“The LTC4365 is a unique solution that compactly and robustly protects sensitive circuits from unexpectedly high or negative supply voltages. The LTC4365 can isolate positive voltages up to 60V and negative voltages as low as C40V. Only voltages within the safe operating power supply range are delivered to the load. The only external active component required is a dual N-channel MOSFET connected between an unpredictable source and a sensitive load.
What would happen if someone connected a 24V power supply to your 12V circuit?
Will the circuit be safe if the power and ground wires are inadvertently reversed?
Does your application circuit operate in a harsh environment where the input power supply can transient to very high voltage or even below ground potential?
Even though the probability of such an event is low, any occurrence of any one of them will completely damage the board.
To isolate negative supply voltages, designers routinely place a power Diode or P-channel MOSFET in series with the supply. However–
Diodes both take up valuable board space and dissipate large amounts of power at high load currents.
The power dissipation of a P-channel MOSFET is lower than that of a series diode, but the cost of the MOSFET and the required driver circuitry will increase.
Both solutions sacrifice low power operation, especially the series diode. Also, neither approach provides protection against excessive voltages — protection that requires more circuitry, including a high-voltage window comparator and charge pump.
Undervoltage, overvoltage and power reverse protection
The LTC4365 is a unique solution that compactly and robustly protects sensitive circuits from unexpectedly high or negative supply voltages. The LTC4365 can isolate positive voltages up to 60V and negative voltages as low as C40V. Only voltages within the safe operating power supply range are delivered to the load. The only external active component required is a dual N-channel MOSFET connected between an unpredictable source and a sensitive load.
Figure 1 shows a complete application circuit. A resistive divider sets the overvoltage (OV) and undervoltage (UV) trip points for connecting/disconnecting the load from VIN. If the input supply drifts outside this voltage window, the LTC4365 will quickly disconnect the load from the supply.
Figure 1: Complete 12V automotive undervoltage, overvoltage and power reverse protection circuit
Dual N-channel MOSFETs are responsible for isolating positive and negative voltages on VIN. During standard operation, the LTC4365 provides a boosted 8.4V to the gate of the external MOSFET. The effective operating range of the LTC4365 is from as low as 2.5V to as high as 34V – the OV and UV windows can be within this range. For most applications, a protective clamp on VIN is not required, further simplifying board design.
Accurate and fast overvoltage and undervoltage protection
Two accurate (?1.5%) comparators in the LTC4365 are used to monitor overvoltage (OV) and undervoltage (UV) conditions on VIN. If the input voltage rises above the OV threshold or falls below the UV threshold, respectively, the gate of the external MOSFET turns off quickly. An external resistive divider allows the user to select an input supply range compatible with the load on VOUT.In addition, the UV and OV inputs have very low leakage current (typically at 100?C
Figure 2 shows the response of VIN in the circuit of Figure 1 when it slowly ramps up from C30V to 30V. The UV and OV thresholds are set to 3.5V and 18V, respectively. When the supply voltage is within the 3.5V to 18V window, VOUT follows VIN. Beyond this window, the LTC4365 turns off the N-channel MOSFET and disconnects VOUT from VIN, even when VIN is negative.
Figure 2: Load protection when VIN rises from C30V to 30V
Novel power reverse protection
The LTC4365 utilizes a novel negative supply protection circuit. When the LTC4365 detects a negative voltage on VIN, it quickly connects the GATE pin to VIN. There is no diode drop between the GATE and VIN voltages. When the gate of the external N-channel MOSFET is at negative Z potential (VIN), the leakage current from VOUT to the negative voltage on VIN is minimal.
Figure 3 shows what happens when VIN is live plugged into C20V. Immediately before connection, VIN, VOUT and GATE start at ground potential. Due to the parasitic inductance of the VIN and GATE connecting lines, the voltage on the VIN and GATE pins will drop significantly below C20V. The external MOSFET must have a breakdown voltage specification that can safely withstand this overshoot.
Figure 3: Hot Swap Protection from VIN to C20V
Obviously, the speed at which the LTC4365 reverse protection circuit operates depends on how closely the GATE pin follows VIN during negative voltage transients. On the scale shown, the waveforms of the two are almost indistinguishable. Note that no other external circuitry is required to provide reverse protection.
And many more features!
AC isolation, reverse VIN hot swap (Hot Swap?) control while VOUT is energized After an OV or UV fault (or when VIN goes negative), the input supply must return to a valid operating voltage window for at least 36ms to turn the external MOSFET back on. This will effectively isolate 50Hz and 60Hz unrectified AC power.
In addition, the LTC4365 provides protection against negative VIN connections, even when VOUT is driven by a separate supply. Reverse polarity on VIN will not affect the 20V supply on VOUT as long as the breakdown voltage of the external MOSFET (60V) is not exceeded.
By using back-to-back MOSFETs (rather than using diodes), the LTC4365 controller provides overvoltage, undervoltage and reverse power supply protection for sensitive circuits. The supply voltage is passed to the output only after passing through the user adjustable UV and OV trip gates XS. Any voltage outside this window is isolated and protected from up to 60V and as low as C40V.
The LTC4365’s novel frame construction results in a rugged, small form factor solution with minimal external components, and is available in tiny 8-pin 3mm x 2mm DFN and TSOT-23 packages. The LTC4365 has a wide 2.5V to 34V operating range and consumes only 10µA during shutdown.