50 Silicon chip Australia’s electronics magazine siliconchip.com.au
grated Li-ion/LiPo charge management
controller. It charges the cell at a con-
stant current, up to a charge termina-
tion voltage of 4.2V. The charge current
is set by the resistance at pin 5. and for
our circuit, this is set to 100mA by the
10kΩ resistor. The charge LED (LED3)
lights when the cell is charging.
The 433MHz UHF transmitter (TX1)
and receiver (RX1) can operate from
2.5-5V. Since the transmitter will have
more output power and thus a better
range when powered from 5V, rather
than the 3.2-4.2V from the LiFePO 4
cell, we use a step-up (boost) regula-
tor to generate a 5V to power these
modules.
However, the circuit can be built
without this step-up regulator, if maxi-
mum range is not required. This saves
time and money. The rest of the circuit
will then be powered directly from the
LiFePO 4 cell. This would also extend
the cell life as the step-up regulator
is only around 70% efficient, and the
lower supply voltage will also mean
that less current is drawn by IC1, IC2,
TX1 and RX1.Jumper link JP2 is used to select
whether these components are pow-
ered from the 5V boosted supply, or
directly from the cell.
The voltage step-up is performed
by TL499A switching regulator REG2.
It comprises a switching control cir-
cuit, a transistor and a series diode. It
requires inductor L1 to perform the
boost function and a 470μF low-ESR
output capacitor for energy storage
and filtering.
A simplified circuit showing the op-
eration of the boost converter is shown
in Fig.2. Initially, internal transistor
Q1 is on and current flow begins to
build through the inductor L1 (at a
rate limited by its inductance), until it
reaches a particular value. This maxi-
mum current is set by the resistor con-
nected to pin 4 of REG2.
When Q1 switches off, L1’s magnetic
field collapses and so current contin-
ues to flow to the load and output ca-
pacitor CL via diode D1. This current
flow causes a voltage to appear across
L1, which adds to the supply voltage
(VIN), charging CL up to a higher volt-
age than the input supply.
The process continues with Q1
switching on again, once L1’s mag-
netic field has mostly dissipated, and
thus the field builds back up until Q1
switches off again.
The output voltage is sampled via
a voltage divider comprising trimpot
VR3 and a 10kΩ resistor. This deter-
mines the proportion of the output
voltage applied to pin 2 of REG2,
which it compares against an internal
1.26V reference. The duty cycle of Q1
is controlled to maintain 1.26V at the
pin 2 input.
Therefore, by changing the resist-
ance of VR3, we can vary the output
voltage. The greater the attenuation of
this resistive divider, the higher the
output voltage must be to maintain
1.26V at pin 2. If VR3 is set to 29.68kΩ,
the divider formed with the 10kΩ re-
sistor reduces the output by a factor of
3.97. That means that the output will
be 3.97 x 1.26V = 5V.
Should the output voltage rise
slightly above 5V, the TL499A will
cease switching Q1 until the volt-
age falls slightly below the 5V level.
Should the voltage fall below 5V, the
transistor will be driven with a higher
duty cycle, to deliver more current to
the output and bring it back up to 5V.
Note that the 1.26V reference is only
a nominal value and could be any volt-Fig.3: this PCB overlay diagram and the photo below show how the
components are fitted to the board. There are two possible locations for IC2,
depending on whether you’re using the through-hole (DIP) or SMD (SOIC)
package version. Be careful to orientate the diodes, ICs, cell holder, transmitter
and receiver correctly, as shown here. Some components can be left off if the
solar battery charging function is not needed (see the text for details).