Negative-input synchronous buck-boost / inverting DC-DC controller

A new synchronous PWM controller for negative-to-negative or negative-to-positive DC-DC conversion, the LT8709 has been developed by Linear technology. The device is 'unique' in solving the problem of regulating a negative voltage with respect to system ground, without the need of complicated level shifting circuitry, says the company. The device’s synchronous operation means that the output diode is replaced with a high efficiency P-channel MOSFET, thereby increasing efficiency, allowing for higher output currents (up to 20A), and eliminating the heat sink typically required in medium to high power applications. The LT8709 can be configured in buck, boost, buck-boost, and inverting topologies, making it highly versatile for a wide range of power supply designs. The LT8709 operates over a -4.5V to -806V input voltage range and produces an output voltage from -0.1V to as high as 60V or from -1.4V to as low as -80V. Its rail-to-rail output current monitor and control enables the device to be configured as a current source. The LT8709 features innovative EN/FBIN pin circuitry for slowly varying input signals and an adjustable under-voltage lockout function. This pin is also used for input voltage regulation to avoid collapsing a high impedance supply. The fixed operating frequency is selectable from 100kHz to 750kHz and can be synchronized to an external clock. Current mode control provides excellent line and load regulation. The LT8709 is configurable for either forced continuous or pulse-skipping operating modes during light load conditions. Additional features include a power good indicator, thermal shutdown and integrated soft-start circuitry. The LT8709 is available in a 20-lead TSSOP package, says the company.