I am a Toryo who is still struggling with electric circuits.
Today's task is to use two LM 317 regulators to output 3.3 V and 2.5 V. The structure using the LM 317 is shown in the figure below.
The output voltage is expressed by the following equation.
Vout = 1.25 (1 + R2 / R1) + Iadj / R2
The value of Iadj ･ R2 are extremely small, so it will be ignored for the present purpose.
The theoretical values of the resistors mounted on each regulator and the output voltage that can be expected from them are as follows.
･In the case of R1 = 240 Ω and R2 = 390 Ω... 3.28 V
･In the case of R1 = 240 Ω and R2 = 240 Ω... 2.5 V
However, the 3.3 V output was almost the same as the theoretical value, but the 2.5 V voltage was remarkably low, not even 1 V was output.
I asked my senior to take a look at the board...
Senior "Oh, you are connecting regulators in series."
I "Oh, yes."
Senior "Oh, I see..."
That was him reaction.
I was curious, so I read the manual of LM 317 and found that the operating condition was "the difference between input voltage and output voltage: 3 V or more".
In my circuit diagram, the 12 V voltage from the AC adapter was supplied by connecting two regulators in series.
However, this would have used the output voltage of the first regulator, 3.3 V, as the input voltage of the second regulator,
Since "voltage difference = output voltage - input voltage," 3.3 V - 2.5 V = 0.8 V, the operating conditions are not met.
In other words, each regulator had to be connected in parallel.
After reconnecting the regulators in parallel and measuring the voltages, we confirmed that the output voltages of each regulator were within the "operating conditions" of the FPGA.
What do you mean by "within the operating conditions" here?
In my previous blog entry, I discussed the Absolute Maximum Ratings for Intel's FPGAs / CPLDs,
In addition, Recommended Operating Conditions are defined as recommended operating conditions.
For example, in the case of Cyclone® IV E, the Absolute Maximum Ratings for VCCA operating at 2.5 V is 3.75 V, but the maximum value for Recommended Operating Conditions is 2.625 V.
If the supply voltage to the VCCA exceeds 2.5 V, there is no risk of device damage as long as the supply voltage is 3.75 V or less, but normal operation is not guaranteed.
The range of the maximum and minimum values depends on the type of voltage and should be checked one by one.
In the previous voltage measurement, the output voltage was 2.55 V, which is within the VCCA recommended operating conditions.
Good!
Toryo, who had been told by his seniors that “if you burn a device, you pay for it,” felt a little relieved again after completing today's task.
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