Further Understanding Datasheet Information

[raphaelriv]

I was looking at a voltage regulator, specifically the LM7805 (digikey part number: 296-1365-1-ND, it is a 5V regulator that output a max of 100mA. I am trying to further understand this table on the datasheet. Its has some parameter with test conditions. For the 7805C, if i understand it correctly it states that with an input range from 7V to 20V Io (operating current?) =1mA to 40mA. My questions are:

what's input voltage regulation? Ripple Rejection? and output voltage regulations numbers of? Dropout voltage ? bias current? how crucial are these numbers?
humm i am also trying to google these terms.

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[audioguru]

No, it is not a 7805 that has a much larger package and is rated at 1A. You posted the spec's for a little 78L05.

1) Its output voltage is spec'd with an input voltage up to only 20V and an output current up to only 40mA so it doesn't get too hot.
2) Input voltage regulation is the amount of voltage change at the output that is caused by a certain amount of voltage change at the input. It says the output voltage changes typically 32mV or 200mV max when the input voltage changes from 7V to 20V. It shows that the voltage regulation is better when the input voltage changes from 8V to 20V.
3) Ripple rejection is the amount of reduction of 120Hz input ripple (from the filtered rectifiers) that appears at the output. It is 40dB minimum which is a reduction of 100 times.
4) Output voltage regulation is the amount of voltage that the output drops with a certain amount of load current.
5) Dropout voltage is the minimum voltage from input to output where it does not regulate. It is spec'd at 1.7V but as shown in item #2 above its regulation is better when it has at least 3V more voltage at its input as at its output.
6) Bias current is its operating current without a load. It changes a little when the input voltage is changed and when its load current is changed.

 

[Papabravo]

Let's take "Input Voltage Regulation": The value listed is 32 Typical and 200 Max. The units are mV or millivolts. The conditions are Vi belongs to the range [7..20] Volts and the temperature is 25 degrees C. Datasheet numbers assume that all of the voltage regulators produced will have parameters that follow a standard normal distribution unless there is a reason to suspect some other distribution. The typical number of 32 mV is the mean of the distribution and it means that as the input voltage is varied from 7V to 20V the nominal 5V output will change by about 32 mV. Some voltage regulators will change by less than 32 mV. The maximum value of 200 mV suggests that some voltage regulators will change their output by up to 200 mV as the input voltage is varied between 7 V and 20 V. At this point I would begin to suspect that this parameter is not normally distributed because the chance of one with 200 mV seems much greater than one with zero.

Ripple rejection. This refers to an AC signal on the input at a frequency of 120 Hz. This would be typical for a full wave bridge rectifier followed by a capacitor filter. The value is 49 dB. This means that any AC signal on the input at 120 Hz. will show up on the output attenuated by 49 dB. How much is 49 dB?

-49 dB = 20 * log (Vo/Vi)
-49/20 = log(Vo/Vi)
3.55e-3 = Vo/Vi
Vo = 3.55e-3*Vi

So 1V p-p of ripple on the input will be knocked down to 3.55 mV p-p on the output.

Output voltage regulation. Here we are changing the load current from 1 mA to 100 mA at 25 degrees C, you should see that the output voltage will change by 15 mV Typical and 60 mV Maximum. Again we suspect an asymmetrical, non normal distribution with a mean of 15 and a spread from zero to 60 mV.

The dropout voltage of 1.7V is the minimum difference between the input voltage and the output voltage for the device to regulate. With a 5V output this means the input needs to be at 6.7 Volts. What happens with the input below 6.7 volts? Well the output is a linear function of the input, but there is NO regulation going on.

Bias current is how much current is required from the input in order for the regulator's circuits to operate. You can think of this as the current required to operate the voltage regulator even when the ouput current is zero.

How crucial are these numbers? I guess dropout voltage is critical in a battery situation and ripple rejection is crucial in sensors and instrumentation. BTW I'm sure you can find better parts than these. They are almost half a century old designs. That might as well be forever in this business.