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 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.
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, enabling 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 schematic in Figure 2 shows an LT8705 circuit that charges a 48V battery and operates from an input voltage that can vary from 36V to 72V. Multiple circuits can be paralleled for higher power applications. There are four external MOSFETs that enable this circuit to be used as a synchronous buck/boost converter and are configured as a current source to charge four each 12V lead acid batteries in series for this application.
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