use.

Hello rajakumar.r,

it is better for you to have an isolation transformer for your oscilloscope power. In this way you can connect your probe ground to the neutral.

Best Regards