For infrequent surges a linear surge protection regulator is fine, but in the face of frequent surges dissipation from a linear regulator could become an issue.
For situations where this dissipation is a problem, Analog Devices has announced LTC7862, a switching dc-dc converter specifically for surge limitation.
In normal operation it is held at 100% duty cycle through an external n-mosfet – with an on-chip charge pump providing the over-Vcc gate voltage.
Once the input climbs above the chosen maximum output voltage, duty cycle is reduced from 100% to keep the output at that maximum allowable voltage.
“Power handling capability of a surge stopper is limited by the power lost in the protection circuit,” said ADI. “In a linear surge stopper, power capability is determined by the size of the pass mosfet, while in a switching surge stopper the power capability is determined by conversion efficiency. Due to its lower power loss, a switching surge stopper can operate through longer input voltage surges at higher output current than a linear solution.”
Input voltage range for the protection chip is 4-140V (150V abs max). And outputs up to 60V can be clamped.
While applications for this kind of circuit might seem rare, the firm points at equipment intended to meet MIL-STD-1275 and, for vehicles, ISO 7637 and that in need of load-dump protection.
“In MIL-STD-1275 applications the LTC7862 protects devices operating from a 28V vehicle power bus, where this bus voltage can reach as high as 100V and last up to 500ms,” said ADI. “It also fits into industrial, avionics, and automotive power, including ISO 7637.”
An external capacitor configures an internal fault timer which limits the time spent in over-voltage mode, putting a limit on the amount of heat external components have to take allowing the size of the solution to be limited.
Input current limiting and soft-start are included.
The chip is available in a 4 x 5mm 20-pin QFN, or a TSSOP package.
Temperature options are -40 to 125°C or -40 to 150°C.