I have designed a PCB with Altera Cyclone IV GX FPGA, ADI's AD7626 ADC and some other components. I used ADP5024 regulator for powering the FPGA. I followed the recommendations given by the following document:
I am using the first buck (VOUT1) to derive 1.2V for the core voltage, and second buck (VOUT2) to derive 2.5V for the IO. The LDO (VOUT3) is being used for the PLL analog supply (2.5V). I have followed all the recommendations and chosen the resistor values appropriately. The input supply is 3.3 V from the motherboard PCIe slot. Unfortunately, after powering up, I am seeing strange behaviors:
VFB1 ~ 1.2V!!!
VFB2 ~ 0.7V!
VFB3 ~ 0.7V!
VOUT1 ~ 1.2V (in spite of misbehavior with VFB1!!!!)
VOUT2 ~ 3.3V
VOUT3 ~ 3.3V
According to the datasheet, VFB should be 0.5V, but I am stunned by this strange behavior. Does this happen when the load is heavy. I have just created a design with single flip-flop inside the FPGA (just to test the behavior after power-up). Fortunately, the FPGA is working fine, but I am really at a loss of words to understand the behavior of the regulator. I have physically measured the values of the resistors, and they are the same as I worked out using the formula given in the datasheet.
Can someone provide me some tips as to what may be the issue. I am attaching the schematic for reference.
There is an update from my side. VFB1 is actually 0.5V. I must have been probing at the wrong location.
But BUCK2 and LDO are still misbehaving.
I thought of bringing down the values of VOUT2 and VOUT3 by reducing R17 and R19. Hence I introduced 102K resistors in parallel with these resistors (R17 and R19 become 102/2 = 51K). I was expecting the output voltage to come down. But the big surprise is that VOUT2 and VOUT3 still stubbornly hold on to 3.3V!! It is the VFB2 and VFB3 which are changing to maintain VOUT constant!!! Both of them have risen to 1V.
After I removed the parallel resistor for VOUT2 (R17 brought back to 102K), the VFB2 came back to its previous level (~0.65V), with the output VOU2 still at 3.3V.
It is very strange behavior. I am expecting the VFB to be constant at 0.5V, while the output can be adjusted. But I am seeing the output constant, while VFB is changing. Just the reverse. Can someone throw some light on this.
thanks and regards,