Narrow down by specifying conditions
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This is Sasaki in charge of power supply dock technology.
In this column, I would like to introduce some examples of problems related to power supplies that I encountered multiple times during various customer support sessions.
I received an inquiry from a customer, saying, "I'm seeing an inexplicable phenomenon due to the characteristics of the power supply after revising the board."
"Ripple is small" is an assumption! ?
It's not a bug, but I had a problem with the power supply of the board I was developing, so I contacted you to let you know.
What is the content?
The other day, when I estimated the current consumption of the FPGA on the development board, I found that the current consumption was not that large, so I changed the power supply from a switching regulator to a linear regulator.
That's right. It is true that if the current consumption is low, it would be a good idea to change to a low-cost, low-noise linear regulator.
Yes, it was good up to that point, but when I actually got the revised board and checked the operation, something inexplicable happened. . .
What happened?
The board itself is working normally, but when I checked the output voltage of the linear regulator just to be sure, the ripple voltage was larger than before the change (when using a switching regulator).
A linear regulator should produce less noise, but I'm having trouble figuring out why this happened. . .
That's certainly strange. . .
By any chance, are you measuring the output of the linear regulator while the board is operating?
Yes, I observed the waveform while the FPGA was working.
So that's it. Doesn't that cause the current to fluctuate quite frequently?
Yes, the current fluctuates significantly when the FPGA switches from low power mode to burst operation.
That's the reason.
Linear regulators have poor load response compared to switching regulators, so if the current fluctuates rapidly, the voltage fluctuations will also increase.
I think that's what Ripple looked like.
That's right.
I thought that with a linear regulator, voltage noise and fluctuations would always appear to be small. Now we can safely mass-produce the revised board.
The point of this time
In general, linear regulators have poorer load response than switching regulators, so care must be taken when using them in areas where there are large fluctuations in current value.
(If the noise becomes large, we recommend using a switching regulator.)
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