### December 2009 - Designing a Solar Cell Battery Charger

L DESIGN FEATURES
Designing a Solar Cell Battery Charger
by Jim Drew
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 VIN_REG pin, falls
12
2.4
24
2.2
IPANEL (A)
22
1000W/m2
2
20
1.8
18
1.6
16
1.4
14
1.2
12
1
10
0.8
8
0.6
6
0.4
4
0.2
0
100W/m
0
2
4
2
2
6
8
10
VPANEL (V)
12
14
16
0
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 current 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 VIN and VIN_REG pins, the current
control voltage is reduced, thus reducing the charging current. This action
causes the voltage from the solar panel
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
100
80
VSENSE – VBAT (mV)
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
panel, the input voltage regulation
loop is used to maintain the panel at
near peak power output.
PPANEL (W)
Introduction
60
40
20
0
2.65
2.67
2.69
2.71
VIN_REG (V)
2.73
2.75
Figure 2. Charger current control voltage (VSENSE – VBAT) vs proportional input voltage, as
measured via voltage divider at VIN_REG pin. VIN (solar panel voltage) only affects charging current
when VIN_REG is between 2.67V and 2.74V. In this range, the charger will reduce the charging
current if necessary to run the panel at peak power output.
Linear Technology Magazine • December 2009
DESIGN FEATURES L
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 VIN_REG
control characteristic curve. As the
voltage on the VIN_REG pin increases
beyond 2.67V, the voltage VSENSE
– VBAT, across the current sensing
resistor, increases until it reaches a
maximum of 100mV, when VIN_REG
is above 2.74V. As VIN_REG increases
further, VSENSE – VBAT remains at
100mV. The expression for the input
voltage control range is:
(
2.67 • RIN1 + RIN2
RIN2
)
(
RIN2
 V •R

1.43 •  IN IN2 – 2.67 V 
 RIN1 + RIN2

Eq.1
)
If we linearize the portion of the
curve in Figure 2 for VIN_REG between
2.67V and 2.74V, the following expression describes the current sensing
voltage VSENSE – VBAT:
VSENSE – VBAT =
Eq.2
1.43 • (VIN_REG – 2.67V)
Eq.3
The charging current for the battery
would then be:
ICHARGE =
1.43
RSENSE
 V •R

•  IN IN2 – 2.67 V 
R +R

IN1
IN2
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:
IIN = ICHARGE •
< VIN CONTROL RANGE <
2.74 • RIN1 + RIN2
VSENSE – VBAT =
VBAT
η • VIN
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:
PIN =
1.43 • VBAT
RSENSE • η
 V •R
 Eq. 6
•  IN IN2 – 2.67 V 
 RIN1 + RIN2

Once RSENSE is selected for the
maximum charging current and RIN1
and RIN2 are determined to select the
CMSH1-40MA
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, VP(MAX), and peak power current, IP(MAX). The short circuit current,
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 = (VBAT(FLOAT) + VFORWARD(D1) + 3.3V) • 1.15
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.
OPTIONAL (SEE TEXT)
SOLAR PANEL INPUT
RNTC
390µF
50V
RSHDN1
787k
RIN1
280k
RIN2
100k
10µF
50V
VIN
LT3652
VIN_REG
CMSH3-40MA
1µF
50V
SW
10µH
IHLP-2525CZ-01
BOOST
FAULT
SENSE
CHRG
RSHDN2
100k
RSENSE
0.05Ω
CMSH1-4
SHDN
BAT
NTC
TIMER
GND
RFB1
619k
VFB
RFB2
412k
10µF
16V
100µF
10V
+
2-CELL Li-ION (2 = 4.1V)
0.1Ω
Figure 3. 2A Solar-powered battery charger
Linear Technology Magazine • December 2009
13
L DESIGN FEATURES
VP(MAX) =
VIN CONTROL RANGE (VREG)
VREG(MIN)=10.65V
(VBAT(FLOAT) + VFORWARD(D1) + 0.75V) •
1.15
20
RSENSE = 0.05Ω
The output feedback voltage divider network of RFB1 and RFB2 are
determined next. The voltage divider
network must have a Thevenin’s equivalent resistance of 250k to compensate
for input bias current error. The VFB
pin reference voltage is 3.3V.
RFB1 =
=
VBAT(FLOAT) • 250k
PIN (W)
10
4
0
=
619k • 250k
619k − 250k
= 419.2k
Let RFB2 = 412k
The next step is to set the peak
power tracking voltage using the volt-
10
E
11
VSHDN V
Let RIN2 = 100k
12
13
PIN FOR VBAT(PRE)
5.7V AT 0.3A
C
14
IN (V)
VP(MAX ) − VFORWARD(D1) − 2.74V
2.74V
( VREG(MIN) − VF(D1) ) − ( VSHDN(MAX) − VSHDN(HYST) )
VSHDN(MAX ) − VSHDN(HYST)
RSHDN1 =
• RIN2
10.9 V − 0.5V − 2.74V
=
• 100k
2.74V
= 279.6k
Let RIN1 = 280k
Verify the minimum and maximum
peak power input tracking voltages.
VREG(MAX ) = 2.74V •
RSHDN1 = RSHDN2 •
Let RSHDN2 = 100k
RIN1 =
= 10.65V
RFB2 • 250k
RFB2 − 250k
9
age divider network of RIN1 and RIN2
connected between the VIN and the
VIN_REG pins.
8.2V • 250k
3.3V
RFB2 =
100W/m2
Figure 4. Action of the solar battery charger circuit in Figure 3. Power-intensity curves for
various illumination levels are shown for 100W/m2 to 1000W/m2 in 100W/m2 steps. The VIN
control range (VREG) is also shown. The VIN control loop extracts maximum possible power from
the solar panel by steering VIN to the top of the panel’s power-intensity curve when VIN is in the
VREG range.
VREG(MIN) = 2.67 V •
Let RFB1 = 619k
PIN FOR VBAT(MIN)
5.7V AT 2A
8
3.3V
= 621.2k
14
12
2
The solar panel characteristics can
be seen in Figure 4.
The current sensing resistor,
R SENSE , is determined from the
maximum VSENSE – VBAT of 100mV
divided by the maximum charging
current of 2A
A
14
η • VP(MAX )
VP(MAX) = 10.9V
IP(MAX) = 1.8A
D
16
6
VOC = 13.8V
PIN FOR VBAT(FLT)
8.2V AT 2A
B
18
VBAT(FLOAT)
Solving for these three equations,
we can define the minimum requirements of the solar panel:
LIGHT INTENSITY = 1000W/m2
22
The peak input power current is the
product of the float voltage and the
maximum charging current divided by
the peak power input voltage and the
efficiency of the charging circuit.
IP(MAX ) = ICHARGE •
VREG(MAX)=10.9V
24
RIN1 + RIN2
+ VF(D1)
RIN2
RIN1 + RIN2
+ VF(D1)
RIN2
= 10.9 V
The final step in selecting resistor values is to determine the VSHDN
voltage divider network consisting of
RSHDN1 and RSHDN2. The VSHDN rising
threshold is 1.2V ± 50mV with a hysteresis of 120mV. The voltage divider
network wants to be set such that,
when the voltage on the VIN pin is at
VREG(MIN), VSHDN is at its maximum
possible value.
(10.65V − 0.5V ) − (1.25V − 0.12V ) • 100k
1.25V − 0.12V
= 798.2k
Let RSHDN1 = 787k
The VSHDN limits are now determined as:
VSHDN Rising Threshold
VSHDN(MIN) = 10.7V
VSHDN(MAX) = 11.6V
VSHDN Falling Threshold
VSHDN(MIN) = 9.6V
VSHDN(MAX) = 10.5V
The LT3652 automatically enters
a battery precondition mode if the
sensed battery voltage is very low.
In this mode, the charge current is
reduced to 15% of the programmed
maximum, as set by the current
sensing resistor, RSENSE. Once the
battery voltage reaches 70% of the
fully charged float voltage (VFB = 2.3V),
the LT3652 automatically increases
maximum charge current to the full
programmed value. The battery voltage
threshold level between precondition
Linear Technology Magazine • December 2009
DESIGN FEATURES L
mode and maximum charge current
is determined as follows:
VBAT(PRE) < VBAT(MIN) = VBAT(FLOAT) •
2.3V
3.3V
VBAT(MIN) = 5.7V
VBAT(PRE) < 5.7V
VCHRG(PRE) = 0.15 • ICHRG
VCHRG(PRE) = 0.3A
Using and efficiency of 0.85, plot PIN
over the range of VIN that is current
controlled. This is the regulated VIN,
or VREG, power line. The intersection
of the VREG power line with the solar
panel power curve is the operating
point. As the battery charges, the
slope of the VREG power line increases,
indicating the increase in input power
required to support the increasing
output power. The intersection of the
VREG power line continues to follow
up the solar panel’s power curves
until the charger exits constant current mode.
The resulting plots are shown in
Figure 4.
approaches full charge (point B). The
LT3652 transitions from constant current mode to constant voltage mode
and the charging current is reduced.
The solar panel operating point moves
back down the light-power-intensity
curve to the open circuit voltage (point
C) when the battery reaches its final
float voltage.
During the charging of the battery, if
the light intensity diminishes, the operation point moves across a constant
The input voltage regulation
loop of the LT3652 has
the ability to seek out the
maximum power operating
point of a solar panel’s
power characteristic, thus
utilizing the full capacity of
the solar panel.
Figure 4 shows the power output of the
solar panel plotted at light intensity
levels from 100W/m2 to 1000W/m2
in 100W/m2 steps. At maximum light
intensity (top curve in Figure 4) and
the battery voltage just above the preconditioning level (VBAT(MIN) at 2A), the
solar panel is producing more power
than the charger needs. The solar
panel voltage rises above the VREG
control voltage and travels across the
constant power line until it intersects
the light-power-intensity curve for
that intensity level (point A in Figure
4). As the battery charges, the input
power increases and the solar panel
operating point moves up the lightpower-intensity curve until the battery
power line for the battery voltage until
it reaches the new power-intensity
curve. If the light intensity level continues to diminish, the operating point
travels along this constant power line
until it reaches the VREG power line.
At this point the charging current is
reduced until the operating point is at
the intersection of the light-power-intensity curve and the VREG power line
(point D for constant current charging
at VBAT(FLOAT) with 800W/m2 illumination). As the battery continues to
charge at this light intensity level, the
operating point moves along the new
light-power-intensity curve until the
battery approaches full charge.
As darkness approaches, the operating point moves down the VREG
power line until charging current
ceases (point E) and the solar panel
output voltage drops below the SHDN
LTC3612, continued from page 11
Conclusion
The Circuit in Action
inductor current measured through
the bottom MOSFET increases beyond
6A, the top power MOSFET is held off
and switching cycles are skipped until
the inductor current is reduced.
Linear Technology Magazine • December 2009
The LTC3612 is well suited for a wide
range of low voltage step-down converter applications, including DDR
memory termination applications
requiring ±1.5A of output current. Its
falling threshold at which point the
LT3652 turns off.
The remaining elements of the
design, selection of output inductor,
catch rectifier and timer capacitor,
are outlined in the design procedure
in the LT3652 datasheet along with
PCB layout considerations.
The maximum power voltage, for
a monocrystalline solar cell, has a
temperature coefficient of –0.37%/K
while the maximum power level is
–0.47%/K. This may be compensated
for by letting RIN1 be a combination
of a series resistor and a series NTC
thermistor. The ratio of the two elements that comprise RIN1 and the value
of RIN2 need to be adjusted to achieve
the correct negative temperature of
VIN while still maintaining the control
range of VIN.
∆VIN(NTC) =
VREG RNTC • ∆RNTC
•
RIN2
RIN1 • RNTC
Conclusion
The input voltage regulation loop of
the LT3652 has the ability to seek out
the maximum power operating point
of a solar panel’s power characteristic,
thus utilizing the full capacity of the
solar panel. The float voltage regulation loop and its adjustable charging
current enable the LT3652 to be used
with many battery chemistries, making
it a versatile battery charger. The added
features of a wide input voltage range,
an auto-recharge cycle to maintain a
fully charged battery, a battery preconditioning mode, NTC temperature
sensing, selectable C/10 or timed
charging termination, a FAULT and
a charging status pins fills out the
full feature set of the LT3652. The
LT3652 is available in a 3mm × 3mm
12-lead plastic DFN, package with an