What are these parameters on the 7805 voltage regulator datasheet?

Question

From the datasheet for the 7805 voltage regulator this information is available.

I have multiple questions about this information, which I haven't been able to find clear answers to on Google.

1. What does an input voltage regulation of \$100 \: \text{mV}\$ mean? What exactly is input voltage regulation?

2. What does an output voltage regulation of \$100 \: \text{mV}\$ mean? What exactly is output voltage regulation?

3. Does a ripple rejection of \$62 \: \text{dB} \$ mean that if the input voltage contains ripples these will be attenuated by \$62 \: \text{dB} \$ at the output?

4. How can the peak output current be higher than the short-circuit current? In what situation can this ever happen?

Answer 1

1. When input voltage is (slowly) changed between the two voltages listed, the output voltage changes by the amount stated.

2. When the output load is (slowly) changed between the two currents listed, the output voltage changes by the amount stated.

3. Yes. The number of 62dB is the worst-case specification, typically it will be better (78dB). However note the input voltage must remain higher than the output voltage by 3V for that to hold. You can expect ripple rejection to get worse as the input voltage approaches the dropout voltage.

4. The output current into a short circuit is not necessarily the same as the current into a load during regulation, it can be less or more. The 78xx regulators have a peak output current that's maximum at about 13V in and 5V out for a 5V regulator, and is less on either side (of input voltage). There is no published information that I can see on typical short-circuit behavior other than the one number you're asking about. The number also varies quite a bit between models and manufacturers- the pre-1982 NS datasheet says 2.1A (sc) and 2.4A (peak).

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A more directed question specifically regarding sc/peak current limits was asked here however no answer was selected.

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Here is a plot of an LM7805 model showing the output current typically varying from 1.7A to a peak of 2.7 depending on the output voltage. At least the shape illustrates the type of behavior we are discussing- the peak current is only reached when the output voltage is just below the desired regulated output voltage.

Answer 2

1. They're writing about what is often known as line regulation:

Line regulation is the ability of a power supply to maintain a constant output voltage despite changes to the input voltage, with the output current drawn from the power supply remaining constant.

2. That's what is commonly known as load regulation:

Load regulation is the capability to maintain a constant voltage (or current) level on the output channel of a power supply despite changes in the supply's load (such as a change in resistance value connected across the supply output).

3. Yes, and TI have a short article about power supply ripple rejection:

\$ \rm{PSRR} = 20\;log\frac{Ripple_{input}}{Ripple_{output}} \$

4. The short-circuit current is the long-term current when the output is shorted, but the peak current is for a pulse of a small length of time (they don't appear to specify how long).

Answer 3

The "Test Conditions" column of that table should give you some clues to what the various lines mean.

The "Input Voltage Regulation" line indicates how much the output voltage may vary when the input voltage varies over the indicated range.

The "Output Voltage Regulation" line indicates how much the output voltage may vary when the output current varies over the indicated range.

Answer 4

The 7805 uses a thermal shutdown mechanism as well as current limiting. A technique called “Foldback current limiting” actually reduces the current to less than the threshold.

The maximum current is available under normal operating conditions. When exceeded, the fold-back circuitry takes over to limit and reduce the current.