Linear Technology
has introduced a chip for solar-powered battery charging, the
LT3652.
Solar panels are notoriously non-linear, and maximum power
extraction under all conditions requires a fairly complex technique
called peak power tracking (PPT) in the panel power converter.
Avoiding the complexity of PPT, Linear instead varies the
effective load current on the solar panel to keep its terminal
voltage constant.
"The LT3652's simple input voltage regulation loop circuitry
delivers virtually the same charging efficiency as more complex and
expensive PPT techniques," said Steve Pietkiewicz, v-p of power
products.
Input voltage regulation works for a sub-set of solar
panels.
"Bypass diodes inside a panel can create complex power/current
characteristics that are not easily optimised when partial shading
exists on the panel, however, virtually all of the 12V system solar
panels currently on the market that are specified with maximum
output power less than 25W are constructed from a simple series
cell arrangement with no bypass diodes," said the firm. "This type
of arrangement yields peak output power within a narrow band of
panel output voltages, regardless of lighting conditions. Peak
powers in excess of 95% may be produced from panel voltages of
12.5-18.5V, depending on the characteristics of the panel."
A resistive potential divider to a chip sensing pin sets the
input voltage for any particular design.
"An input voltage regulation loop also allows optimised charging
from poorly regulated sources where the input can collapse under
over-current conditions," added Linear Tech.
Operating voltage is 4.95 to 32V, with a 40V absolute
maximum.
The LT3652 needs no external microcontroller and can charge a
variety of series-connected cell configurations, including one to
three Li-ion or Li-polymer cells, one to four LiFePO4
cells, and sealed lead-acid batteries up to 14.4V.
Charge current is programmable up to 2A and there are charge
termination options including C/10 and an on-board timer.
Float voltage feedback accuracy is +/-0.5%, charge current
accuracy is +/-5% and C/10 detection accuracy is +/-2.5%.
"Once charging is terminated, the LT3652 automatically enters a
stand-by mode, which reduces the input supply current to 85uA,"
said Linear. "In shutdown, the input bias current is reduced to
15uA."
Auto-recharge starts a new charging cycle if the battery voltage
falls 2.5% below the programmed float voltage.
Other features include: 1MHz switching frequency, a thermistor
interface, and a bad battery detector with auto-reset.
The device is packaged in a 0.75mm tall 12pin 3x3mm DFN, and is
offered in both E and I grade versions, guaranteed from -40 to
125C.