Troubleshooting the Texas Instruments TPS7A4700 LDO Regulator
Lately I’ve been working on the Arduino precision 16-bit DAC and the Arduino AD9951 VFO projects, which both use the Texas Instruments TPS7A4700 ultra-precision ultra-low noise LDO voltage regulator. As I was assembling the latest boards, of course I was getting anxious and I wanted to check to see if I had soldered this QFN package correctly. The QFN [Quad Flat No-leads] package is one of those that looks like a big chocolate chip. As the name and acronym suggest, there are no traditional leads on this package, and the spacing between the lead frames is very thin. Couple that with using a bulk PCB fabrication service, which *may* not pay much attention to detail, and the question arises as to if the device can be properly placed and soldered without a machine. Because I just couldn’t wait to put the rest of the components onto the board before checking the entire circuit, I decided to power on this chip and measure the output voltage. On these devices, one sets the output voltage of this device by grounding the pins associated with the desired voltage. Those pins that are not selected should be left floating. If the output voltage is not a voltage that is desired, then logic tells us that perhaps there is a solder bridge somewhere. This is my first time dealing with these devices and with the exception of the page on the datasheet that details how to set the voltage, I didn’t really study it. I should have.
As I measured the output voltage of the chip all by itself, I wanted 1.8V, but I was getting 4.08V. WHAT!!!!&*(*^&*%&!!! I re-checked the schematic and I even double checked the library for this device in Eagle to ensure that the pinouts were correct. I checked the board, I checked the schematic again, and then I decided to desolder and resolder thinking that I had a solder bridge. After desoldering and resoldering, three times, changing devices thinking the device was bad, and losing all hope that these projects were going to come together, I took a break to calm down and decided to put the rest of the supporting components for this device onto the board. This device requires four capacitors, minimum, but I just thought the capacitors were to reduce noise and ripple. After putting the rest of the supporting components for this device onto the board, I powered it on and VOILA! It worked! I was getting the 1.800V that I had expected. So now I wondered why.
With more common voltage regulators like the 7805 and 7815, if one supplies the “in” pin with power, and connects ground, then the device will output 5 and 15-volts, respectively, without additional components. Simple! And I guess this is what I was expecting with the TPS7A4700. So then I decided to look at the TPS7A4700 datasheet to see what was up; I was doing the studying that I should have done in the first place. It appears that inside this wonderful device is an op-amp. What is known about op-amps is that their gain is controlled by a RC circuit. This lovely device also has an error amp that helps regulate based on demand, which is also assisted with a RC circuit. The short of it is: Without the capacitors, the gain of the op-amp was through the roof! Adding the capacitors brought the voltage back to reality.
With all of the above in mind, I’ve decided to purchase some better quality capacitors so the two aforementioned projects will be put on hold until probably this weekend. What a great feeling to have solved this mystery that has puzzled me for days now.