Linear Technology’s approach
The LT8705 is a high efficiency (up to 98 percent) synchronous buck-boost DC/DC controller that operates from input voltages above, below or equal to the regulated output voltage. This device has four feedback loops to regulate the input current/voltage, along with the output current/voltage. The input current and voltage feedback loops can prevent overloading of solar cells. The output current loop provides a regulated output current for a battery charger or as a current source. The LT8705 operates over a wide 2.8V to 80V input voltage range and produces a 1.3V to 80V output, using a single inductor with 4-switch synchronous rectification. Output power up to 250W can be delivered with a single device. Higher output power can be achieved when multiple circuits are configured in parallel. The LT8705 employs a proprietary current-mode control architecture for constant frequency operation in buck or boost mode and has powerful onboard quad N-channel MOSFET gate drivers. The user can select among forced continuous, discontinuous and Burst Mode operation to maximize light load efficiency.
Additional features include servo pins to indicate which feedback loops are active, a 3.3V/12mA LDO to power external devices, adjustable soft-start, onboard die temperature monitor and an operating junction temperature range of -40°C to 125°C. The LT8705 is available in a 38-pin 5mm x 7mm QFN, and a 38-lead TSSOP package. An LTspice circuit model for the LT8705 is also available and can be used to evaluate all kinds of creative applications quickly.
The LT8705 contains four error amplifiers allowing it to regulate or limit the output current, input current, input voltage and output voltage. In a typical application the output voltage might be regulated, while the remaining error amplifiers are monitoring for excessive input or output current or an input undervoltage condition. In other applications, such as a battery charger, the output current regulator can facilitate constant current charging until a predetermined voltage is reached where the output voltage control would take over. The block diagram in Figure 1
shows a solar panel input where the LT8705 is utilized in a battery charging application.
Figure 1: LT8705 solar panel battery charger block diagram
Click on image to enlarge
In the block diagram shown in figure 1
, the LT8705 monitors the input voltage of the solar panel with its onboard EA3 error amplifier to make sure it does not collapse. It also charges four lead acid batteries in series with a constant current source utilizing the EA1 error amplifier. Even though the LT8705 has four servo pins this example only shows two of the servo pins for clarity purposes. The battery charger current source is in control of the LT8705 to supply a 5A charging current as shown by the red LED indicator. However, if the solar panel voltage dips below a predetermined voltage, the LT8705 will reduce the charge current to prevent the solar panel voltage from collapsing. The other two servo pins not shown in the block diagram are the input current and output voltage pins. All these servo indicators are very useful if maximum power point tracking (MPPT) is required.
A complete LT8705 schematic is shown in figure 2
that operates from a wide 28V to 72V input voltage range which is wide enough to cover the solar panel input voltage for this application. There are 4 external MOSFETs that allow this circuit to be used as a synchronous buck/boost converter and are configured as a current source to charge 4 each 12V lead acid batteries in series.
Figure 2: Complete LT8705 solar battery charger applications schematic
Click on image to enlarge