Switchmode
Replacement for
78xx regulators
By
Tim Blythman
The 78xx series of three-terminal linear regulators
started as the LM109/309 in 1969. So they’ve been
around for over 50 years, and are still useful today.
Their biggest disadvantage is inefficiency, especially with a large input/
output voltage difference. If only there was an efficient, drop-in alternative...
W
e have been using 78xx
and voltage differentials. It’s built Our replacement device
series linear regulators for on a board that’s roughly the same The 78xx we know and love is the
decades, and we still use size as a TO-220 package and has the one we find in a TO-220 package. This
same three connecting leads. And it’s version alone appeared in half a dozen
them extensively today.
There is no doubt that they are a relatively inexpensive and doesn’t circuits that we published last year.
simple and effective way of getting use very many components.
There are also variants in the smaller
However, we must point out that TO-92 package (the 78Lxx) and SMD
a well-controlled fixed voltage supply between 3.3V and 24V. They’re sometimes, a linear regulator is pre- TO-252 (78Mxx in surface-mounting
cheap, they’re available everywhere ferred, mainly because its output does D-PAK) packages.
not have switching artefacts (such as
and they’re easy to use.
It’s the TO-220 package that we’re
For example, our 45V Bench Supply high-frequency ripple). Linear regula- targeting, because if you can get away
(see PE, October and November 2020) tors may also have better line and load with one of the smaller variants, the
used three 78xx series positive regu- regulation. Switchmode regulators are chances are that you don’t have too
lators and one 7905 −5V regulator to continually improving in this regard, much dissipation to worry about.
provide regulated rails for its circuitry. but there will always be cases where Also, it’s harder to cram the necessary
But being linear devices, they can a linear regulator is required.
parts into the smaller spaces that these
The ideal solution is often to com- packages offer.
be inefficient, and this causes two
major problems. Not only is much bine a switchmode pre-regulator with
If your intended application has a
of the supplied energy wasted, but a low-dropout linear post-regulator. 78xx bolted onto a chunky heatsink,
it must be adequately removed from That gives you the best of both worlds. then you’re going to benefit most from
Thus, in the space taken up by two our upgrade. And that’s precisely what
the device to prevent overheating. In
other words, more inefficiency means TO-220 parts, you can even implement this project is; a drop-in replacement
more dissipation and more dissipation such a hybrid regulator arrangement regulator for the hot, inefficient IC
by using our design and then passing that’s wasting energy in your circuit.
means more heatsinking is required.
Of course, there are plenty of switch- its output to a discrete linear regulator.
Our design is easily adaptable for
The latter should ideally be a low- many voltages; it can be used in place
mode ICs which do a similar job, but
they almost always require quite a few dropout type, but a 78xx could be used of a 7833 (3.3V), 7805, 7806, 7808,
extra ‘support’ components, possibly if maximum efficiency is not required. 7809, 7810, 7812, 7815 or 7824. It
including a bulky inductor.
might also be suitable to
And sometimes selecting
replace one of the many
Features and specifications
the right components is a bit
low-dropout three-terminal
of a ‘black art’. Even then,
fixed regulators on the market
• Input voltage ........ 4-30V
the result may not match
(although their pinouts don’t
• Output voltage ...... 2-24V
the performance of a 78xx;
always match the 78xx).
• Output current ...... up to 1A
for example, the allowable
The IC at the centre of this
• Quiescent current . around 80µA
range of input voltages may
design can deliver any voltage
• Efficiency .............. typically 90-96%
be more limited.
from 2V to 24V, with the out• Dropout voltage .... 0.5V
This article describes a
put voltage of our Regulator
• Size....................... equivalent to a TO-220-package device
switchmode regulator that
set by just one resistor value.
• Heatsinking........... not required
can be used as a direct reSo this design can replace not
• PWM frequency .... 500kHz; lower at light loads
placement for a 78xx type
just one part, but many.
• External capacitance required: 1µF+ at input, 22µF+ at output
regulator in most cases,
You might also wonder
• Other features ........under-voltage lockout (4V), thermal shut
but with much greater efabout parts like the ubiquitous
down, over-current/short-circuit protection
ficiency at higher currents
LM317 adjustable regulators.
32
Practical Electronics | August | 2021
They have a different pinout to the
78xx series, so it isn’t possible to
make a one-size-fits-all solution that addresses both
of these families.
While it is possible to
fit this device in place of
an LM317 in many cases,
you would need to make some
changes to the surrounding circuitry, including deleting the external resistors which set its voltage.
The design
We wanted our Regulator to be as
close as possible to a direct substitute
for the 78xx in a TO-220 package,
and the first item we considered was
the size.
The body of a TO-220 part is around
10mm × 15mm; a minuscule size for
a PCB. But it would not be a drop-in
replacement if it doesn’t fit in the
same space.
We decided to leave off the tab
mounting hole, since our design will
not need to dissipate anywhere near
as much heat. So there is no need to
attach it to a heatsink.
While this does also remove the
option of using a mounting screw to
secure the part, our Regulator uses
sturdy pin headers which are thicker
than the leads on most discrete parts.
If absolutely necessary, silicone sealant or other adhesive can be used to
provide mechanical support.
In any case, the Regulator PCB with
all its parts is typically around half
the weight of a TO-220 device, so the
mechanical stresses will be less.
Our Switchmode Regulator
has a very similar outline to the 78xx
linear regulator it is intended to replace.
With careful choice of parts, the
thickness can be kept much the same
too. If you have space available, you may
wish to use a larger inductor or larger
capacitors to improve its performance.
With the PCB size set, we started
looking for the best switchmode regulator IC to use. We needed to choose
one which we could fit on this small
PCB, including all the required supporting components. We found it difficult to find suitable parts that could
work up to the nominal 35V input that
the 78xx series can tolerate. In the end,
we settled for a part with a 30V rating,
as this covers most use cases.
We considered using a device in a
user-friendly SOIC-8 SMD package,
but one of these would take
up around a quarter of the
available space on the PCB.
Other parts we found came
in QFN (quad flat no-lead)
and DFN (dual flat no-lead)
packages, but we decided that
these would be too difficult
for many people to solder.
You need a reflow oven or hot
air station to have much chance
of success, so we limited our
search to parts with leads.
A decent compromise between
size and ease of soldering is the
MSOP-8 package (eight-pin micro
small outline package). We found
a device that came in this package,
with a good compromise of most of
the features we wanted.
By the way, MSOP packages have
varying pin pitch, sometimes 0.635mm
(the same as SSOP and TSSOP) and
sometimes even smaller, at 0.5mm. But
they’re also narrower than SSOP and
TSSOP, so are one of the most compact
packages that can be hand-soldered
without too much difficulty.
Switchmode operation
If you aren’t familiar with the operation of switchmode regulators, see our
panel, How switchmode regulators
work. This also explains some of the
other design considerations we had
to take into account.
While sorting through the (huge
number of) switchmode regulator
ICs that are available, we looked at
several features. First, high-frequency
operation is necessary. This means
How switchmode regulators work
There are many types of switchmode regulators in use today. This
includes step-down (buck), boost, flyback, buck/boost, SEPIC,
resonant and fully isolated types.
But step-down/buck is probably the most common configuration
and is also, in a sense, the simplest (with boost not far behind).
This is a step-down/buck design.
A linear regulator reduces its output voltage by simply introducing a controlled resistance in series with the load. If the input
voltage is twice the output voltage, this means that 50% of the
power going into the regulator is turned into heat. That means
poor efficiency.
If your aim was to vary the power to something like an LED or
lamp, which only responds to the average current, you could get
much greater efficiency by applying the full input voltage to it but
only doing so 50% of the time. This could be done using pulsewidth modulation (PWM), and indeed that is how most DC lamp
dimmers and simple motor-speed controllers work.
The voltage is normally switched by a transistor, with the transistor either fully off (and passing no current) or fully on (dropping
no voltage). Little power is lost in the switching element, with
real-world efficiencies coming quite close to 100%.
But such an arrangement is not suitable for powering ICs or
other devices which expect a more-or-less constant supply voltage. Thus, to get a similar efficiency to the PWM approach when
Practical Electronics | August | 2021
powering such devices, we need to ‘filter out’ the rapidly changing
part of the waveform (the AC component), giving us just an average
voltage level (the DC component).
An LC low-pass filter is a simple way to do this. We can’t use
an RC filter since we would have half the voltage across the resistor, so efficiency would be no better than a linear regulator. But
with an LC filter, energy is stored in both elements (the inductor
and the capacitor). Most of that is returned later, so losses and
heating are minimal.
In the case of the inductor, excess energy is briefly stored in
its magnetic field.
One way to think of this approach is that applying pulses of
voltage to an inductor forms something like a constant-current
source. At the same time, the capacitor makes the load impedance
very low at high frequencies, resulting in a fairly unchanging voltage across the load, despite the pulses applied by the transistor.
There will still be some amount of ripple present at the load, but
with the correct choice of components, it can be reduced to a manageable amount. In fact, the amount of tolerable ripple dictates the
required switchmode frequency and capacitor and inductor values.
The best way to reduce ripple is to use the largest inductor and
capacitor values possible. In practice, size is an issue, particularly
with inductors, so we are forced to compromise (too large an inductor can also affect the regulator’s response to load transients).
33
CON1
OUTPUT
GND
INPUT
3
2
L1 22 H /1A
4
1
3
1 F
35V
2
1 F
6.3V
X7R
VIN
EN
VOUT
6
7
REG1
BOOST
MCP16311
VFB
VCC
AGND
PGND
8
5
100nF
R1
52.3k
1 F
1
6.3V
X7R
10k
SC HIGH EFFICIENCY
High-efficiency
Swtchmode
SWITCHMODERegulator
REGULATOR(5V)
(5V)
2020
Fig.1: the circuit of the Regulator is just about straight out of the MCP16311 data
sheet, except that the input and output capacitors are lower than recommended.
That’s because these are supplemented by external capacitance on the host
board. The values in red need to change for a different output voltage.
The circuit
The circuit for our design is shown in
Fig.1, with the components for a 5V
output. IC1 is the MCP16311 integrated
switchmode controller. It works with
4.4-30V at its input (pin 4, VIN) and
can deliver 2-24V at up to 1A. Pin 3,
the enable (EN) input, is tied to VIN so
that the IC is enabled as long as there
is a sufficiently high supply voltage.
The input supply is bypassed by a
1µF capacitor. While this is less than
34
the recommended capacitance in
IC1’s data sheet, any application using a 7805 requires an external bypass
capacitor anyway, which will supplement the capacitance fitted to the PCB.
Pins 5 and 8 are connected to
ground, with pin 5 being the highcurrent return for the synchronous
switch, while pin 8 is the low-current
reference ground to which the output
voltage is referred. Both are connected
to large copper pours on the PCB.
IC1 has an internal low-voltage
regulator for its control circuitry,
which should be bypassed by a 1µF
capacitor connected between pin 2
and ground. This pin sits around 5V,
so a 6.3V capacitor is adequate.
Pin 1 is connected to IC1’s internal
regulator feedback circuitry. The voltage at pin 1 is compared to a precision
0.8V reference, so this pin should be
connected to the midpoint of a voltage divider between the output and
ground. The ratio of this divider sets
what fraction of the output voltage
6
5
OUTPUT VOLTS
that a lower inductor value is needed,
which reduces its physical size. A
higher frequency also means less ripple and noise.
We also looked for parts which
operate synchronously, rather than
requiring an external diode. While it
is only one extra part, the diode does
carry a fair amount of current, so
choosing a synchronous part means
that some space and dissipation is
saved. The voltage drop across the
low-side MOSFET (which replaces
the diode’s function in synchronous
designs) is less than that of the diode.
Ultimately, we settled on the Microchip MCP16311. It has a switching
frequency of 500kHz and operates synchronously with a minimum number
of external components for an adjustable output. As noted earlier, it can
operate with up to 30V on its input.
We initially tried to lay out the
PCB using 3216-size (1206 imperial)
passive components. These measure
3.2mm × 1.6mm, but were too large,
so we switched to 2012-size (0805 imperial) parts measuring 2.0 × 1.2mm.
These save a significant amount of
space on the PCB, but aren’t too much
harder than 3216-size parts to solder.
The footprints that we’ve provided
on the PCB are actually a tiny bit larger
than 3216/1206 parts, so you might
be able to use the slightly larger 1206
parts anyway.
is seen at pin 1 and thus dictates the
output voltage.
The MCP16311 data sheet recommends a 10k resistor for the lower
part of the divider, so changing the
output voltage is simply a case of setting the upper resistor.
For a 5V output, the upper resistor should ideally be 52.5k . While
changing this resistance will set a
different output voltage, for optimum
performance, other components must
be adjusted too.
In practice, 52.3k is the closest
commonly available value, from the
E96 (96 values per decade) series. This
gives a nominal 4.984V output. For
comparison, a 51k resistor (found in
the more common E24 series) would
give a nominal 4.88V output.
Unless you need a precision voltage reference, either of these would
be close enough for most 5V supplies. You probably should not use
a switchmode device as a precision
reference anyway!
Pin 6 is the switch (SW) terminal,
which is connected to the two internal MOSFETs. One switches the
output to ground (pin 5), the other
to VIN (pin 4). A non-synchronous
part would require an external diode
(typically a schottky diode) in place
of the lower transistor, to allow inductor current to circulate while the
upper MOSFET is off.
Between the switch terminal and
the output is an LC low-pass filter
comprising a series inductor and a
capacitor to ground. Like the input
capacitor, we’re using a lower capacitor value than recommended given
that more external capacitance will be
fitted. However, it would be possible
to fit a higher capacitance in the space
available if necessary.
SWITCHMODE
4
7805
3
2
1
0
0
1
2
3
4
5
6
7
8
9
INPUT VOLTS
Fig.2: the Switchmode Regulator does not operate with an input supply below
4V. At 4V and above, though, it has a much lower dropout voltage than the 7805
and attains a 5V output with only 5.5V at its input (ie, 0.5V dropout). The 7805
needs nearly 7V on its input before it is in regulation.
Practical Electronics | August | 2021
The output of the LC filter is fed to
the output pin of CON1, which forms
the interface with the external circuitry; its other two pins are connected to
the VIN pin of IC1 and the ground pour.
This output voltage is also fed to the
upper resistor in the feedback voltage
divider mentioned earlier.
The final component on the board
is a 100nF capacitor between pins 6
(switch or SW) and pin 7 (BOOST).
Because the internal high-side MOSFET is an N-channel device for maximum efficiency, it needs its gate to be
brought above its source to conduct.
As the source is connected to the SW
pin, a voltage above SW (and possibly
above VIN) is required to drive its gate.
An internal charge pump provides
this higher voltage, which is stored in
this 100nF capacitor until it is needed
to switch the MOSFET.
The overall operation is as follows.
IC1 produces a pulse-width modulated (PWM) square wave at the SW
pin (pin 6) which is filtered by the LC
circuit. The output voltage is monitored by the voltage divider connected
to pin 1, which causes IC1 to change
its PWM duty cycle to maintain the
desired output voltage.
With a light load at its output, IC1
can also ‘drop’ or skip PWM cycles,
reducing power consumption.
Three-pin header CON1 has 0.1inch (2.54mm) spacing, to match a
TO-220 package.
Setting the output voltage
The MCP16311 data sheet recommends different inductors for different
output voltages. The rule-of-thumb
value is 4.5µH per volt at the output.
In choosing an inductor, keep an
eye on the DC resistance specification too. Values around 100m are
Desired
Vout
R1
(E96)
Nominal
Vout
R1
(E24)
Nominal
Vout
3.3V
31.6k
3.328V
30k
3.2V
5V
6V
8V
9V
10V
12V
15V
24V
52.3k
64.9k
88.7k
102k
115k
140k
178k
287k
4.984V
5.992V
7.896V
8.96V
10V
12V
15.04V
23.76V
51kV
62k
5.76V
91k
8.08V
100k
110k
130k
180k
300k
8.8V
9.6V
11.2V
15.2V
24.8V
15µH
(eg, SRN6028-150M)
22µH
(eg, SRN6028-220M)
27µH
(eg, ASPI-6045S-270M)
39µH
(eg, ASPI-6045S-390M)
39µH
(eg, ASPI-6045S-390M)
47µH
(eg, SRN6028-470M)
56µH
(eg, SRN6045TA-560M)
68µH
(eg, TYS6045680M-10)
120µH
(eg, SRN6045TA-121M*)
* current rating is 850mA, so don’t draw more than this (the output voltage
may drop before reaching that level). For more current, you can probably get
away with a 100µH inductor, part code ASPIAIG-S6055-101M.
Table 1: Component choices
recommended, meaning that the
inductor will drop 0.1V, dissipating
100mW when the regulator is supplying 1A. If you are planning to run your
regulator near 1A, this will probably
be the biggest loss.
Another critical point is the voltage
rating of the output filter capacitor.
You need a 6.3V or higher rating for a
5V output, but we’ve specified 50V for
all capacitors to keep things simple.
Advanced constructors may wish
to use devices with a lower voltage
rating but higher capacitance, as long
as they still have a sufficient voltage
rating for their particular role.
Table 1 shows some choices for both
the top resistor value (from both the
E24 and E96 series) and also a suggested inductor value.
Left to right: 3.3V, 5V Regulator and 12V Regulator. Note the inductor is much
larger for higher-voltage versions. This version is only 6mm thick, which is
more than the 5mm of many 78xx regulators, but still slim enough to fit in most
places where one would be used, especially as no heatsink is normally required.
Practical Electronics | August | 2021
4.88V
L1
Note that the E24 resistor values do
not allow for a high level of accuracy,
but may still be close enough, depending on your application.
Performance
Naturally, we ran some tests to ensure
that the Regulator has equivalent
performance to its linear predecessor.
As per the data sheet recommendations, we connected around 10µF
extra capacitance at the input and
22µF at the output.
Efficiency is very high compared
to a linear device. We connected our
prototype 5V Regulator to an 8
load (a wirewound power resistor),
drawing a nominal 625mA. For low
values of input voltage (up to around
12V), efficiency was 96%, dropping
off above 12V.
This agrees well with the information in the MCP16311 data sheet. Our
calculations suggest that well over
half of these losses are simply due
to dissipation in the inductor’s DC
resistance. Hence, the importance of
low DC resistance in this part.
During this test, we noted the Regulator was warming up above ambient,
but was never too hot to touch.
Another quick measurement indicated that the quiescent current of the
Regulator (under no-load conditions)
is around 80µA, close to the value
from the data sheet, and a lot less than
a 78xx regulator at around 5mA (60
times higher!).
Fig.2 shows how the output voltage
varies with the input voltage, comparing the Regulator with the expected
performance of a standard 7805. This
also indicates the dropout voltage.
35
Construction
Taking note of what is described
above, choose your components before starting construction. Many of
the components are quite small, and
their marking will be barely legible.
The capacitors will probably be unmarked, so take care not to mix them
up (or lose them!).
Check that you have the appropriate
tools for working with small surfacemounted components.
At a minimum, we recommend a
fine-tipped soldering iron (preferably
with adjustable temperature), a pair
of fine-tipped tweezers, a magnifier
as well as some flux paste and solder
braid (wick).
36
TOP VIEW
1 F
1
18105201
BOTTOM VIEW
L1
1
52.3k 10k
1 F
100nF
REG1
1
1 F
REG1
1
1 F
CON1
1
R1
R1
L1
52.3k 10k
1 F
100nF
18105201
CON1
Interestingly, the 7805 passes more
voltage at very low input voltages.
This is not unexpected, as the
MCP16311 does not even come out
of the under-voltage lockout until its
input reaches around 4V. Once it starts
up, it has a much lower dropout, needing an input of only 5.5V to supply
5V at the output; a dropout voltage
of around 0.5V.
On the other hand, the 7805 is not
regulating correctly until its input
reaches around 7V; a 2V dropout.
In battery-powered applications,
both the lower quiescent current
and the low dropout voltages are big
advantages. Not only does the higher
efficiency mean that less energy is
wasted, but the Regulator is also capable of operating with much lower
input voltages, making better use of
the same battery.
One advantage of the MCP16311’s
low-voltage shutdown feature is that
in a battery situation, the 7805 would
continue to pass current, completely
flattening the battery (which could be
fatal if it’s a rechargeable type), while
the MCP16311 will switch off when
its input gets too low, preventing this.
Since the output is below 5V by
the time the input reaches 4V, the
connected circuit will probably not
be operating to specification anyway.
Scope1-Scope4 show more details
of the circuit’s performance. Scope1
shows that the Regulator takes around
350µs to start up, which is quick
enough for most applications. Scope2
shows output ripple.
This is one area where the 7805 will be
superior, although this small amount of
ripple is tolerable for most applications.
Scope3 and Scope4 show the response to load and line changes; the
output varies by around ±150mV for a
625mA load step, recovering in less than
100µs, while line regulation is around
1%, ie, an output variation of around
17mV for an input ripple of 1.88V.
1 F
1
(WITHOUT LABELS)
Fig.3: we’ve shown the component
overlays same size (above) as we
IN GND OUT
OUT GND IN
normally do but thought a veryBOTTOM VIEW (300%)
TOP VIEW (300%)
much-enlarged view (at right)
would help you with assembly.
Inductor L1 is fitted to the top side of the PCB, opposite to the other parts. It is
easiest to solder IC1 first, as access to its pins is not as good once the surrounding
parts are in place. The part that controls the output voltage is resistor R1 at upper
left. Here it is a 52.3k resistor, to give a 5V output. Pin header CON1 can be fitted
to either side, depending on the needs of your application. This can be fitted last,
so you can test fit the board before soldering it.
Something to secure the very small
PCB would be handy. If you don’t have
a PCB clamping tool, then Blu-Tack
may be sufficient.
The Regulator is built on a doublesided PCB coded 18105201, available
from the PE PCB Service. It measures
15 × 10mm and is 0.6mm thick (a
standard PCB is 1.6mm thick, which
would make the device 1mm thicker).
Refer to the PCB overlay diagram
(Fig.3) during construction, to see
which parts go where.
IC1 has the finest pins, so start by
fitting it. Check and confirm where
the pin 1 dot is and align it with the
markings on the PCB. If you have
CON1 at the bottom then IC1’s pin 1
is at lower left. If you cannot find a
dot, then the part may have a chamfer
along one edge; this edge should be
closest to CON1.
IC1’s pins are on a 0.65mm pitch
with only a 0.2mm spacing. You will
probably bridge some of the pins
while soldering it, so the solder braid
is essential.
Apply some flux to the pads and
hold the IC in place with the tweezers. Solder one pad down (or even
one side if your iron tip is broad).
Check and double-check that all the
pins are entirely within their pads; if
they are not, then they may short to
adjacent pads even after any solder
bridges are removed. Also check that
the part is flat.
Once you are sure of this, solder
the pins on the other side. Don’t be
concerned about bridges; in this case,
they are almost inevitable. Just ensure
that each pin is soldered to its correct
pad in some fashion.
With the IC soldered in place on
both sides, you can clean up any
bridges. Apply some more flux paste
to the pins and press the braid against
the pins with the soldering iron on one
side. Gently draw the braid away from
the part. It should draw up any excess
solder, leaving a clean fillet. Inspect
this with the magnifier and compare
it to our photo above.
Apart from IC1, none of the parts
are polarised, so do not be concerned
about the orientation after IC1 is installed. Follow with the 100nF capacitor which goes near IC1’s pins 1 and
8. Apply flux to the pads and solder
one lead only. Confirm that the part is
flat and square within the pads before
soldering the other lead. Go back and
retouch the first lead with some fresh
solder or a bit of extra flux.
Use the same technique to fit the
three 1µF capacitors. While they don’t
all need to be 50V types, the price difference is small, so it’s easier to just use
50V types for all three as stated in the
parts list. That also makes assembly
easier since you don’t have to worry
about which one goes where.
The two remaining parts on this side
are the resistors. Fortunately, these are
usually marked so are more difficult
to mix up. The 10k resistor will be
marked as ‘103’ or ‘1002’. The other
resistor value will vary depending on
your selected output voltage. For the
52.3k resistor we’ve recommended
for a 5V output, expect a code of 5232.
The last component, inductor L1, is
on the other side of the PCB. So now
is a good time to clean up any flux
residue on the top before flipping the
PCB over. If you don’t have a dedicated
Practical Electronics | August | 2021
discrete part, but a more rigid option would be to mount a straight
header at the back.
This may not work if you have
components very close to where
the Regulator will need to mount,
but will be a lot more secure as
the shorter leads will not be able
to flex as much.
Again, reproduced very much larger
than real life, these photos show front
and back of the Regulator – in this
case set up to replace a 7805 (5V)
regulator. Changing the regulation
voltage is as simple as changing R1
and L1 to suit.
flux solvent, isopropyl alcohol may
work (assuming you can get some at
a reasonable price!).
In any case, take care, as many of
these cleaning substances are flammable. Allow the PCB to thoroughly
dry before resuming soldering.
L1 is a larger part and will generally have more thermal mass, so may
require more heat. We’ve sized the
pads for a nominal 6mm × 6mm part
although an inductor up to 8mm ×
8mm footprint may fit. In this case,
you may need to apply heat to the
inductor leads.
The technique is much the same
as for other two-lead parts. Apply
flux, solder one lead, check that the
part is where you would like it and
then solder the remaining lead. Then
clean up the flux that’s been applied
to this side of the PCB.
You may need to install straight
or right-angle headers for CON1, depending on how you wish to use the
Regulator. We’ve fitted right-angle
headers to our units to make them
install just like a TO-220 device. This
is also ideal for use on a breadboard.
If using right-angle headers, check
which side is the best fit (they can
be soldered on either side), in case
space is tight in your application.
We fitted the headers at the rear
(IC1 side) of the PCB by removing
the pins from the plastic frame and
threading them into the frame from
the other side. This allows the pins to
be held in position while soldering.
This arrangement can also be used
to mount the Regulator flat against
the PCB by bending the leads a
further 90°, just as you would for a
Practical Electronics | August | 2021
Testing
One of the worst things that could
happen is that R1 is open circuit,
which would mean that IC1 is
not able to regulate the output
as it cannot see any voltage at
its output; effectively, the input
voltage will appear at the output.
If this is your first foray into
surface-mounted parts, you might
want to double-check your soldering against our photos. You should
ideally also test that the Regulator
works correctly before deploying it
to your circuit. 3-pin header CON1
will make it easy to plug into a breadboard or use jumper wires to rig up
a test circuit.
Note that the front of the Regulator
is the side with the inductor and the
CON1 header and pin 1 markings are
on this side.
Apply 4-30V to pin 1 of CON1
(with respect to GND at pin 2). Use
a current-limited supply if possible
(eg, a bench supply) or series resistor
to limit the current; this will minimise damage in the event of a fault
with the circuit.
You should be able to measure the
desired output voltage at pin 3. You
may also like to load the output (for
example, with a resistor) to check
that the circuit works under load. If
it works as expected, you are ready
to solder it into your final circuit.
Installation
Because it is intended to replace a single component, the Regulator could
be used in any number of designs,
so we can only offer general advice.
Any design using a 78xx or similar
should have separate bypass and
filter capacitors already included.
We’ve put some modest capacitance
on the Regulator PCB, but as mentioned earlier, not as much as recommended by the MCP16311 datasheet;
mostly due to space considerations.
The MCP16311 should ideally
have at least 2µF at its input and
20µF at its output; we’ve provided
around 1µF for each. Thus an extra
1µF on the input and at least 22µF at
the output is recommended.
One option to add more capacitance directly to the Regulator PCB is
to stack capacitors vertically. We’ve
Parts list – ‘78xx’
1 double-sided PCB coded 18105201,
15 x 10 x 0.6mm available from the
PE PCB Service
1 3-pin right-angle header, 2.54mm
pitch (or straight header,
depending on application) (CON1)
1 22µH 6mm x 6mm 1.1A inductor*
(eg, BOURNS SRN6028-220M)
1 MCP16311 switchmode IC, MSOP-8
package (Digi-key, Mouser)
Capacitors (all X7R SMD ceramics,
size 2012/0805)
3 1µF 50V^
1 100nF 50V
(code 105)
(code 104)
Resistors (all 1% SMD size 2012/0805)
1 52.3kΩ (R1)* (code 5232)
1 10kΩ
(code 1002)
* parts for 5V output; see Table 1 for
other voltages
^ increase to 2.2µF if an external lowESR input bypass capacitor of at
least 1µF is not possible
even seen part manufacturers do this
to create discrete capacitors with
more capacitance!
You might also be able to get discrete capacitors with a higher value
that will fit onto the board, depending on the actual input and output
voltages you’ll be using.
Check what parts are available in
the 2012/0805 size (or 3216/1206
size, if you’re willing to jam them
in). We recommend that you stick
with types having an X5R, X6S or
X7R dielectric.
For example, 2.2µF 50V X5R capacitors are available in 2012/0805
size, if you can’t fit a 1µF external
ceramic bypass capacitor on your
host board.
We’ve also built some variants
with larger 1206 (3216 metric) sized
input and output capacitors; you can
see these in the photos.
On the other hand, if your design
can tolerate some ripple at the output, then you may be able to reduce
the output capacitance below the
recommended value.
Just be careful to check that this
doesn’t affect stability under the
range of load conditions the Regulator will experience.
When fitting the Regulator to your
PCB, keep in mind that there are
bare component leads on the back of
the Regulator PCB which may short
against (for example) the existing
78xx mounting hole. Some insulating tape (eg, polyimide) applied to
the PCB should be sufficient to avoid
problems here.
37
Scope1: this shows the response of the Regulator to having
8V applied with an 8 load (625mA). Its startup time is
limited mostly by having to charge the output capacitance,
which would be the case for most regulator circuits.
Under low load conditions, thermal dissipation will be quite low,
so you could probably even seal the
entire part in heatshrink tubing, although we haven’t tested this.
Alternatively, if you have space, extend the headers pins of CON1 so that
there is clearance between the Regulator PCB and the PCB underneath.
If your design is subject to vibration,
some neutral-cure silicone sealant
between the two will reduce mechanical fatigue.
If you are using the right-angle
mounting arrangement, then you will
also lose the option to mechanically
secure the Regulator PCB because it
lacks a mounting hole. You should
also ensure clearance between the
Regulator PCB and any case parts that
might short against the components
on the Regulator PCB.
Again, some tape and sealant may
be required to maintain clearance
and insulation. If you have space, the
right-angle connector (CON1) can be
mounted at the front (rather than the
back) of the PCB. This will increase
the clearance behind it.
Reproduced by arrangement with
SILICON CHIP magazine 2021.
www.siliconchip.com.au
Scope3: here we connected a 68 load to the Regulator
and switched a second 8 load in and out using a MOSFET
(gate voltage in blue, the yellow trace is the supply voltage).
Thus the output current jumps from 75mA to 700mA and
back. The green trace shows the output voltage, which in
all cases stays within 200mV of the setpoint. More output
capacitance would stabilise this further.
38
Scope2: same test conditions as Scope1, we’ve zoomed into
the output waveform after it has had time to stabilise to
show the output ripple. We see around 50mV of ripple at
the MCP16311’s 500kHz PWM frequency; more output filter
capacitance would reduce this. This ripple is the main
downside of using a switchmode regulator.
For some
variants, we
squeezed
in slightly
larger 3216/
1206-sized
capacitors
across the
input and
output pins.
It’s generally
easier to get
these largervalued or
higher-rated parts
in the larger part
sizes, so it is worth
considering if space
is not critical.
Scope4: the same 8 load as before but with the input
supply being fed from an AC transformer and bridge
rectifier with a 1000µF filter capacitor. Around 2V of ripple
(at 100Hz) from the supply produces less than 20mV of
ripple at the Regulator’s output, an attenuation of around
100 times.
Practical Electronics | August | 2021
