Hi,
I'm designing a SEPIC based on the LM3478. Vin = 10 to 30V. Vout = 12V. Iout max = 700mA. SW Freq = 1M.
I've calculated all the values required for this features, but as you can see, when I connect a load for 100mA I obtain some spikes on the output and the switching freq is not the desired one (1MHz).
Have someone had this type of problems with the prototiping of this chip? What could I do or where could I start looking for an error?
Thank you.
Hi Richard,
Thank you very much for your quick answer, it has been a great help for me.
Image 1: Vout and DR with pigtail measuring method
Now, the next step is to find the reason of the switching frequency change. I've a Resistor of 12kOhm for a SWFreq = 1MHz. As you can see, the freq is around 400kHz.
I will try to measure FB and COMP signals as you've said and I'll tell you how are them working.
Here is my schematic, I've changed C2 to 1uF and added a parallel Cap with R6 in my prototype:
Image 2: LM3478 SEPIC Schematic
Best,
Ignacio.
Hi Richard,
I've measured FB and COMP signals. The switiching freq is still wrong. These are my results.
COMP pin:
The voltage is around 1.3 ~ 1.4V
FB Pin:
The voltage is around 1.26V
Regards,
