LINEAR TECHNOLOGY LT3652 Technical data
P
PANEL
(W)
I
PANEL
(A)
V
PANEL
(V)
160
2.4
0
24
0
2 4 6 8 10 12 14
2.2
0.2
0.4
0.6
0.8
1
2
1.8
1.6
1.4
1.2
22
2
4
6
8
10
20
18
16
14
12
100W/m
2
1000W/m
2
V
IN_REG
(V)
2.65
V
SENSE
– V
BAT
(mV)
0
20
80
60
40
2.67
2.69
100
2.71
2.73 2.75
L DESIGN FEATURES
Designing a Solar Cell Battery Charger
by Jim Drew
Introduction
The market for portable solar powered
electronic devices continues to grow
as consumers look for ways to reduce
energy consumption and spend more
time outdoors. Because solar power
is a variable and unreliable, nearly
all solar-powered devices feature
rechargeable batteries. The goal is to
extract as much solar power as possible to charge the batteries quickly
and maintain the charge.
Solar cells are inherently inefficient
devices, but they do have a point of
maximum power output, so operating
at that point seems an obvious design
goal. The problem is that the IV characteristic of maximum output power
changes with illumination. A monocrystalline solar cell’s output current
is proportional to light intensity, while
its voltage at maximum power output
is relatively constant (see Figure 1).
Maximum power output for a given
light intensity occurs at the knee of
each curve, where the cell transitions
from a constant voltage device to a
constant current device. A charger
design that efficiently extracts power
from a solar panel must be able to steer
the panel’s output voltage to the point
of maximum power when illumination
levels cannot support the charger’s full
power requirements.
The LT3652 is a multi-chemistry
2A battery charger designed for solar
power applications. The LT3652 employs an input voltage regulation loop
that reduces the charge current if the
input voltage falls below a programmed
level set by a simple voltage divider
network. When powered by a solar
panel, the input voltage regulation
loop is used to maintain the panel at
near peak power output.
LT3652 Input Voltage
Regulation Loop
The input voltage regulation loop of the
LT3652 acts over a specific input voltage range. When VIN, as measured via a
resistor divider at the V
12
IN_REG
pin, falls
Figure 1. A solar cell produces current in proportion to the amount of sunlight falling on it, while
the cell’s open-circuit voltage remains relatively constant. Maximum power output occurs at
the knee of each curve, where the cell transitions from a constant voltage device to a constant
current device, as shown by the power curves.
below a certain set point, the charge
current is reduced. The charging cur rent is adjusted via a control voltage
across a current sensing resistor in
series with the inductor of the buck
regulator charging circuit. Decreased
illumination (and/or increased charge
current demands) can both cause the
input voltage (panel voltage) to fall,
pushing the panel away from its point
of maximum power output. With the
LT3652, when the input voltage falls
below a certain set point, as defined by
the resistor divider connected between
the V
and V
IN
pins, the current
IN_REG
control voltage is reduced, thus reducing the charging current. This action
causes the voltage from the solar panel
Figure 2. Charger current control voltage (V
measured via voltage divider at V
when V
current if necessary to run the panel at peak power output.
is between 2.67V and 2.74V. In this range, the charger will reduce the charging
IN_REG
pin. VIN (solar panel voltage) only affects charging current
IN_REG
to increase along its characteristic VI
curve until a new peak power operating point is found.
If the solar panel is illuminated
enough to provide more power than
is required by the LT3652 charging
circuit, the voltage from the solar panel
increases beyond the control range of
the voltage regulation loop, the charging current is set to its maximum value
and a new operation point is found
based entirely on the maximum charging current for the battery’s point in
the charge cycle.
If the electronic device is operating directly from solar power and the
input voltage is above the minimum
level of the input voltage regulation
– V
SENSE
) vs proportional input voltage, as
BAT
Linear Technology Magazine • December 2009
DESIGN FEATURES L
2 67
2 74
1 2
2
. •
. •
R R
R
V CONTROL RANGE
R
IN IN
IN
IN
+
(
)
< <
IIN IN
IN
R
R
1 2
2
+
(
)
V – V
SENSE BAT
=
+
1 43 2 67
2
1 2
. •
•
– .
V R
R R
V
IN IN
IN IN
I
CHARGE
=
+
1 43
2 67
2
1 2
.
•
•
– .
R
V R
R R
V
SENSE
IN IN
IN IN
I
V
V
IN
BAT
IN
= I
CHARGE
•
•η
P
V
R
V R
R R
IN
BAT
SENSE
IN IN
IN IN
=
+
1 43
2 6
2
1 2
. •
•
•
•
– .η77V
SW
V
IN
SOLAR PANEL INPUT
V
IN_REG
V
FB
BOOST
SENSE
NTC
BAT
TIMER
GND
1µF
50V
R
FB1
619k
2-CELL Li-ION (2 = 4.1V)
+
390µF
50V
CMSH1-40MA
OPTIONAL (SEE TEXT)
10µF
16V
10µH
IHLP-2525CZ-01
LT3652
R
IN1
280k
R
NTC
R
IN2
100k
SHDN
CHRG
FAULT
10µF
50V
R
SHDN1
787k
R
SHDN2
100k
R
SENSE
0.05Ω
R
FB2
412k
0.1Ω
100µF
10V
CMSH1-4
CMSH3-40MA
loop’s control range, the excess power
available is used to charge the battery
at a lower charging rate. The power
from the solar panel is adjusted to its
maximum operating power point for
the intensity level.
Figure 2 shows a typical V
IN_REG
control characteristic curve. As the
voltage on the V
beyond 2.67V, the voltage V
– V
, across the current sensing
BAT
pin increases
IN_REG
SENSE
resistor, increases until it reaches a
maximum of 100mV, when V
is above 2.74V. As V
further, V
SENSE
– V
increases
IN_REG
remains at
BAT
IN_REG
100mV. The expression for the input
voltage control range is:
Eq.1
If we linearize the portion of the
curve in Figure 2 for V
IN_REG
between
2.67V and 2.74V, the following expression describes the current sensing
voltage V
V
– V
SENSE
1.43 • (V
IN_REG
SENSE
=
BAT
– 2.67V)
– V
BAT
:
Eq.2
Eq.3
The charging current for the battery
would then be:
Eq.4
Since the charging circuit of the
LT3652 is a current controlled buck
regulator, the input current relates to
the charging current by the following
expression:
Eq. 5
where η is the efficiency of the
charger
The input power can now be determined by combining Equations 4 and
5 with the input voltage, resulting in
the following:
Eq. 6
Once R
maximum charging current and R
and R
are determined to select the
IN2
is selected for the
SENSE
IN1
input voltage current control range,
Equation 6 can be plotted against the
solar panels power curves to determine the charger’s operating point for
various battery voltages. An example
follows.
Design Example
Figure 3 shows a 2A, solar powered,
2-cell Li-Ion battery charger using
the LT3652.
First step is to determine the minimum requirements for the solar panel.
Important parameters include the
open circuit voltage, VOC, peak power
voltage, V
rent, I
P(MAX)
ISC, of the solar panel falls out of the
calculations based on the other three
parameters.
The open circuit voltage must be
3.3V plus the forward voltage drop
of D1 above the float voltage of the 2cell Li-ion battery plus an additional
15% for low intensity start-up and
operation.
VOC =
(V
BAT(FLOAT)
The peak power voltage must
be 0.75V plus the forward drop of
D1 above the float voltage plus an
additional 15% for low intensity operation.
, and peak power cur -
P(MAX)
. The short circuit current,
+ V
FORWARD(D1)
+ 3.3V) • 1.15
Linear Technology Magazine • December 2009
Figure 3. 2A Solar-powered battery charger
13
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