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
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-Î¼F, 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!
The pinouts of the 78M00 series:
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
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.
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
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
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!
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
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.
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
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.
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.
Both devices have thermal shutdown if they get too hot.
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
Contains 18 active transistors.
Note in thisdatasheet LM340 is the same device as LM7805.
MC78M00 series Linear Regulator
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
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.
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
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:
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