60V chip converts regulators into solar chargers
Linear Technology has introduced a 60V power controller and power manager “that converts virtually any externally compensated DC/DC power supply into a full-featured battery charger,” said the firm.
Called LTC4000-1, it operates across 3 to 60V input and output range.
“It is capable of driving typical dc-dc converter topologies, including buck, boost, buck-boost, SEPIC, fly-back and forward,” said Linear. “It only requires the dc-dc converter to have a control or external-compensation pin whose voltage level varies in a positive monotonic way with its output. The output variable can be either output voltage or output current.”
Output voltage and current regulation is offered, as well as input voltage and current regulation.
The two output measurements are used to charge different batteries.
“It charges a variety of chemistries, including lithium-ion. lithium polymer, lithium iron phosphate, sealed lead acid (SLA), and nickel-based,” said the firm. The device provides charge status pins, and includes +/-0.2% programmable float voltage [+/-1% over temperature], selectable timer or C/X current termination, temperature-qualified charging using an NTC thermistor, automatic recharge, C/10 trickle charge for deeply discharged cells and bad battery detection.”
The two input measurements are used to match the controller with difficult power sources.
In particular, input voltage regulation is use to approximate maximum power point tracking for solar panels.
The output impedance of solar panels depends on illumination conditions (and temperature), with the effect that every level of illumination needs a different load impedance to extract maximum power from the panel.
This load-illumination-temperature relationship is so complex that the only practical way to extract maximum power reliably involves iterative circuits that dither the operating point of a regulator to make small changes in the load it offers to the panel.
By measuring power delivered to the final load during these dithers, the operating point of the regulator can be servoed-in on the value that extracts most power from the panel in any particular condition.
This technique is known as ‘maximum power point tracking’, and various similar expressions.
Linear Tech has instead implemented an approximation to the above technique that it calls “input maximum power point control”.
This takes advantage of a fixed panel output voltage, declared by the panel manufacturer, that gives a significant part of maximum output over a significant range of illumination.
The LTC4000’s solar power control loop sets the regulator output current to a value that holds the solar panel output voltage to the manufacturer’s suggested voltage.
Battery disconnect and reverse current control comes from the chip through two external p-FETs wired as ‘perfect diodes’. These also provide “instant-on operation to ensure that system power is available at plug-in, even with a deeply discharged battery”, said Linear Tech.
Operation is guaranteed from -40 to 125°C, and by choosing appropriate external FETs and external regulator, power from milliwatts to kilowatts can be handled by the 0.75x4x5mm 28 contact QFN, which is also available as a SSOP.