TQFP
Junior Member level 3
Hello,
I have a circuit board that plugs into an existing computer system so the input is 5VDC (4.8 to 5.25 is about the range). My board carries an FPGA and I need 1.2VDC, 2.5VDC, and 3.3VDC. I have two LDOs (MCP1801 and MIC5320) that provide the required voltages to the FPGA, each with a 1uf input and output ceramic cap (XR7) as per the "typical application" examples in the datasheets.
To date I have not had any problems with my design until tonight. I was testing my board in a system (these are all old computers) and the FPGA would not configure. I have the "configuration done" signal from the FPGA wired to an LED so I know when it is up and running. In this particular system the LED stayed off. That really surprised me because to get the FPGA to configure only requires 5V input (to the board) and ground. I could (and have) plug my board into a battery and at least get it to configure. Checking the host system voltages indicated a 5.07VDC which is easily within range.
After pulling my hair out for hours, I finally put my circuit board on a breadboard, ran one wire to ground and another to the host system's 5VDC. It still did not work, but at least this test isolated the problem to power and not some strange issue with the I/O pins.
Then, for some reason, I put a 10-ohm resistor inline with the input, powered on, and it worked! Now I was confused because the max input of the LDOs are 10V and 6V, so 5.07V should not be causing a problem (and never caused a problem in any of the 20 or more systems I have used in the past).
Next I pulled the 5VDC wire from the breadboard, powered on the system, then plugged the 5VDC wire back into the breadboard (without the 10-ohm resistor), and the FPGA configured correctly again!
Finally (and I only did this once because I really don't want to blow anything up) I put my board back in the system (i.e. via a 40-pin socket) powered on, then *quickly* powered off/on (click click click). That worked after 2 or 3 off/on tries.
So how can I fix this? It seems to me that the host system's power supply is either coming up too slow for the FPGA and it gets stuck during initial configuration, or the host power supply is over-shooting the 5VDC at power-on. I don't know which it is though, and I can't come up with a way to capture the first few milliseconds of the power-on period. I have a decent multimeter and an o-scope, but that's about it.
But even with the small 10-ohm inline resistor, the input to my board was reduced to about 3.2 volts (I'm not sure how the 3.3V LDO was even working at that point, but it was). I was thinking maybe an inductor would help limit the input without the voltage loss, but I could not find any examples on the 'Net where anyone was using an inductor with a small LDO.
Any insight, theories, or suggestions would be greatly appreciated.
Thanks,
Matthew
I have a circuit board that plugs into an existing computer system so the input is 5VDC (4.8 to 5.25 is about the range). My board carries an FPGA and I need 1.2VDC, 2.5VDC, and 3.3VDC. I have two LDOs (MCP1801 and MIC5320) that provide the required voltages to the FPGA, each with a 1uf input and output ceramic cap (XR7) as per the "typical application" examples in the datasheets.
To date I have not had any problems with my design until tonight. I was testing my board in a system (these are all old computers) and the FPGA would not configure. I have the "configuration done" signal from the FPGA wired to an LED so I know when it is up and running. In this particular system the LED stayed off. That really surprised me because to get the FPGA to configure only requires 5V input (to the board) and ground. I could (and have) plug my board into a battery and at least get it to configure. Checking the host system voltages indicated a 5.07VDC which is easily within range.
After pulling my hair out for hours, I finally put my circuit board on a breadboard, ran one wire to ground and another to the host system's 5VDC. It still did not work, but at least this test isolated the problem to power and not some strange issue with the I/O pins.
Then, for some reason, I put a 10-ohm resistor inline with the input, powered on, and it worked! Now I was confused because the max input of the LDOs are 10V and 6V, so 5.07V should not be causing a problem (and never caused a problem in any of the 20 or more systems I have used in the past).
Next I pulled the 5VDC wire from the breadboard, powered on the system, then plugged the 5VDC wire back into the breadboard (without the 10-ohm resistor), and the FPGA configured correctly again!
Finally (and I only did this once because I really don't want to blow anything up) I put my board back in the system (i.e. via a 40-pin socket) powered on, then *quickly* powered off/on (click click click). That worked after 2 or 3 off/on tries.
So how can I fix this? It seems to me that the host system's power supply is either coming up too slow for the FPGA and it gets stuck during initial configuration, or the host power supply is over-shooting the 5VDC at power-on. I don't know which it is though, and I can't come up with a way to capture the first few milliseconds of the power-on period. I have a decent multimeter and an o-scope, but that's about it.
But even with the small 10-ohm inline resistor, the input to my board was reduced to about 3.2 volts (I'm not sure how the 3.3V LDO was even working at that point, but it was). I was thinking maybe an inductor would help limit the input without the voltage loss, but I could not find any examples on the 'Net where anyone was using an inductor with a small LDO.
Any insight, theories, or suggestions would be greatly appreciated.
Thanks,
Matthew