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By NICHOLAS VINEN
Low-Power Car/Bike
USB Charger
Looking for an efficient USB charger that can operate from a
12V car battery? This unit functions at up to 89% efficiency and
can charge USB devices at currents up to 525mA. Best of all, it
won’t flatten the battery if it’s left permanently connected, as
long as you remember to unplug the USB device.
T
HERE ARE LOTS of USB chargers
on the market but this device has
two stand-out features: high efficiency
and low standby current. In fact, its
standby current is just 160µA, a figure
that’s well below the self-discharge
current of most lead-acid batteries.
This means that you can leave the
device permanently connected and it
will not cause that battery to go flat (or
at least, not much faster than it would
of its own accord).
Why is this useful? Well, in September 2009’s “Ask SILICON CHIP” section,
D. E. of Ainslie, ACT asked if it was
possible to connect a 12V-to-5V USB
charger directly to the battery on a motorbike. His reason for wanting to do
this is that doing anything else might
void the warranty. Our reply was that
it is possible but that it would need to
66 Silicon Chip
have a quiescent current (IQ) of less
than 1mA to avoid draining the battery
between uses.
While USB car chargers are cheap
and plentiful, finding one with a low
enough quiescent current for permanent battery attachment is difficult.
Even those marketed as “low idle
power” devices don’t specify how
much current they draw on standby.
We tested a regular charger and
found that it consumed 13mA with
no load. Like many others, it has an
integrated power LED and that would
contribute significantly to the standby
current consumption. However, since
the cigarette lighter socket is only
powered when the engine is running,
there is no real reason for the designers of these car supplies to keep the
quiescent current low.
Cigarette lighter plugs are also pretty
lousy DC connectors. They often don’t
fit well and can easily fall out. With
this project, you can use whatever type
of connector is most convenient. In
many cases, this will mean input wires
terminated in spade or eyelet lugs.
While this may seem like a very
specific application, there are many
other uses for a low-quiescent current
12V DC to 5V DC converter. For example, remote monitoring stations often
run from a 12V SLA battery topped
up by a solar panel. These stations
invariably contain a microcontroller
and other circuitry which needs a
3.3V-5V supply.
The current consumption in these
devices will be low most of the time
but occasionally the microcontroller
will wake up and activate a radio
siliconchip.com.au
5.05
100%
90%
80%
Efficiency %
60%
50%
5.00
40%
30%
Output Voltage (V)
70%
20%
10%
0%
0
100
200
300
400
Output Current (mA)
500
4.95
600
Fig.1: this graph plots the efficiency and output voltage over the full
output current range. As shown, the efficiency is over 80% for any
output current above 10mA.
module or other circuitry which can
draw more current. This charger can
deliver that current – up to 500mA –
while still being miserly with battery
power when the load is light.
In addition, because its efficiency is
high (up to 89%), hardly any battery
power is wasted even when the load
is drawing 500mA.
What is quiescent current?
So what exactly is quiescent (or
standby) current? This term often
comes up in IC data sheets. Its simple
meaning is “idle current”, although
when talking about regulators, it sometimes refers to the current consumed
by the device itself, rather than by what
it is supplying.
In most fixed regulators, this is the
same as the “ground pin current”.
There are typically two current flows
in a regulator – from the input to the
output and from the input to ground.
The ground pin current is the power
consumed by the regulator itself.
At higher currents, most regulators
consume more current than they do at
idle. As a result, the quiescent current
may be specified for different output
currents, including the no-load case.
Although the device is arguably no
longer “quiescent” when it is delivering an output, the term is often used
this way.
Since we primarily want to minimise power consumption with no
USB device attached, the idle current
is critical for this design. What’s more,
siliconchip.com.au
a device with low idle current will
usually also have low ground pin current at higher loads. This is just what
we want since the overall efficiency
is determined by the combination of
the conversion efficiency and ground
pin current.
USB charging issues
Basically, this device is a DC-DC
converter. You feed 12V DC (or there
abouts) in at one end and it delivers a
5V DC output at the other end. It complies with the USB 2.0 specifications
with regard to power, ie, it supplies at
least 500mA at 4.75-5.25V.
However, for some devices, this
current level is insufficient for them
to operate and charge their battery
simultaneously. Many of these devices
require a custom cable or special USB
data pin connection arrangement before they will attempt to draw more
than 500mA so that they can do both
at the same time.
This shouldn’t be a big problem
since such devices should be able to
operate without simultaneously charging the battery. The battery can then be
charged when they are switched off (ie,
no longer being used). Unfortunately,
many USB-powered devices provide
no way to switch modes like that.
However, if your device can operate
normally from a computer’s USB port,
it should work fine with this charger,
since they supply the same amount
of power.
There’s just one wrinkle here. If your
USB device switches to a data transfer
mode when plugged into a computer
USB port, it may behave the same
way when connected to this charger,
even though the data lines (D+ and D-)
aren’t connected. Its battery will still
charge but the device may have to be
unplugged to be used.
Devices which typically behave in
this manner are car GPS units. Plug
them into a PC’s USB port and they
immediately switch to data transfer
mode (ie, for downloading software
upgrades and map updates). This
doesn’t stop the internal battery from
charging via the USB port – it’s just
the the device must be unplugged in
order to use it as a GPS.
Design considerations
The first step in designing this
device was to find an appropriate
switchmode regulator IC. One candidate that satisfies all the requirements
is the Linear Technology LTC1174HV.
The HV (high-voltage) version can
run from 6-17.5V (for 5V output) and
consumes only 130µA at idle, with a
maximum output of around 500mA
(this is also the most current that can
be drawn from a single USB port). The
LTC1174HV is quite efficient too.
Unfortunately, it’s hard to get the HV
version in a DIP package. None of our
usual vendors stock it, so we had to
order the low-voltage version, which
has an absolute maximum rating of
only 13.5V.
This problem was solved by adding
a low quiescent current linear preregulator to the design. This prevents
the IC’s supply from exceeding 13V,
regardless of the battery voltage. The
only drawback is that it reduces the
efficiency slightly at higher battery
voltages, although it doesn’t add much
to the idle current.
However, since the battery will only
be above 13V while it is being charged,
the loss of efficiency under this condition doesn’t really matter.
The other issue is that while the data
sheet says that switching will occur at
around 100kHz with the components
we are using, at light loads the burst
mode causes switching to occur at
much lower frequencies – in some
cases, well into the audio range. As a
result, the inductor used in the circuit
makes some noise with light loads.
We managed to tweak the design
to minimise this noise. If you listen
carefully you can hear it but once the
May 2010 67
C
A
3
LB IN
1N5819
GND
4
K
VFB 1
D2
1N5819
IC1
LTC1174
100nF
USB CHARGER FOR CARS & BIKES
SC
C
–
12V
IN
E
B
1M
Q2
BC549
C
10M
B
Q3
BC559
E
TVS1
1.5KA
36CA
+
CON1
2010
VR1
200k
2x
22 µF
470nF
270k
2
A
ZD2
12V
A
ZD1
15V
Fig.2: the circuit is based on an LTC1174 switching regulator IC (IC1), while Mosfet Q1 and transistors Q2 &
Q3 form a pre-regulator circuit. The pre-regulator prevents the supply to IC1 from exceeding 13V, regardless
of battery voltage.
A
A
K
5
SW
LB OUT
6
7
8
VIN SHTDWN IPGM
2.2M
1k
K
G
K
Output voltage: 4.75-5.25V
Output current: approximately 525mA
Input voltage range: 6-16V DC
Input current requirement: maximum 300mA at 12.0V
Input current with output shorted: 4.3mA
Output ripple: 110mV p-p, 30mV RMS at 500mA
Load regulation: 50mV at 12V, 0-500mA (1%)
Line regulation: 16mV at 450mA, 7.0V-13.0V (0.32%)
No load input current: 160µA
Efficiency: up to 89% (see Fig.1)
Switching frequency: 10Hz – 120kHz
K
ZD1, ZD2
470nF
E
B
BC549, BC559
2x
47 µF
110k
330k
G
D
S
CON2
IRF9540
D
USB
TYPE A
SOCKET
OUTPUT
–
+
CON3
L1 100 µH
A
K
D3 1N5819
CON4
D
Q1 IRF9540
S
K
A
D1 1N5819
68 Silicon Chip
Performance
board is mounted in a box and placed in a moving vehicle, it will be inaudible.
Circuit description
Refer now to Fig.2 for the circuit details. IC1
is the main switching regulator IC, while Mosfet
Q1 and its associated parts form the pre-regulator
circuit.
Power from the external 12V DC source is fed
in via CON1. Immediately following this, a 36V
AC transient voltage suppressor (TVS1) across
the input damps any positive voltage spikes that
may appear on the supply line (eg, due to devices
switching on or off). Diode D1 then provides
reverse polarity and negative spike protection.
The pre-regulator circuit (based on Q1) was
published previously in Circuit Notebook for
March 2010. It is a low quiescent current Mosfetbased design, especially developed for this type
of application. Its operation was fully explained
in the Circuit Notebook entry, so we’ll just cover
the basics here.
Essentially, the transconductance of the Mosfet
Q1 is controlled so that the voltage at its drain
will not exceed a preset value. This is done using
zener diode ZD2, trimpot VR1 and transistors
Q2 & Q3.
In this case, the voltage on Q1’s drain is set
to 13V and VR1 allows you to trim this value.
We need to make sure the LTC1174 can’t be
damaged and this provides a small safety region
(ie, 0.5V) between its supply voltage and its
maximum rating.
The circuit works as follows. When power
is applied, Q1’s gate is pulled low via a 1MΩ
resistor, turning it on. Q1’s output voltage then
rises until ZD1, a 12V zener diode, begins to
conduct and pass current to trimpot VR1. Once
VR1’s wiper exceeds 0.65V, Q2 turns on and this
then turns on Q3.
As a result, current now flows though Q3 and
the 1MΩ resistor. This in turn increases Q1’s gate
voltage and switches it off. By suitably adjusting
VR1, Q1’s output can be accurately set to 13V.
siliconchip.com.au
The nominal 13V supply from the
pre-regulator is decoupled using two
22µF 16V tantalum capacitors and a
470nF MKT capacitor. Tantalum capacitors were chosen for two reasons:
(1) they have much lower leakage than
aluminium electrolytics and (2) they
have a lower ESR at high frequencies
than other electrolytics.
Any capacitor leakage across the
input or output of the switchmode
regulator adds to the quiescent current
of the circuit and we want to keep leakage to a minimum. The switchmode
circuit can operate at frequencies in
excess of 100kHz (occasionally as high
as 1MHz) in burst mode, so we need
to make sure the capacitors will be
effective at high frequencies.
The switchmode regulator section is
based on the schematic shown in the
LTC1174 data sheet (“High Efficiency
3.3V Regulator”). However, the 50µH
inductor has been increased to 100µH
and we’ve added a voltage divider
since we need a 5.0V output instead
of 3.3V.
Pins 7 & 8 of IC1 are tied to the positive supply rail. Keeping pin 8 high
ensures that the IC is always enabled,
while pulling pin 7 high selects the
higher peak current limit (600mA).
That way, the current limiting will
not kick in until an average of almost
exactly 500mA is being supplied.
The 330kΩ and 110kΩ resistors
across the output form a 4:1 voltage
divider. This sets the output voltage.
In operation, the LTC1174 adjusts its
output voltage so as to keep its VFB pin
(pin 1) at 1.25V. This means that the
output voltage will be 1.25 x 4 = 5.0V.
If you want to change the output
voltage, use the formula R3 = R4 x
((VOUT/1.25) - 1), where R4 is 110kΩ.
For example, to set the output to 3.3V,
replace R3 with 180kΩ. In this case,
the output would be taken from CON3
(which is a polarised 2-pin header)
rather than from the USB socket.
The 2.2MΩ and 270kΩ resistors form
a voltage divider which is applied to
the LBIN (Low Battery Input) pin of IC1.
If the supply falls below 11V, pin 2 will
sink current (ie, it goes low). Header
CON4 enables a high-brightness LED
to be fitted to indicate the low-battery
condition but note that once it comes
on, it will then run the battery flat
even faster!
In short, this LED is optional and
should be left out unless you have a
specific reason for using it.
siliconchip.com.au
By contrast, diode D3 is necessary.
It’s included to protect IC1 from an
input supply short circuit – as unlikely as that may be. Without it, if an
input short were to occur, IC1 could
be destroyed.
Following L1 (which serves as the
switchmode energy storage element),
the output voltage is filtered by two
47µF tantalum capacitors and a parallel 470nF MKT capacitor. This is not a
great deal of capacitance but thanks to
the good high-frequency performance
of tantalum capacitors, the output ripple is typically no more than 110mV
peak-to-peak and 30mV RMS. Larger
capacitors could be used here but their
leakage currents would be higher.
The 5V output is fed to two different
output sockets connected in parallel.
CON2 is a Type A USB socket for
recharging USB devices. For other
devices, the output can be taken from
2-pin polarised header CON3.
Note that the operating temperature
range for the LTC1174CN8 is specified
as 0-70°C. If you live in a cold or extremely hot climate and will be using
this device outdoors (eg, mounted
outside the cabin of a vehicle), then
you may need to use the LTC1174IN8
IC instead. This can operate from -40°C
to 85°C.
Input limitations
Normally, the supply voltage will be
in the range of 12-14.4V. However, the
regulator will operate just fine over a
range of at least 9-15.6V. In a vehicle,
it is not unusual to get short-term voltage spikes in both directions. TVS1,
D1 and the pre-regulator combine to
protect the device from these spikes.
Voltages between -36V and 0V will
not harm the regulator since D1 will
not conduct. D1’s reverse breakdown
voltage is -40V but TVS1 should absorb
spikes below -36V anyway.
Above 15.6V, the regulator will
continue to operate normally, all the
way up to 36V at which point the
TVS clamps the supply voltage. We
tested the regulator to 30V and it ran
normally. However, if you were to run
the regulator at high current and high
voltage, Q1 would eventually overheat
since it has no heatsink.
This means that while the regulator
will run off voltages above 15.6V, as
can happen in a vehicle from time to
time, it must not be run at high voltages
for extended periods. With a maximum
input current of about 220mA at up to
Parts List
1 PC board, code 14105101, 62
x 49mm
1 2-pin terminal block (5.08mm
pitch)
1 PC-mount horizontal Type A
USB socket (Jaycar PS0916,
Altronics P1300)
2 2-pin polarised headers
(2.54mm pitch)
2 2-pin polarised header connectors (2.54mm pitch)
1 100µH high-frequency 1.13A
bobbin inductor (Altronics
L6222)
1 small rubber grommet
1 M3 x 6mm machine screw
1 M3 star washer
1 M3 nut
1 8-pin machine tooled socket
(optional)
1 200kΩ horizontal single-turn
trimpot (VR1)
Semiconductors
1 LTC1174CN8 (IC1) (available
from Farnell)
1 IRF9540 Mosfet (Q1)
1 BC549 transistor (Q2)
1 BC559 transistor (Q3)
1 1.5KE36CA transient voltage
suppressor (TVS1)
1 12V 1W zener diode (ZD1)
1 15V 1W zener diode (ZD2)
3 1N5819 Schottky diodes
(D1-D3)
Capacitors
2 47µF 16V tantalum
2 22µF 16V tantalum
2 470nF MKT
1 100nF MKT
Resistors
1 10MΩ
1 300kΩ*
1 2.2MΩ
1 270kΩ
1 1MΩ
1 110kΩ
1 360kΩ*
1 1kΩ
1 330kΩ
* May be necessary to adjust
regulator output – see text
15.6V, Mosfet Q1’s dissipation will not
normally exceed 572mW.
Buck regulation
The LTC1174 has several modes
but works similarly to a normal “buck
converter” at high output currents.
A “buck converter” is the most
common type of step-down DC/DC
May 2010 69
SWITCH S1
There are losses in this process,
which is why switchmode regulator
efficiency is never 100%. However, it
is still a great deal better than linear
regulation. With a 13V input, a 5V
output and 500mA output current, the
input current is around 220mA. This
gives an efficiency of (5 x 0.5)/(13 x
0.22) = 87%. A linear regulator under
these conditions would have just 5/13
= 38.5% efficiency (assuming that the
input and output currents are equal).
If the instantaneous current through
the inductor exceeds the IC’s internal
current limit (nominally 600mA), the
internal transistor switches off and
the switch off-time is extended from
4µs to around 12µs. This gives the inductor time to discharge if the output
is shorted.
One reason for this current limit,
apart from stopping IC1’s internal
transistor from overheating, is that
inductors with non-air cores can “saturate”. Essentially, the core can only
hold a certain amount of magnetic flux
and its inductance rapidly drops when
that level is reached. When it drops far
enough, the inductor is essentially just
a wire and if the switch is still on, a lot
of current can flow through it.
Because the current through the
inductor is ramping up and down as
the transistor switches, the average
current is less than the peak current.
That is why, with a 600mA limit, we
can only draw up to 500mA. The current limit kicks in soon after that and
the output voltage drops until the current draw decreases below the limit.
This protects against short-circuits
INDUCTOR L1
+
+
iL
PATH 1
DIODE
D1
VIN
PATH 2
C1
VOUT
LOAD
Fig.3: basic scheme for a switchmode buck converter. Voltage regulation
is achieved by rapidly switching S1 and varying its duty cycle. Current
flows via path 1 when S1 is closed and path 2 when it is open.
converter. It requires a single switch
(normally a transistor), an inductor
and a capacitor. Fig.3 shows the basic
scheme and it works as follows.
When the switch is closed, current
flows through inductor L1 into the load
(Path 1). This current slowly builds up
from zero to a peak value. When this
peak current is reached, the switch
opens and current flows through diode
D1 to discharge the inductor’s energy
into the load (Path 2).
C1 is included to act as a reservoir,
to smooth out the voltage produced
across the load. This voltage is dependent on the load and duty cycle of
switch S1 (ie, the time that it is closed
compared to the time that it is open).
It’s also dependent on the peak current
through L1 and the input voltage.
This type of circuit can be very
efficient because voltage control is
achieved by rapidly switching the
input. The small amount of power dissipated is mainly due to voltage losses
in the switching device (in practice,
S1 is a switching transistor or Mosfet)
and in L1 and D1.
The USB Charger operates in similar
fashion but in this case the the switching is performed inside IC1 (LTC1174).
Many buck regulators operate at a
fixed frequency, using PWM to control
the switch duty cycle and thus the output voltage. By contrast, the LTC1174
has a “fixed off-time” configuration. It
varies the switch duty cycle by controlling the length of the “on-time”, ie,
how long the switch is kept on for each
pulse. This is a power saving feature
– it means that the frequency drops
at light loads and the less the internal
Mosfet has to switch, the less power
is consumed by the IC itself.
When the internal Mosfet switches
on, current flows from VIN (pin 6) to
SW (pin 5) and through inductor L1,
charging the output capacitors. During
this period, the magnetic field generated by the inductor increases.
Conversely, when the internal Mosfet switches off, the magnetic field collapses and this continues driving current into the output capacitors. Since
the internal transistor is off, the current
instead flows from ground through D2
and then through the inductor.
It is this charging and discharging
of the inductor’s magnetic field which
allows for efficient voltage conversion.
When the internal transistor is on,
the inductor nominally has 12V at its
switch end and 5V at the output end.
If the inductor was a resistor, then
more than half the power would be
wasted as heat.
Table 2: Capacitor Codes
Value µF Value IEC Code EIA Code
470nF 0.47µF 470n
474
100nF 0.1µF
100n
104
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
No.
1
1
1
1
1
1
1
1
1
70 Silicon Chip
Value
10MΩ
2.2MΩ
1MΩ
360kΩ
330kΩ
300kΩ
270kΩ
110kΩ
1kΩ
4-Band Code (1%)
brown black blue brown
red red green brown
brown black green brown
orange blue yellow brown
orange orange yellow brown
orange black yellow brown
red violet yellow brown
brown brown yellow brown
brown black red brown
5-Band Code (1%)
brown black black green brown
red red black yellow brown
brown black black yellow brown
orange blue black orange brown
orange orange black orange brown
orange black black orange brown
red violet black orange brown
brown brown black orange brown
brown black black brown brown
siliconchip.com.au
ZD1
15V
Q2
Q3
BC559
D2
D3
L1
100 µH
5819
5819
BC549
VR1
200k
1
3
2
4
100nF
+
+
47 µF
CON3
330k
IC1
LTC1174
5819
10M
1M
+
12V
TVS1
1.5KA
D1
1k
2.2M
110k
CON1
270k
+
10150141
CON2
470nF
CON4 +
Q1
IRF9540
–
B SU ra C
12V
IN
22 µF
22 µF
USB OUTPUT SOCKET
47 µF
470nF
ZD2
©
0102
Fig.4: follow this parts layout diagram and the accompanying photo to assemble the PC board. Make sure that all
polarised parts are correctly oriented and don’t get the transistors mixed up.
at the output as well as inductor
saturation.
Burst mode
At lower currents, the IC goes into
“burst mode”. What it does is deliver
several very fast pulses of current to
the inductor over a short period, bringing the output voltage slightly above
5V. It then switches off and waits for
the output voltage to drop below 5V
and then starts pulsing again.
As it is waiting for the voltage to
drop, the IC is in “sleep mode” and
consumes very little power. The result is that at light loads, ground pin
current is substantially lower than
it would otherwise be without this
burst mode.
While the delay between the bursts
makes the effective frequency of operation much lower than at full power,
the frequency of the bursts themselves
is actually quite high. We measured
frequencies as high as 1MHz.
This means that the noise generated
by the inductor is a sub-harmonic of
the switching frequency and is caused
by magnetostriction of the inductor’s
core.
If there is nothing attached to the
regulator’s output, the feedback volt-
age divider becomes the only load.
Because the output voltage decays
very slowly, the period during which
the IC sleeps in burst mode becomes
several hundred milliseconds. It is
this long sleep period which allows
the regulator to have a very low quiescent current with light loads or no
load (approximately 140µA).
Construction
Building this unit is easy. All the
parts mount on a small PC board
coded 14105101 (62 x 49mm) and this
snaps into the integral channels in a
standard UB5 plastic box. The USB
socket is accessed through a hole cut
in one side of the box, while a hole at
one end provides access to the input
screw-terminal block.
If you want something that’s a bit
more robust, a small IP67-rated box
can be used instead. In this case,
the board can be mounted on M3 x
12mm tapped stand-offs and secured
using M3 x 6mm machine screws and
washers.
Note that because this unit is likely
to be exposed to a lot of vibration, we
have not specified a socket for the IC.
You can use one if you prefer but make
sure it is a machine-tooled type, as the
IC is less likely to work its way loose.
Before starting the assembly, carefully check the PC board for defects.
Most of the underside is covered by a
ground plane. Make sure that there are
no unintentional connections between
this ground plane and any of the other
tracks, as could occur if the board is
under-etched.
If you are going to install the board
in a UB5 case, check that it fits correctly by snapping it into place. It may
be necessary to file the edges slightly
if it is too large. Even if it’s just 0.1mm
too wide, that can make the plastic
case bulge slightly when it is in place.
Once you are satisfied the board is
OK, install the resistors. Check each
resistor with a DMM before installing
it on the board, to ensure the values are
correct. That done, install the diodes,
starting with the two zeners (ZD1 &
ZD2), then the three 1N5819 diodes
(D1-D3). Don’t mix them up and be
careful with their orientation.
Next, bend the Mosfet’s leads down
by 90° exactly 5mm from its body and
mount it on the PC board. Check that
its tab mounting hole lines up with
the board, then fasten it to the board
using a 3mm machine screw from the
top and a star washer and M3 nut on
82
6
7
12
28.5
15
(SIDE OF UB5 BOX)
ALL DIMENSIONS IN MILLIMETRES
Fig.5: this diagram can be copied and used as a drilling template for the USB socket cut-out in the side of the case.
siliconchip.com.au
May 2010 71
Fig.6: this shows the output voltage (yellow) and switching (green) waveforms at 10mA. The long off-time relative
to the on-time can be seen. The device is operating in
discontinuous mode – the inductor current falls to zero,
causing the oscillations in the green trace.
the underside. Do the nut up firmly,
then solder and trim the leads.
Note: don’t solder the Mosfet’s leads
first. If you do, you could stress and
crack the the copper tracks on the PC
board as the mounting screw is tightened. Always install the mounting
screw before soldering.
Next, install the IC socket if you
have decided to use one. Follow this
with the transient voltage suppressor
(TVS1) – its orientation doesn’t matter – then install the two small-signal
transistors (Q2 & Q3). Note that Q2
& Q3 are different types, so don’t get
them mixed up. Q2 is a BC549 NPN
transistor, while Q3 is a BC559 PNP
type.
Fig.7: this scope shot shows the output voltage waveform
at 450mA. The device is switching continuously and
so the frequency is much higher. There is evidence of
occasional burst-mode operation, as can be seen near the
centre of the trace.
If their leads are too close to fit
through the holes, bend them outwards near the body of the transistor
using small pliers, then back down
again.
The PC-mount USB socket (CON2)
is next on the list. Be sure to press it
down firmly so that it sits flush against
the board, then solder its two metal
tabs to secure it in place. That done,
solder the four pins, taking care to
avoid bridging them.
Trimpot VR1 and the three MKT
capacitors can now go in, followed by
the four tantalum capacitors, inductor
L1 (this can go in either way around)
and the screw terminal block (CON1).
Push the terminal block down firmly
onto the board and make sure its entry
holes face outwards before soldering
its pins.
Be careful also with the orientation
of the tantalum capacitors. A “+” will
be printed on the case above the positive lead – just line it up with the “+”
sign on the board overlay.
Vibration proofing
If the unit is to be used in a vehicle,
it’s a good idea to apply some silicone
sealant around the base of each tantalum capacitor and TO-92 transistor.
The idea is to glue them to the PC
board so that they can’t vibrate and
break their leads.
Be sure to use neutral-cure silicone
Recharging Apple USB Devices
+5V
+5V
+5V
27k
22k
2.5V
D–
2.0V
22k
D+
Vcc
USB TYPE A
SOCKET
GND
16k
~3.3V
1
2
3
4
0V
18k
0V
Fig.8: the data pin biasing arrangement for iPOD NANO 2nd generation players.
Some USB devices require their D+ and
D- pins to be biased for charging to occur.
These devices include the iPOD NANO
1st generation and 2nd generation music
72 Silicon Chip
30k
D–
D+
Vcc
GND
USB TYPE A
SOCKET
2.8V
1
2
3
4
D–
2.0V
47k
10k
0V
33k
33k
D+
Vcc
USB TYPE A
SOCKET
GND
1
2
3
4
22k
0V
Fig.9: the biasing arrangement for
iPOD NANO 1st generation players
and 5th generation iPOD video.
Fig.10: the biasing arrangement for
the iPhone 3G and iPOD Touch 2nd
generation player.
players, the 5th Generation iPOD video,
the iPhone 3G and the iPOD Touch 2nd
generation player.
This biasing can be achieved using
resistors, as shown in the accompanying
diagrams. All resistors are 0.25W and
they can be installed by adding them to
the copper side of the PC board.
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sealant (ie, the stuff without acetic
acid).
Set-up & testing
Before soldering in the IC, it’s a
good idea to adjust the pre-regulator
voltage. To do this, connect a power
supply which can provide somewhere
between 14-30V to the input terminal
block, with an ammeter
in series. It’s best to
start at the lower end
of that voltage range.
Turn on the supply and check the
current. It should
be less than 1mA. If
it is more than 1mA, then something
is wrong – turn it off and check for
assembly errors.
Now check the voltage between
pads 8 & 4 for IC1. It should be in the
range of 12-14V. Adjust trimpot VR1
until it reads 13V (or just under). If you
want to be extra cautious, you can set it
to 12.5V for a slight loss in efficiency.
Once the reading is correct, disconnect the power and install the IC to
the PC board. Make sure it goes in the
right way around!
Now power the board using a 9-16V
supply and check the output voltage.
The easiest way to do this is to check
the voltage across pin header CON3.
The output should be very close to
5.0V, or if you have changed the output
divider, your target voltage. It will be
moving up and down slightly due to
the burst mode regulation but should
not vary by more 0.2V. If it is not being
properly regulated to 5V, disconnect
the power and check for faults.
It’s possible that the output voltage could be below 4.85V, due to a
combination of the tolerance of the
voltage feedback divider resistors and
the tolerance of the LTC1174’s internal
reference voltage. If this is a case, replace the 330kΩ feedback resistor with
a 360kΩ resistor. This will increase the
output voltage by 6.8%, ensuring that
it never drops below the minimum
USB supply limit of 4.75V.
Conversely, if the output is above
5.2V, replace the 330kΩ feedback resistor with a 300kΩ resistor, to reduce the
output voltage by 6.8%. However, in
most cases, the output will be within
50mV of the programmed voltage with
the recommended 330kΩ resistor.
Installation
If you are going to install the board
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The PC board snaps into the side channels
of a standard UB5 plastic case. A blob
of hot-melt glue can be used to stop the
grommet for the input leads from working
loose.
Fig.11: this shows
the output voltage
during standby
operation. Note
the low frequency
of operation due to
the long sleep time
and burst mode.
in a UB5 box, you will first need to
make a cut-out for the USB socket.
Fig.5 shows the cutting details and
this diagram can be copied and used
as a template. You will also have to
drill a hole in one end of the box to
accept a grommet for the input leads
or connector.
After that, the board should simply
snap into place. It’s best to introduce
the side with the USB socket first and
then gently push the board into place.
Alternatively, as previously stated,
you can mount the board in the case of
your choice and secure it on threaded
standoffs using M3 x 6mm machine
screws. A 500mA in-line fuse on the
input side is a good idea, although the
IC’s current limiting should normally
protect the power supply.
As a final check, once the supply
is wired up, it’s a good idea to use a
multimeter to measure the voltage at
the USB socket before attaching any
devices. There are four pins in the USB
socket – touch the multimeter probes
to the two outer pins, being careful to
avoid shorting them to adjacent pins or
the surround. If the multimeter reads
close to 5.0V (or your target voltage),
then it’s working properly.
That’s it! If you are using the USB
Charger to power USB devices in a
vehicle, don’t forget to unplug them
when they are not in use, or you could
still flatten the battery.
Alternatively, if you power the device via the cigarette lighter socket,
it will be automatically switched off
when the ignition is switched off. SC
May 2010 73
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