### Battery Charger’s Unique Input Regulation Loop Simplifies Solar Panel Maximum Power Point Tracking

```Battery Charger’s Unique Input Regulation Loop Simplifies
Solar Panel Maximum Power Point Tracking
Jay Celani
Solar panels have great potential as
energy harvesting power sources—they
just need batteries to store the harvested
power and to provide carry-though during dark periods. Solar panels are relatively expensive, so extracting maximum
power from the panels is paramount to
minimizing the panel size. The tricky part
is a balancing of solar panel size with
required power. The characteristics of
solar panels require careful management
of the panel’s output power versus load
to effectively optimize the panel’s output
power for various lighting conditions.
(see Figure 1). Maintaining this peakpower point during operation as lighting
conditions change is called maximum
peak power tracking (MPPT). Complex
algorithms are often used to perform this
function, such as varying the panel’s load
periodically while directly measuring panel
output voltage and output current, calculating panel output power, then forcing
the point of operation that provides the
peak output power as illumination and/or
temperature conditions change. This type
of algorithm generally requires complex
circuitry and microprocessor control.
For a given illumination level, a solar
panel has a specific operating point that
produces the maximum amount of power
There exists, however, an interesting
relationship between the output voltage of a solar panel and the power that
the panel produces. A solar panel output
voltage at the maximum power point
remains relatively constant regardless of
illumination level. It follows that forcing operation of the panel such that the
output voltage is maintained at this peak
power voltage (VMP) yields peak output
power from the panel. A battery-charger
can therefore maintain peak power
In Depth
For an in-depth discussion of the maximum
power point tracking feature of the LT3652,
see “Designing a Solar Cell Battery Charger”
in the December 2009 issue of LT Magazine.
data sheet at www.linear.com/3652.
INCREASING ILLUMINATION
2.2
2
24
VMP
22
20
I VS V
1.8
18
P VS V
1.6
16
1.4
14
1.2
12
1
10
0.8
8
0.6
6
0.4
4
0.2
2
0
0
0
2
4
6
8
10
VPANEL (V)
12
14
16
PPANEL (W)
2.4
IPANEL (A)
Advances in battery technology and device performance
have made it possible to produce complex electronics that
run for long periods between charges. Even so, for some
devices, recharging the batteries by plugging into the grid
buoys, and remote weather monitoring stations are just a
so they must harvest energy from their environment.
Figure 1. Current vs voltage and power vs voltage
for a solar panel at a number of different illumination
levels. The panel output voltage at the maximum
power point (VMP) remains relatively constant
regardless of illumination level.
transfer by exploiting this VMP characteristic instead of implementing complex MPPT circuitry and algorithms.
A FEW FEATURES OF
THE LT3652 BATTERY CHARGER
The LT3652 is a complete monolithic stepdown multi-chemistry battery charger
that operates with input voltages as high
as 32V (40V abs max) and charges battery
stacks with float voltages up to 14.4V.
The LT3652 incorporates an innovative
input regulation circuit, which implements a simple and automatic method
for controlling the charger’s input supply voltage when using poorly regulated sources, such as solar panels. The
LT3652HV, a high voltage version of the
charger, is available to charge battery
stacks with float voltages up to 18V.
design features
The LT3652 is a versatile platform for simple and efficient
solar-powered battery charger solutions, applicable to
a wide variety of battery chemistries and configurations.
The LT3652 is equally at home in conventionally powered
applications, providing small and efficient charging solutions
for a wide variety of battery chemistries and stack voltages.
Input Regulation Loop Maintains
Peak Power Point for Solar Panels
The LT3652 input regulation loop linearly reduces the output battery charge
current if the input supply voltage falls
toward a programmed level. This closedloop regulation circuit servos the charge
current, and thus the load on the input
supply, such that the input supply voltage
is maintained at or above a programmed
the LT3652 implements MPPT operation by
simply programming the minimum input
voltage level to that panel’s peak power
voltage, VMP. The desired peak-power voltage is programmed via a resistor divider.
current or drop in solar panel illumination levels. In either case the regulation
loop maintains the solar panel output
voltage at the programmed VMP as set
by the resistor divider on VIN_REG.
The input regulation loop is a simple
and elegant method of forcing peak
power operation from a particular solar
panel. The input voltage regulation
loop also allows optimized operation
from other types of poorly regulated
sources, where the input supply can collapse during overcurrent conditions.
Integrated, Full-Featured
Battery Charger
The LT3652 operates at a fixed switching frequency of 1MHz, and provides
a constant-current/constant-voltage
(CC/CV) charge characteristic. The part
is externally resistor-programmable to
provide charge current up to 2A, with
charge-current accuracy of ±5%. The IC is
If during charging, the power required
by the LT3652 exceeds the available
power from the solar panel, the LT3652
input regulation loop servos the charge
current lower. This might occur due to
an increase in desired battery charge
particularly suitable for the voltage ranges
associated with popular and inexpensive
“12V system” solar panels, which typically
have open-circuit voltages around 25V.
The charger employs a 3.3V float voltage
feedback reference, so any desired battery float voltage from 3.3V to 14.4V (or
up to 18V with the LT3652HV) can be
programmed with a resistor divider. The
float-voltage feedback accuracy for the
LT3652 is ±0.5%. The wide LT3652 output
voltage range accommodates many battery
chemistries and configurations, including
up to three Li-ion/polymer cells in series,
up to four LiFePO4 (lithium iron phosphate) cells in series, and sealed lead acid
(SLA) batteries containing up to six cells
in series. The LT3652HV, a high-voltage
version of the charger, is also available.
The LT3652HV operates with input voltages
up to 34V and can charge to float voltages of 18V, accommodating 4-cell Li-ion/
polymer or 5-cell LiFePO4 battery stacks.
CMSH1-40MA
523k
VIN
LT3652
VIN_REG
100k
10k
10k
LED
SHDN
1N4148
10µH
0.05Ω
BAT
NTC
LED
FAULT
VFB
TIMER
Figure 2. A 2A solar panel power
manager for a 2-cell LiFePO4 battery
with 17V peak power tracking
1µF
SENSE
CHRG
CMSH3-40MA
SW
BOOST
549k
22
SYSTEM
CMSH3-40MA
10µF
10µF
464k
10k
B = 3380
MURATA NCP18XH103
2-CELL LiFePO4 (2 × 3.6V) BATTERY PACK
+
INPUT REGULATION VOLTAGE (V)
SOLAR PANEL INPUT
(<40V OC VOLTAGE)
TA = 25°C
20
18
100% TO 98% PEAK POWER
16
98% TO 95% PEAK POWER
14
12
10
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
CHARGER OUTPUT CURRENT (A)
2
Figure 3. A 17V input voltage regulation threshold
tracks solar panel peak power to greater than 98%
The LT3652 incorporates an innovative input regulation
circuit, which implements a simple and automatic method
for controlling the charger’s input supply voltage when
using poorly regulated sources, such as solar panels.
Energy Saving
Low Quiescent Current Shutdown
The LT3652 employs a precision-threshold
shutdown pin, allowing simple implementation of undervoltage lockout functions using a resistor divider. While in
low-current shutdown mode, the LT3652
draws only 15µ A from the input supply.
The IC also supports temperature-qualified
charging by monitoring battery temperature using a single thermistor attached
to the part’s NTC pin. The device has two
binary coded open-collector status pins
that display the operational state of the
LT3652 battery charger, CHRG and FAULT.
These status pins can drive LEDs for visual
signaling of charger status, or be used as
logic-level signals for control systems.
REPLACED BLOCKING DIODE
Si2319
SOLAR PANEL
INPUT
(14.5V TO 40V)
442k
PFET SUBCIRCUIT
TO REPLACE
BLOCKING DIODE
VIN
LT3652
VIN_REG
10k
49.9k
SENSE
SHDN
49.9k
Figure 4. A 2A 3-cell LiFePO4 charger
using a P-channel FET for input
blocking to increase high-current
charging efficiencies
FAULT
SW
1µF
1N4148
10µH
BOOST
0.05Ω
3.3V
BAT
CHRG
VFB
TIMER
NTC
0.68µF
226k
180k
10µF
100k
10k
B = 3380
MURATA NCP18XH103
+
3-CELL LiFePO4 (3 × 3.6V) BATTERY PACK
SIMPLE SOLAR POWERED
BATTERY CHARGER
Figure 2 shows a 2A 2-cell LiFePO4 battery charger with power path management. This circuit provides power to the
system load from the battery when the
solar panel is not adequately illuminated
and directly from the solar panel when
Figure 5. Comparative efficiencies for blocking
Schottky diode vs blocking FET as battery voltage
rises for 15V to 10.8V 3-cell LiFePO4 charger
the power required for the system load
is available. The input voltage regulation loop is programmed for a 17V peak
power input panel. The charger uses
C/10 termination, so the charge circuit is
disabled when the required battery charge
current falls below 200m A. This LT3652
charger also uses two LEDs that provide
status and signal fault conditions. These
binary-coded pins signal battery charging, standby or shutdown modes, battery
temperature faults and bad battery faults.
90
89
88
FET
87
86
85
84
SCHOTTKY
83
82
81
80
CMSH3-40MA
10µF
10V
100k
EFFICIENCY (%)
The LT3652 contains a programmable
safety timer used to terminate charging
after a desired time is reached. Simply
attaching a capacitor to the TIMER pin
enables the timer. Shorting the TIMER pin
to ground configures the LT3652 to terminate charging when charge current
falls below 10% of the programmed
maximum (C/10), with C/10 detection
accuracy of ±2.5%. Using the safety timer
for termination allows top-off charging
at currents less than C/10. Once charging is terminated, the LT3652 enters a
low-current (85µ A) standby mode. An
auto-recharge feature starts a new charging cycle if the battery voltage falls 2.5%
below the programmed float voltage. The
LT3652 is packaged in low-profile, 12-lead
3mm × 3mm DFN and MSOP packages.
7.7
8.2
8.7
9.7
9.2
VBAT (V)
10.2
10.7
The input voltage regulation point is
programmed using a resistor divider
from the panel output to the VIN_REG pin.
Maximum output charge current is
reduced as the voltage on the solar panel
output collapses toward 17V, which corresponds to 2.7V on the VIN_REG pin.
This servo loop thus acts to dynamically reduce the power requirements
of the charger system to the maximum
design features
maximum power point tracking (MPPT) operation by
simply programming the minimum input voltage level to
that panel’s peak power voltage, VMP. The specific desired
peak-power voltage is programmed via a resistor divider.
L1
10µH
SOLAR PANEL INPUT
(<40V OC VOLTAGE)
1µF
CIN
390µF
50V
523k
+
1.15M
VIN
SW
SENSE
VIN_REG
FAULT
10µF
50V
CMSH3-40MA
CMSH1-4
0.05Ω
BOOST
LT3652
BAT
IN
CHRG
SHDN
100k
NTC
100µF
10V
10µF
16V
100k
TIMER
VFB
Figure 6. A solar powered, 2A Li-ion charger with
ideal diode output pass element; the LTC4411 ideal
diode IC prevents reverse conduction during lowlight conditions
OUT
LTC4411
GND
CTL
STAT
316k
1.18M
0.1Ω
+ Li-ION
BATTERY
4.2V
L1: IHLP-2525CZ-01
power that the panel can provide, maintaining solar panel power utilization
close to 100%, as shown in Figure 3.
WANT BETTER EFFICIENCY?
REPLACE THE BLOCKING DIODE
WITH A BLOCKING FET
The LT3652 requires a blocking diode
when used with battery voltages
higher than 4.2V. The voltage drop
across this diode creates a power loss
term that reduces charging efficiency.
This term can be greatly reduced by
replacing the blocking diode with a
P-channel FET, as shown in Figure 4.
Figure 4 shows a 3-cell LiFePO4 2A charger with a float voltage of 10.8V. This
charger has an input voltage regulation
threshold of 14.5V and is enabled by the
SHDN pin when VIN ≥ 13V. Charge cycle
termination is controlled by a 3-hour
timer cycle. The blocking diode normally
used in series with the input supply for
reverse voltage protection is replaced
by a FET. A 10V Zener diode clamp is
used to prevent exceeding the FET maximum VGS . If the specified VIN range does
not exceed the maximum VGS of the
input FET, this clamp is not required.
During the high-current charging period
of a normal charge cycle (ICHG > C/10), the
CHRG status pin is held low. In the charger shown in Figure 4, this CHRG signal
is used to pull the gate of the blocking FET low, enabling a low-impedance
power supply path that eliminates the
blocking diode drop to improve conversion efficiency. Figure 5 shows that the
addition of this blocking FET improves
efficiency by 4% compared to operation with a blocking Schottky diode.
Should the timer be used for termination, the body diode of the FET provides
a conduction path once charge currents
of < C/10 is achieved, and the CHRG pin
becomes high-impedance. If desired, a
blocking Schottky diode can be left in
parallel with the blocking FET to improve
conversion efficiency during the top-off
portion of the timer-controlled charge
cycle. Use of a FETKEY as the blocking
element also increases top-off efficiency.
SCARED OF THE DARK?
USE AN IDEAL DIODE FOR
LOW-LIGHT APPLICATIONS
When the LT3652 is actively charging,
the IC provides an internal load onto the
switching loop to ensure closed-loop
operation during all conditions. This is
accomplished by sinking 2m A into the
BAT pin whenever a charging cycle is
active. In a solar-panel-powered battery
charger, low-light conditions can make the
input solar panel voltage collapse below
the input regulation threshold, causing
SOLAR PANEL INPUT
VP(MAX) = 3.8V
L1
4.7µH
C1
2.2µF
VIN VS L
L2
10µH
MBRS230LT1
470pF
SW
FBN
38.3k
SHDN
30.9k
LT3479
VREF
SS
RT
0.1µF
GND
17.8k
1µF
50V
169k
100pF
0.1µF
10V
SENSE
100k
4.32k
10nF
CMSH3-40MA
SW
VIN_REG
10µF
10V
210k
FBP
VC
VIN
CMSH1-4
BOOST
FAULT
0.068Ω
LT3652
CHRG
BAT
SHDN
NTC
100k
TIMER
VFB
1.18M
L1: CDRH-6D28-3R0
L2: IHLP-2525CZ-01
VBAT
4.2V
1.5A
316k
100µF
10V
10µF
16V
0.1Ω
Figure 7. Low-voltage solar panel powers 1.5A single cell Li-ion buck/boost battery charger. The LT3479 boosts the solar panel’s 3.8V output to operate an LT3652
charger. The LT3652’s closed loop operation includes the boost converter, thus regulating the LT3479’s input to the solar panel’s VMP of 3.8V.
output charge current to be reduced to
zero. If the charger remains enabled during this condition (i.e., the panel voltage remains above the UVLO threshold),
the internal battery load results in a net
current drain from the battery. This is
undesirable for obvious reasons, but this
condition can be eliminated by incorporating a unidirectional pass element that prevents current backflow from the battery.
Linear Technology makes a high-efficiency pass element IC, the LTC4411 ideal
diode, which has an effective forward
drop close to zero. The effect on overall
charger efficiency and final float voltage is negligible due to the extremely
low forward drop during conduction.
Figure 6 shows an LT3652 solar-powered
battery charger that employs low-light
reverse protection using an LTC4411
ideal-diode IC. During a low light condition, should the panel voltage collapse
below the input regulation threshold,
the LT3652 reduces battery charge current to zero. In the case where the input
voltage remains above the UVLO threshold, the charger remains enabled but is
held in a zero charge current state. The
LT3652 attempts to sink 2m A into the
BAT pin; however, the LTC4411 prevents
reverse conduction from the battery.
NEED TO STEP-UP? NO PROBLEM.
A 2-STAGE BUCK-BOOST
BATTERY CHARGER
The LT3652 can be used for step-up
and step-up/step-down charger applications by incorporating a front-end
step-up DC/DC converter. The frontend converter generates a local highvoltage supply for the LT3652 to use
as an input supply. The LT3652 input
regulation loop functions perfectly when
wrapped around both converters.
Figure 7 shows a low-voltage solar panel
powered 1.5A single-cell Li-ion charger
with a 4.2V float voltage. This charger is
designed to operate from a solar panel
that has a peak power voltage of 3.8V.
An LT3479 switching boost converter
running at 1MHz is used on the front-end
to create an 8V supply, which is used to
power the LT3652. This charger operates
with input voltages as low as the input
regulation threshold of 3.8V, up to 24V, the
maximum input voltage for the LT3479.
When input voltages approach 8V (or
higher), the LT3479 boost converter no
longer regulates, ultimately operating at
0% duty cycle and effectively shorting the
input supply through the pass Schottky
diode to the LT3652. Because the input
regulation loop monitors the input to the
LT3479, when the input voltage collapses
toward the input regulation threshold,
the LT3652 scales back charge current,
reducing the current requirements of
the LT3479 boost converter. The input
voltage servos to the regulation point,
with the boost converter and LT3652
charger combination extracting the peak
power available from the solar panel.
NEED MORE CHARGE CURRENT?
USE MORE LT3652s
Multiple LT3652 chargers can be used in
parallel to produce a charger that exceeds
the charge current capability of a single
LT3652. In the application shown in
Figure 8, three 2A LT3652 charger networks are connected in parallel to yield a
6A 3-cell Li-ion charger with a float voltage
of 12.3V that uses C/10 termination. This
charger is solar power compatible, having
an input regulation threshold of 20V. This
charger also implements an input blocking FET to increase charging efficiencies.
The three LT3652 charger ICs share a
common float voltage feedback network
design features
SOLAR PANEL INPUT
(20V TO 32V)
Si4401DY
649k
10V
100k
CIN
390µF
50V
10k
LED
+
MBRS340
VIN_REG
100k
VIN
SW
1µF
SHDN
CHRG
BAT
FAULT
NTC
VFB
0.05Ω
TIMER
28.7k
280k
10k
B = 3380
and a common input regulation network.
A feedback network with an equivalent
resistance of 250k W is recommended
to compensate for input bias currents
into the LT3652 VFB pin. Since the three
LT3652s share the same feedback network in this charger, and the input bias
currents are also shared through the
network, the network equivalent resistance is reduced to 250k/3, or ~83k W.
Due to tolerances in reference voltages,
one of the ICs will likely power up before
the other during an auto-recharge event.
In this case, the battery auto-recharges
at a maximum of 2A. Should the battery
continue to discharge due to a >2A load,
the second charger engages. Higher
discharge currents will engage the third
charger IC, allowing the charger to produce the full 6A system charge current.
The CHRG pins on all of the LT3652s are
tied together to enable the input blocking
FET, so the FET is low-impedance regardless of which order the ICs auto-restart.
The NTC and status functions are shared
by all three LT3652s, with each IC using a
dedicated NTC thermistor. The open collector status pins of the ICs are shorted
together, so engaging any or all of the
individual chargers lights the CHRG status
indicator. Likewise, an NTC fault in any of
the ICs lights the FAULT status indicator.
The individual LT3652 NTC functions are
slaved to each other via a diode connected from the common FAULT pins to
the common VIN_REG pins of all three ICs.
10µH
SENSE
LT3652
10k
4.7V 1N4148
BOOST
1N4148
10k
LED
4.7µF
x3
1.4M
100k
MBRS340
VIN_REG
VIN
SW
1µF
SHDN
4.7V 1N4148
BOOST
10µH
0.05Ω
SENSE
LT3652
CHRG
BAT
FAULT
NTC
VFB
TIMER
28.7k
10k
B = 3380
MBRS340
VIN_REG
VIN
SW
1µF
SHDN
4.7V 1N4148
BOOST
0.05Ω
SENSE
LT3652
CHRG
BAT
FAULT
NTC
VFB
TIMER
100k
10k
B = 3380
10µH
28.7k
3-CELL Li-ION
(3 × 4.2V)
BATTERY
+
MURATA NCP18XH103
Figure 8. A 6A 3-cell Li-ion battery charger using three LT3652 charger ICs
This diode pulls the VIN_REG pin below
the VIN_REG threshold should any of the
ICs trigger an NTC fault, which disables
all output charge current until the temperature fault condition is relieved.
CONCLUSION
The LT3652 is a versatile platform for
simple and efficient solar-powered battery charger solutions, applicable to a
wide variety of battery chemistries and
configurations. The LT3652 is equally at
home in conventionally powered applications, providing small and efficient
charging solutions for a wide variety of
battery chemistries and stack voltages.
Solar-powered charger solutions maintain
panel utilization close to 100%, reducing solution costs due to minimized panel
area. The compact size of the IC, coupled
with modest external component requirements, allows construction of stand-alone
charger systems that are both tiny and
inexpensive, providing a simple and
efficient solution to realize true gridindependence for portable electronics. n