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MC78M05BDTRKG

The MC78M05BDTRKG is a 5V regulator and is a surface mount version (package: TO-252-3 (DPAK) ) of the ubiquitous 7805 (TO-220 case). It is a linear voltage regulator which shares most of the same parameter specifications of the 7805. One big difference is that it states on the datasheet that no external components are required i.e. no capacitors.

Note: The data sheet states that you don't need external components.

 

When you use a standard 7805 (uA7800 series) you will always see input and output capacitors usually a minimum of 1uF (the datasheet states 0.33uf) at the input and 100nF at the output. These minimize the effect of high input and output capacitance and also improve the ripple characteristics.

Note: For long power supply leads and/or a large capacitive load at the output it is recommended use added capacitors at the input and output.

Note: In the 7805 datasheet it states the following

Input capacitor C1 Required if the regulator is located far from the power supply filter. Output Capacitor C2 Although no output capacitor is needed for stability, it does help transient response. (If needed, use 0.1-μF, ceramic disc).

It s just good practice to add these capacitors but you could get leave them out, even on the 7805!

The MC78M05BDTRKG is actually part of a medium current (M) family of 7805 devices i.e. medium current of 500mA compared to the 7805 at 1.5A!

Pinouts

The pinouts of the 78M00 series:

mc78m00 pinout

Parameters

Input Voltage

In the same way as the standard 7805, you can allow input voltages from7V to 35V (although the max. recommended voltage is 25V 0 from the datasheet) but see the section on power dissipation examples before just plugging in 35V!

Warning: High input voltage and high current output, will kill a voltage regulator.

The key issue with high input voltage is the current that your circuit uses and the voltage dropped across the device (pass transistor voltage drop). Multiply these two together and you get the power dissipation in the voltage regulator. The higher this value the worse it gets - you need a large heatsink for high voltage inputs, either that, or use a switch mode power supply.

Output Current

The output current capability is 500mA whereas for the 7805 it is 1.5A.

Remember that since this is a SMD (Surface Mount Device) there is less metal to get rid of heat so you can only get rid of it through the PCB pad (which is an plane area of copper that the back of that the device solders onto - connected to pin 2 or ground).

For this reason the datasheet talks about "Thermal Resistance and Maximum Power Dissipation" versus P.C.B. Copper Length. So for this device, how you layout the tracks, how thick and how long they are determines your maximum power dissipation ability. This is quite different to the 7805 where you just bolt on a bigger heatsink as needed.

Power Dissipation

One thing to be aware of is how quickly power dissipation rises for seemingly trivial operation.

9V input, 5V output and 100mA output

Lets assume that you have a 5V output device (you can get different output voltages e.g. 12V etc by selecting a different part). You plug in a 9V battery and draw 100mA from it. The voltage difference across the device is the problem.

The input is 9V and at the output is 5V so the difference is 4V and current that flows from input to output is 100mA (none is lost). Therefore 100mA * 4V is the power that the device must dissipate:

100mA * 4V = 400mW

...nearly half a Watt - this is within the dissipation curve so it is acceptable use.

9V input, 5V output and 500mA output

500mA * 4V  = 2.0W  (Just within allowed dissipation for the MC78M05BDTRKG if you set the pcb pad big enough).

12V input, 5V output and 100mA output

What about increasing the input voltage to 12V? (a common wall outlet value) :

100mA *7  = 700mW

12V input, 5V output and 500mA output

Maybe you need more current. What happens at the maximum? 12V - 5V =7V

Here dissipation increases quickly and at 500mA:

500mA * 7 = 3.5W (This is beyond the allowed dissipation for the MC78M05BDTRKG).

...which will destroy it.

The power dissipation curve below shows the maximum power versus copper pad area (for an area of dimensions L * L). The diagram shows you the maximum allowed power output before the device is destroyed! i.e. before internal temperatures become too great.

The top curve is the power dissipation allowed and the bottom is the Thermal resistance from Junction to Air. As the copper pad size increases so power dissipation increases because thermal resistance is reduced.

You can see that the maximum power output you can ever get to is about 2.1W for L=30mm and 2.0W with L=16.25mm. So for a ~doubling of dimension you only get 0.1W extra available - so its probably best to keep to around 16.25mm!

MC78M00 series power dissipation vs copper area

MC78M05BDTRKG Maximum current for specific input voltages

Assuming L=16.25mm  - this is (approx) where the curve crosses the 2.0W power point.

For 9V input you can get to

2.0/4 = 500mA i.e. this is the maximum capability for this layout and input voltage. This also happens to be the max current output of the MC78M05BDTRK.

For 12V input you can get to:

2.0/8 = 250mA

At the maximum input voltage (35V) you can get to:

2.0/30 = 66mA i.e. not much

Analysis of Digispark USB connected ATTiny85

This is a small board with the MC78M05BDTRKG on board and just a quick look at its current output capabilities.

Assuming a 9V battery as the power source.

The device is from here:

attiny85 digispark with USB TABS

Source [https://www.kickstarter.com/projects/digistump/digispark-the-tiny-arduino-enabled-usb-dev-board]

Note the large DPAK-3 regulator (MC78M05BDTRKG ) that is for high current output. However it needs a large heatsink for the full 500mA. I would estimate that it uses the suggested footprint shown in the datasheet (there is another image from the page in the link above showing the pcb footprint) and the area of this is 6.2 * 5.8 = 35.96 taking the square root gives L=5.99mm (the equivalent square size fo L * L). Max Dissipation 1.55W. Therefore Max current is 1.55/4 = 0.3875A 388mA. See the section on power dissipation for discussion of values used.

The current output will be 22% lower than supposed max of 500mA – still very useful for a small device.

De-rating

The previous calculations conveniently used 2W as they show simple maths for maximum current at 9V however, the device is being operated at the extremes of the design.

This brings up a useful point:

Don't design power systems for maximum output

You should de-rate the system i.e. either allow a maximum current usage of 10% less than you could allow (arbritrarily chosen) so that if something is not quite right the power system will not melt. Things that could go wrong:

  • Plugging in too many outputs that consume too much current (applicable in a plug-and play system) or perhaps a batch of devices consumes a bit more current.
  • The ambient temperature goes up e.g maybe something else gets hot as well e.g. another part of the circuit or the air temperature goes up (used in a different climate) etc.

The other way to de-rate the 9V 500mA design is to increase the dissipation pad to give 10% more power dissipation ability.

De-rating allows you to mitigate problems and keep the hardware functioning.

Note: The above calculations assume an ambient temperature of 50°C which is quite high until you think about the fact that electronics is usually mounted inside an enclosed area so the inside temperature will be high anyway. Also other components in a system can run hot e.g. a processor running at high MHz etc.

Warning: For the SMD version of the MC78M05BDTRKG you are basically stuck with increasing the copper area to increase heat dissipation. For the T0-220 you can add a bolt-on heatsink which allows more power dissipation and therefore more current output.

Short Circuit Protection

If you short the outputs together the MC78M05BDTRKG is current limited to 230mA whereas the 7805 would allow larger current through (750mA)!

In addition to that, the device has "Output Transistor Safe Area Compensation" which means that current is limited even more depending on the input voltage level so that power dissipation will not destroy the pass transistor for increasing input voltage.

Thermal Overload

Both devices have thermal shutdown if they get too hot.

MC78M05BDTRKG Datasheet

Open the datasheet here.

Schematic comparison

The schematics, below, show the LM7805 and the more complex MC78M00 series device. You can see that more long tailed pairs have been added to the MC78M00 series device.

LM7800 series Linear Regulator

LM6#7805 Schematic
Contains 18 active transistors.

LM7805 Datasheet

Note in this datasheet LM340 is the same device as LM7805.

MC78M00 series Linear Regulator


MC78M05BDTRKG Schematic

Drop out voltage

The "drop out voltage" is the voltage required to operate the regulator and is the voltage drop from the input to output of the regulator. For the MC78M05BDTRKG it is 2V. So, for a specific voltage output x you must put x+2Volts into the input. 

Note You can select a specific output voltage by selecting a specific chip (each is fixed output e.g. 5V 12V etc.). The 7805 outputs 5V.

For a battery powered system you really need a low drop out voltage because then you don't need such a large source voltage. If you were to use this device for battery operation you have to account for the fact that the drop out is 2V higher than your desired voltage.

Say you want to supply a 5V device, then you need a 7V voltage source and since batteries don't come with a 7V output you have to use a 9V battery. As you have seen above, higher source voltages cause power dissipation problems.

The other problem is that as soon as the 9V battery drops below 7V the system shuts down.
If you had a lower drop out voltage (100mV say) then the source voltage could drop to 5.1 and the system would keep going. That means you are using the battery more efficiently.

In a real system you would use low voltage chips e.g. 3V3, 3V or 1.8 and a low drop out voltage to squeeze maximum lifetime from a battery.

Conclusions

The MC78M05BDTRKG is a useful voltage regulator that provides medium high current output (500mA) in a very small package. Since it is related to the 7805 its use will be familiar to users of the 7805. In addition it requires no input or output capacitors.

When designing with this part remember that heat is dissipated through the PCB - this is the ground pin (pin 2) which is the back of the case and soldered down. Make this pad larger for higher power operation. See the Figure below:
MC78M00 series power dissipation vs copper area

Also remember this is a replacement for the 7805. Neither of these are low drop out regulators (A drop of 2V from input to output voltage is required so for an output of 5V the input must be greater than or equal to 7V). You can get different devices that operate 50mV ~ 500mV below the input voltage which is useful for battery operated systems.



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