Post Go back to editing

# LT3575 Simulation problems output voltage

I´m about to simulate the LT3575 in LTSpice.

The datasheet provides information about programming the output voltage by adjusting the resistors Rfb and Rtc.

Unfortunately I have some problems by doing so. I want to get an output voltage of 15V by an input voltage of 12V using the Würth Elektronik 750370058 with an inductance of 25uH and a winding ratio 1:1.

I attached my simulation file “LT3575+Wuerth.asc” with 12V input, Rfb = 80.6kOhm and Rtc = 80.6kOhm which should provide an 15V output voltage regarding table 1 of the LT3575 datasheet.

LT3575+Wuerth.asc

But my simulation has an output of about 18V.

I also attached a screenshot of my simulation and circuit diagram showing the 18V output.

Adding an output resistor in parallel to the capacitor changes the output voltage, but nevertheless it does not reach 15V. Moreover I don´t get the relation between the output voltage and output resistor and can´t find information on that in the datasheet.

This circuit provides an 5V output voltage with a winding ratio of 3:1.

Regarding table 3 of the LT3575 datasheet and setting/changing Rfb = 165kOhm and Rtc = 54.9kOhm in the demo file I expected an output voltage of 10V.

But the simulation shows an output of about 5.7V. I attached a screenshot showing the changed demo circuit and the output voltage.

Changing Rfb and Rtc with table 3 to 3.3V output works perfectly.

What am I doing wrong in these circuits and receiving other output voltages than expected?

How do I get 15V output with 12V input?

Kind regards

Parents
• Hi PaS,

Thank you for sending us your feedback. I'm forwarding your design to be checked by the experts on this device.

Regarding the LT3575 LTspice demo file that you used as cross-reference, kindly review whether you are reaching the current limit. I also run it and made the same circuit modifications and got the same result as you. But I tried lower loading and was able to get Vout = 10.44V

I notice that the primary side transformer current peaks at 3.4A.

Best regards,

I set RLoad to 10Ohm and then also received 10V output. The current at L1 was 3.4A.

With RLoad=3.4Ohm the current at L1 was 3.44A and the switch current limit of the datasheet is 4.2A. So it does not reach the current limit.

• Hi PaS,

Thanks. I have forwarded this issue already to the experts on this device.
We will hear from them soon.

• Hi,

I got a message from them.

They say:
"I don’t see a problem.

When there is no load, the Vout cannot be regulated. The part need ~1% dummy load as the datasheet says.

In other cases, the current limit 3.5A is reached. I used 20 Ohm as load, the simulation runs well."