Off grid? On grid with battery backup? How do you monitor the state of your batteries?
Battery
Monitor Logger
By TIM BLYTHMAN
Knowing the condition of your batteries is essential for keeping them
healthy longterm. A system that can monitor and log vital battery statistics
is a great aid, and can help you to avoid having to shell out for expensive
replacements. It can also be used for troubleshooting, such as when you
don’t know which device is responsible for periodically discharging a battery.
S
olar and wind power is growing in use and
getting cheaper, so there is a need to maintain batteries
associated with such systems. You might also have
a large battery in a shed, caravan, boat or another vehicle
that you need to monitor. Backup batteries for mains power
failures are another case where you might need a battery
monitor or logger.
Our new Battery Monitor Logger is versatile and capable,
being able to handle a charger and two separate loads outof-the-box. It is based on a Micromite LCD BackPack, so can
be reprogrammed in MMBasic, Micromite’s variant of the
BASIC language. But as we have written software with many
useful features, you don’t need to do any programming.
New features
Our design supports up to three shunts, so it can monitor
three separate current paths, helping you to split out the
charging or discharging figures across multiple loads and/
or generators. It even includes a fourth internal shunt for
monitoring its own power usage.
For example, you might have a solar panel array and a
wind generator (or several) and want to keep track of the
energy they generate separately. Or you might have several
loads like a fridge, lights and a kettle and want to see which
one is consuming the most energy.
The design allows 100V at its input and the PIC32 we have
used has plenty of storage space,
so it can record more
data for long periods.
The battery voltage and currents are
sampled at 10-second
intervals. That data is
averaged every hour
to give up to two days
of hourly samples. The
hourly samples are also
averaged over each day
to give about a fortnight
of daily values.
22
The flow of both charge and energy is logged, to provide
capacity values in Ah (amp-hours) and Wh (watt-hours). You
specify the full and empty voltages of your battery, plus the
battery capacity, so that the unit can self-calibrate when the
battery is either fully charged or discharged.
A simple, linear voltage state-of-charge value is also calculated, giving a rough indication of battery state when the
more accurate information is not available.
Operating concept
Fig.1(a) shows the simplest way to use the Battery Monitor
Logger. The battery connects to a two-way screw terminal
(CON3) while the positive ends of up to three loads or
charging sources connect to the contacts of three-way screw
terminal CON3a.
The negative ends of those loads/charging sources connect
directly to the battery negative (ground).
This allows the Battery Monitor Logger to independently
measure and display the current flowing to or from each
load or charging source.
It also produces a total current in/out figure and uses this
to keep track of the battery’s state of charge in amp-hours
(Ah). Multiplying this by the battery’s current voltage gives a
nominal watt-hours (Wh) figure for the current state of charge.
If you have more than three external devices to connect,
they can share terminals on CON3a, as shown in Fig.1(b).
For example, one terminal
is shared by two loads
(LOAD1 and LOAD2).
The measurement on that
channel will be the total
load current for these two
devices. Another terminal
is shared by two charging sources (SOLAR and
WIND), and likewise, their
currents will be summed.
The third terminal is
shared by LOAD3 and
a mains charger. In this
Practical Electronics | February | 2022
case, the unit will measure the net current flow in/out – ie, it
will see a flow into the battery if the charger current exceeds
the current drawn by LOAD3, a flow out if the situation is
reversed, and will measure zero if the two currents are equal
(ie, the LOAD3 current is supplied by the charger).
If you need to monitor currents over 10A, you can use the
same arrangement except with external shunts. These will
typically have a lower resistance and also can handle higher
dissipation, both factors allowing greater currents to flow
safely. For example, you can get 100A shunts quite easily,
or even 500A shunts.
Circuit design
The circuit of the Battery Monitor Logger is shown in
Fig.2. It has been designed as a complete Micromitecompatible board, rather than an add-on board for a Micromite LCD BackPack.
This allows us to control its power usage better, reducing
the current drawn from the battery. As with any batteryoperated device, it’s important to consider power consumption during the design phase.
The battery and load/charger terminals are at lower right, with the bottom
half of the right-hand page showing the
sensing circuitry. Other external connections (USB, serial, programming etc)
are arranged along the left-hand side,
with the BackPack circuitry occupying most of the left-hand page, plus the
display at centre-right. The unit’s power
supply is across the top of both pages.
The Micromite V2 BackPack (May
2018) is the closest BackPack variant
to our design. This comparison is only
for the sake of explaining some of our
design choices; it is not important if
you are coming to this circuit without
knowing about the earlier designs.
We’ve opted to use the 2.8in (7cm diagonal) LCD touchscreen in this design,
rather than the 3.5in (9cm) version we’ve
been using more recently (eg, in the V3
BackPack), as the smaller display uses
slightly less power.
The V3 BackPack also has many
features which simply aren’t needed
in this case, hence our choice of the V2
BackPack as the basis for this design.
The main advantage it has compared to
the original Micromite BackPack is the
inbuilt USB-Serial interface.
Battery sensing
The main battery sensing circuitry centres on IC5 (an AD7192) and REF1 (a
MAX6071). IC5 is a four-channel 24-bit
ADC (analogue-to-digital converter) with
an SPI serial interface. It is supplied from
REG2’s 3.3V output, with its analogue
rail filtered by a 10µH inductor. Each
of its 3.3V supply pins is bypassed by a
100nF capacitor.
IC5 shares the SPI bus with the LCD
touchscreen, with IC1’s pin 24 used for
the CS function, to indicate when IC5 is
being addressed.
IC5 needs a stable reference voltage to
convert voltages into digital values, and
this comes from REF1, a MAX6071 2.5V
Practical Electronics | February | 2022
reference. It is a very low-noise and precise voltage reference
chip, and it is supplied with 3.3V from REG2, with 100nF
capacitors on its input and output. Its output supplies IC5’s
REFIN1+ (pin 15), while IC5’s REFIN1− (pin 16) is tied to
analogue ground.
Each of the four analogue inputs to IC5 is fed by a
390kW/10kW divider, bypassed at the bottom by a 100µF
capacitor. This means that the nominal full-scale reading is
100V with a resolution of around 6µV, and settling times of
around ten seconds. We use the ADC to perform a conversion cycle (of all channels) about once every ten seconds, a
slow rate needed to obtain maximum resolution.
One of the dividers is connected directly across the battery
at CON3. The other three monitor the voltage at the load/
charger end of the three shunts which connect between the
BAT terminal of CON3 and the terminals of CON3A. By measuring the difference between the voltages fed to the ADC, we
can determine the current flow into or out of each terminal.
The PCB provides pads for 15mW shunt resistors which
allow a theoretical resolution under 10mA. These are 3W
Fig.1: three examples of how you could use the Battery Logger/Monitor. The
simplest configuration, at top, uses its internal shunts to monitor the currents
(up to 10A) into or out of three loads/charging sources. Or as shown in (B),
you can connect more than three loads/charging sources, with some of them
sharing shunts. For higher-current applications (up to hundreds of amps),
external shunts can be used, as in (C).
23
parts, notionally allowing up to 14A to be sensed. In practice,
the terminals limit this to around 10A.
If larger external shunts are used instead, you just need to
run low-current sensing wires from both their ends, back to
CON3/CON3A. The shunt values can be set in the software
to account for practically any resistance value.
A local analogue ground net separates the analogue
voltages from digital SPI signals.
Supply current
The current drawn by the circuit itself is modest but not
insignificant, and needs to be accounted for to get accurate
measurements. Since it is a fairly low current, we use a
different technique to monitor it. Any current flowing into
our circuit from the battery at CON3 flows out through a
100mW shunt resistor, generating a voltage below ground
proportional to the current.
IC6 is a single-channel op amp in a five-pin SOT23-5 SMD
package. It is wired as an inverting amplifier with a gain of
100 (100kW/1kW), presenting a voltage to IC1’s pin 4 where
the micro’s internal ADC can read it. The 100nF capacitor
and 100kW resistor provide similar smoothing on this signal
(a time constant of around ten seconds) so that it too can be
sampled at similar intervals to the other channels.
When the Battery Monitor Logger is operating, the LED
backlight of the LCD panel consumes the most power, so
l
Battery
Multi-logger
SC
Ó
Fig.2: the circuit includes the equivalent of an entire Micromite V2 BackPack, a precision multi-channel ADC and a
switchmode regulator capable of running the device from a DC supply between 6V and 100V. It monitors the battery
voltage, the current to/from three external points and its own current consumption and logs all this (plus the current
battery state-of-charge) to the internal Flash memory of microcontroller IC1.
24
Practical Electronics | February | 2022
a high PWM frequency is used to ensure that this measurement is accurate.
Power supply
There are two possible power sources in this circuit; USB
socket CON5 can supply 5V, while the battery connection at
CON3 handles up to 100V from the battery being monitored.
(Several components on the board have a 100V maximum
rating, so this is a hard limit and should not be exceeded.)
A switchmode buck regulator chip, IC4 (LM5163) efficiently steps the battery voltage down to 5V. Its supply from
the battery via CON3 is bypassed with a 2.2µF capacitor and
fed into pins 2 (VIN) and 1 (GND).
Practical Electronics | February | 2022
A voltage above 1.5V on pin 3 (EN) enables the regulator,
which is equivalent to a voltage of around 5.5V at CON3
due to the 1MW/390kW resistive divider.
Apart from accepting up to 100V at its input, IC4 also has
an extremely low idle current of just 10.5µA with no load,
and not much more at light loads. Its efficiency varies with
the input voltage and load current, but is typically in the
75-90% range. See the panel below for more details on this
handy little chip.
It switches its pin 8 output (SW) alternately between VIN
and GND using a pair of internal N-channel MOSFETs. The
upper MOSFET has its gate voltage supplied from the 2.2nF
capacitor on pin 7 (BOOST).
25
can cram more onto the PCB, and most of the other
ICs are only available as SMDs anyway. In this case,
its pins are relatively far apart (on a 1.27mm/0.05in
• Battery voltage: 6-100V
pitch) so it is not difficult to solder.
• Current monitoring: up to three chargers or loads,
To save power, the micro can switch 5V power
monitored separately
on and off to the touchscreen via the 14-way LCD
• Current handling: limited only by the shunts used
header. A high level on IC1’s pin 10 turns on N(10A with onboard shunts)
channel MOSFET Q4, which is otherwise held off by
a 10kW pull-down resistor. When Q4 is on, it pulls
• Current resolution: 0.1% (10mA with onboard shunts)
P-channel MOSFET Q3’s gate low, which allows 5V
• Operating current: <1mA while logging (with display off)
to flow from Q3’s source to drain and into the LCD
• User interface: 2.8-inch colour touchscreen
panel’s supply pin.
• Firmware: Programmed in BASIC
A similar arrangement, controlled by IC1’s pin
• Data logging: can be viewed on device graphically,
26 via MOSFETs Q2 and Q1, switches power to
the LCD panel’s LED backlight. Typically, a PWM
or do nloaded as
files
signal is applied to pin 26, modulating the back• Measurements: current charge (Ah) and energy (Wh)
light brightness.
• State of charge: displayed based on voltage and charge.
Unlike the Micromite BackPack V2, which had
PWM brightness control, we have omitted the option
of manual backlight control as the backlight is easily
The pulses are smoothed by the 120µH inductor and a 22µF the biggest user of power in the circuit. So it needs to be
capacitor to provide the output voltage. The voltage on feed- fully shut off during logging and monitoring.
back pin 5 (FB) is internally compared to a 1.2V reference,
so the 30kW/10kW divider sets the output voltage to 4.8V.
Serial communications
This is set to be slightly less than 5V so that if an alterna- IC1 sends display data and gets touch events back from the
tive 5V supply is available, it takes over from the battery. touchscreen using an SPI serial bus on its pins 3, 14 and 25
Schottky diode D2 feeds the 4.8V into a pi filter formed of (MOSI, MISO and SCK). These connect to the LCD panel’s
two further 10µF capacitors and a 10µH inductor.
pin 6 and 12 (MOSI), pin 13 (MISO) and pins 7 and 10 (SCK).
The 1nF capacitor across the 30kW resistor at the top of ‘MISO’ stands for ‘master in, slave out’ while ‘MOSI’ stands
the FB divider helps with the stability of the circuit that for ‘master out, slave in’.
drives the output pulses, by ensuring sufficient ripple at the
The MISO line has a series 1kW resistor so that it can still
FB pin for the circuit to operate correctly. See our panel for operate when the LCD panel is switched off. These signals,
more detail on this.
plus a chip select signal from IC1’s pin 9, also connect to
the SD card header at the other end of the LCD panel PCB
Microcontroller details
via a four-pin header.
This approximately 5V rail then feeds the Micromite section
We had planned to use the SD card to store data, but Flash
of the circuit. MCP1700-3.3 REG2 and its associated bypass memory limitations in the micro mean that there isn’t enough
capacitors provide the 3.3V supply for microcontroller IC1. space to include the (rather large) libraries needed to do this.
This is a 32-bit, 50MHz micro (PIC32MX170F256B) and is
IC2 is an 8-bit PIC16F1455 microcontroller programmed
surrounded by its own complement of bypass capacitors.
with the Microbridge firmware. This allows it to act as a
IC1 is programmed with the MMBasic firmware and USB-Serial bridge, and it can also be used to program the
runs a BASIC program to implement the Battery Monitor PIC32 microcontroller.
Logger functions.
Pushbutton S1 is used to switch IC2 between USB-Serial
While some Micromite BackPacks used the 28-pin DIP and programming modes, with LED1 flashing to indicate
version of this IC, the Battery Monitor Logger uses the 28-pin that it is passing serial data, or lighting up solidly when in
SMD (SOIC) part. It works identically but is smaller, so we programming mode.
Features and specifications
These photos show an earlier prototype, which was missing the MISO
series resistor and CON6 (which is not used by the current version of the software).
Some of the resistor and capacitor values are slightly different too, but overall it looks quite
similar to the final version. Take note of the values shown on the silkscreen PCB overlay
diagram during construction.
26
Practical Electronics | February | 2022
Screen1: The main screen provides all the critical statistics
for your battery, as well as three simple menu options
for accessing other features. The greyed values seen are
capacity calculations which are not yet valid, as the Logger
has not detected a complete charge and discharge cycle;
they will light up brighter when that happens.
Screen2: The Data screen provides a graphical view of
the logged data. Different timespans can be shown, and
the display will automatically scroll once a minute to
show current data. The Weeks option provides around a
fortnight of data. Data can also be dumped as CSV rows
over the console serial port with the Export button.
Mini USB Type-B socket CON5 is used both for USB
communications (D+/D−) as well as optionally supplying 5V power. Schottky diode D1 feeds USB 5V to the
Micromite 5V rail. Jumper JP1 provides the means to
bypass D1 if needed.
REG1 is identical to REG2 and supplies 3.3V to IC2 independently. Serial TX and RX signals are bridged to and from
the virtual USB-Serial port by IC2. These connect between
its pins 5 and 6, via 1kW resistors, to Micromite console pins
11 and 12 on IC1.
IC2’s pins 2, 3 and 7 can be used to program IC1 via its
ICSP interface; they are connected to IC1’s pins 4, 5 and 1
respectively. The PGD signal travels via JP2, which allows
IC1’s pin 4 to be used as an analogue input when it is not
being used for programming.
Both IC1 and IC2 have their in-circuit serial programming
(ICSP) pins broken out to the edge of the PCB at CON2 and
CON1 respectively. This is a feature not seen on the other
BackPacks, but we have included it here because the SMD
ICs used here are more difficult to program out-of-circuit
than through-hole (DIP) chips.
A DS3231 real-time clock (IC3) provides accurate timekeeping over long periods. Its I2C serial bus pins 15 and 16
(SDA and SCL) connect to IC1 at pins 18 and 17, the I2C
pins used by the Micromite firmware. Two 4.7kW resistors
provide the pullups needed by the I2C protocol.
DS3231 MEMS variant
The DS3231 real-time clock IC has been
the go-to choice for keeping track
of time for the last five years or
so. Its appeal is no doubt enhanced by the fact that it is
available in an easy-to-use
module typically sold as an
rduino accessory
Such a module was the subect of our first l heapo odules
feature from anuary 2 1 , hich e used in several pro ects,
typically in combination ith a icromite he module includes
R
and a cell holder
I2 pullup resistors, an 2
The module simplifies connection as it includes
all that is needed for the
2 1 chip to ork, but
sometimes it’s too big. We used the bare DS3231 IC
( hich comes in a ide 1 -pin
package)
in our icromite ack ack
( ugust 2 2 ) and the l imer
clock ( ugust 2 21)
o support those pro ects, e kept a stock of those s ne
day, e ere surprised to receive a package of small -pin
parts instead of the ide 1 -pin
s that e ere e pecting
Had we been conned?
o e had received the
2 1 variant instead hose
familiar ith the
2 1 ill kno that it only uses eight of
its pins the lo er pins are marked
( not connected ) he
Practical Electronics | February | 2022
reason for the large package is not that it needs 16
pins, but because it includes
a temperature-compensated
crystal oscillator inside the
plastic case, hich ould not fit
inside an -pin package chip
ut ith the advance of
technology,
the crystal oscillator inside the
2 1 has
been superseded by a smaller
device
o given their small si e and decent performance, e decided
to try them out in this pro ect We found the
2 1 to ork
the same as the
2 1 he nominal accuracy is slightly orse
at ppm compared to
ppm, but for situations here si e
is of concern, the smaller package is the overriding concern
he
part doesn t appear to suffer from crystal ageing
either, hich means that in the longer term, it could be more
accurate unless this is compensated for in the earlier version
of the chip he backup battery current dra appears to be
higher for the
part in typical cases, but in most cases,
the battery life ill still be close to its shelf life
n this particular pro ect, e ve made allo ances for either
part in the
design, ith a dual footprint that suits both the
ide 1 -pin
part and the narro er -pin
part We
don t kno if the
2 1
ill end up more popular than the
original
2 1, but e re ready for either eventuality
27
– Parts list –
Battery Monitor Logger
1 double-sided PCB coded 11106201, measuring 86mm x 50mm,
available from the PE PCB Service
1 2.8in LCD touch panel with ILI9341 controller
1 UB3 Jiffy box (optional, depending on desired mounting)
1 laser-cut acrylic panel to suit LCD and UB3 box
2 5-pin right-angle headers (CON1, CON2; both optional, for
programming IC2 and IC1)
1 2-way 5/5.08mm-pitch screw terminal (CON3)
1 3-way 5/5.08mm-pitch screw terminal (CON3A)
2 2-pin headers (CON4 and JP1; both optional)
1 SMD mini-USB socket (CON5)
1 3-way pin header (CON6, serial port; optional)
1 3-pin header (JP2)
2 jumpers/shorting blocks (JP1,JP2)
1 SMD coin cell holder (BAT1) [BAT-HLD-001 – Digi-key, Mouser]
1 CR2032/CR2025 cell or similar (BAT1)
1 120µH 6mm x 6mm SMD inductor (L1) [eg, SRN6045TA-121M –
Digi-Key, Mouser etc]
2 10µH 1206/3216-size SMD chip inductors (L2,L3)
1 SMD or through-hole 4-pin tactile pushbutton switch (S1)
1 14-pin header socket strip (for LCD)
1 4-way female socket strip (for LCD)
8 M3 x 6mm panhead machine screws
4 M3 x 12mm tapped spacers
4 M3 x 1mm untapped spacers (eg, stacks of 3mm ID washers)
3 heavy-duty current shunts [eg, Jaycar QP5415, Altronics Q0480 –
optional, see text]
hookup / heavy-duty wiring to suit shunts, batteries, load (see text)
Semiconductors
1 PIC32MX170F256B-I/SO 32-bit microcontroller
programmed with MMBasic or 11110620A.hex, SOIC-28
(IC1)
1 PIC16F1455-I/SL 8-bit microcontroller programmed with
icrobridge firm are,
SOIC-14 (IC2)
1 DS3231/DS3231M real-time clock IC, wide SOIC-16 or
SOIC-8 (IC3)
1 LM5163DDAR synchronous buck regulator, SOIC-8 (IC4)
1 AD7192BRUZ 24-bit ADC, TSSOP-24 (IC5)
1 NCS325 CMOS op amp, SOT-23-5 (IC6)
1 MAX6071AAUT25+TT high-precision 2.5V reference, SOT23-6
(REF1)
2 MCP1700-3.3 low-dropout 3.3V regulators, SOT-23
(REG1,REG2)
2 IRLML2244TRPBF P-channel MOSFETs, SOT-23 (Q1,Q3)
2 2N7002 N-channel MOSFETs, SOT-23 (Q2,Q4)
1 3mm or SMD M3216/1206 LED (LED1)
2 SS14 (or equivalent) 40V 1A SMD schottky diodes, DO-214AC
(D1,D2)
Capacitors (all SMD M3216/1206 size)
4 100µF 6.3V X5R
1 22µF 16V X5R
7 10µF 50V X7R
1 2.2µF 100V X7R
10 100nF 50V X7R
1 2.2nF 50V C0G/NP0
1 1nF 50V C0G/NP0
Resistors (1%
21 /12 si e 1/ W metal film e cept
where noted)
1 1MW (code 105 or 1004) 5 390kW (code 394 or 3903)
2 100kW (code 104 or 1003) 2 30kW (code 303of 3002)
8 10kW (code 103 or 1002) 2 4.7kW (code 472 or 4701)
8 1kW (code 102 or 1001) 1 0.1W (code R100 or 0R10)
3 15mW 1% 3W (M6331/2512 size; not needed if external
current shunts are used)
28
The PCB is also fitted with a SOIC-8 footprint to allow
the similar DS3231M (which uses a MEMS oscillator
rather than a crystal) to be used instead. See the separate
panel explaining the differences.
Software operation
Some of the following may seem obscure to those not
familiar with MMBasic, but this information could come
in handy if you want to change the code.
MMBasic certainly makes driving the LCD (TFT) panel
easy, as it performs startup initialisation and has builtin BASIC commands for drawing on and writing to the
display. But it needs some help to work with our circuit
arrangement, which starts with the LCD panel powered
off, and therefore not ready to accept the initialisation
commands that are automatically sent.
So we need to add a routine (in the MM.STARTUP subroutine) to set pin 10 as an output and set it high, then
re-run the LCD initialisation code. Every time we power
up the display after shutting it down, we need to trigger
that code.
We also need to control the other lines that run to the
LCD panel, as some of these idle high by default and
would therefore waste power. MMBasic does not allow
direct control of these, as the firmware reserves them to
control the LCD panel, so we need to POKE directly to
IC1’s registers and then run a command to reinitialise
the LCD controller.
Similarly, shutting down the controller requires direct
POKEs to shut down those pins. No software deinitialisation
is needed as the LCD can simply be powered down from
any state.
Despite this complication, it’s relatively easy to sense
touches on the LCD panel even if it is shut down. This is
necessary, as the user needs some way to wake the unit
up if it is in a low-power state.
Even when the LCD is powered off, the TIRQ pin
(which is connected to IC1’s pin 15) is pulled to GND
whenever the panel is touched. As the Micromite
firmware provides a weak pullup on this pin, simply
monitoring the state of this pin is sufficient to know if
a touch has occurred.
The main job of the MMBasic program is to read the
battery voltage and the voltage across the three shunts
to infer battery voltages and currents. It logs these to
variables which are kept in RAM and they are regularly
saved to internal Flash memory.
With the circuit running from the battery it is monitoring, it would take a major fault to shut it down and lose
the contents in RAM, so only longer-term samples are
saved to Flash memory hourly. If the unit needs to be
disconnected to work on the battery, at most one hour
of data will be lost.
When saving to Flash, the data is averaged over a period
before being archived. This means that less data needs
to be stored, but a good amount of data can be kept for
historical purposes.
For example, you might like to compare how much
power your solar panels are putting into your battery over
a period of a few weeks. Data about current and power
usage is also used to calculate parameters such as battery
capacity and state of charge.
The MMBasic program also provides a user interface
to allow settings to be changed and values to be graphed
and viewed. Plus there is the option to dump the data
over a serial port so that it can be exported to a PC program for graphing and analysis. We’ll delve further into
the software operation during the setup procedure in
Part 2 next month.
Practical Electronics | February | 2022
The LM5163 switchmode regulator IC
Our initial design plans for the Battery Logger set the ambitious
target of designing it to work at up to 80V, improving on the 60V
limit of an older design. That one used an LM2574HV integrated
s itchmode operating at a fi ed fre uency of k , re uiring a
si eable toroidal inductor and electrolytic capacitor
Hoping that that state of the art had progressed in the last decade,
we decided to look for newer parts. We found plenty of parts capable
of working with a 100V supply, which is impressive.
1
s itching fre uencies are no longer uncommon his
much higher s itching fre uency means that a smaller inductor
and capacitors are needed, helping us to keep our board compact.
Many parts we found could only deliver 100mA. While this might
have been sufficient ith careful control of the
backlighting,
we wanted more headroom. The LM5163 came in as the cheapest
part capable of more than 100mA (500mA) in an easily-soldered
- package, hich is a good compromise bet een si e and
ease of handling.
As is typical of modern buck regulator designs, it is a synchronous
type, meaning it has two internal switches. The incoming voltage is
switched to the inductor by a high-side internal MOSFET. When the
MOSFET is off, a second, low-side MOSFET is switched on to provide
a path for the inductor current to circulate. This removes the need
for an e ternal diode to serve this role and increases its efficiency
The LM5163 is a COT (constant on-time) design; the time that the
high-side
is s itched on is set by an e ternal resistor, after
which it is switched off. The feedback pin monitors the output voltage,
and when the output voltage has decayed, another on-cycle begins.
So the duty cycle is modulated to maintain the desired output
voltage, but the constant on-time means that the s itching fre uency
varies, although it can be predicted.
When e built our first prototype, everything orked as e pected
e ere truly impressed ith ho e ible and easy-to-use this tiny
part as ut then, it started s uealing he tone ould change ith
load ( hich e could easily modulate by ad usting the
backlight
intensity) and input voltage. It was bad enough, especially around
12V, that we needed to do something about it.
The cause was electrical noise, which was affecting when it
would switch on. It might switch on early, which causes the output
voltage to rise his ill cause the ne t s itch-on to be delayed, as
the controller will be waiting for the output voltage to drop below
its threshold.
The output pulses start to cluster into bursts, and it is these
clusters that occur at audible fre uencies, causing the high-pitched
s uealing e ere hearing ( subharmonic oscillation ) see belo
s e found ith our
itchmode
replacement ( ugust
2021), trying to get these sort of parts to operate optimally over a
ide range of input voltages can be tricky n that case, e tra output
capacitance helped.
Fortunately, a section of the
LM5163 data sheet (reproduced in
Fig.4) describes methods to avoid
this. The aim is to increase the ripple
seen by the FB pin, so that the regulator has a clearly defined time to s itch
on, despite the presence of noise.
We tried the Type 1 method, which
involves adding series resistance to the output capacitor. The
e tra resistance means that the voltage seen at the
pin is
in uenced less by the capacitor and more by the pulses from
the inductor. But it also means that the output capacitor is less
effective at filtering the output voltage, and e found it did little
to reduce the s uealing
So we tried part of the Type 2 method (omitting the series resistor from ype 1) and simply added the feedfor ard capacitor in
parallel with the top feedback divider resistor. This means that the
FB pin sees the full amplitude of the output ripple voltage, as it is
coupled directly by the capacitor rather than being simply divided
by the resistor chain.
his effectively uadruples the ripple seen by the pin ith our
30kW/10kW divider, ithout degrading filtering hat eliminated the
s uealing, so e have kept it in our final design
Any switching device which depends on a feedback voltage from
a divider to s itch its output elements can benefit from having a
feedfor ard capacitor t depends on the fre uency of operation,
capacitor value and divider ratio, though.
A word of caution: while this capacitor may appear to be a cure-all,
it does have the side-effect of slowing down response to transients
as it reduces the closed-loop gain for higher fre uency components
Fig.4: Texas Instruments’ recommended solutions for
subharmonic oscillation or ‘squegging’ in the LM5163. We
tried Type 1, and it didn’t work, but Type 2 did. It only
requires the addition of a low-value feedforward capacitor,
Cff, across the upper half of the feedback divider. Type 3
is similar but adds another pole for improved transient
response; that’s overkill in our application.
Fig.3: usually, low
ESR is considered
desirable in a
capacitor as it gives
superior filtering,
but when it filters
out the ripple too
effectively, it affects
the regulator’s
ability to produce
pulses regularly.
Next month
The second and final part of this feature will have the complete PCB assembly details, microcontroller programming
procedures, setup and operation instructions, calibration
information along with the final construction procedure.
Practical Electronics | February | 2022
Reproduced by arrangement with
SILICON CHIP magazine 2022.
www.siliconchip.com.au
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