No Blocking Diode Needed to Protect Sensitive Circuits from Overvoltage and Reverse Supply Connections

What would happen if someone connected 24V to your 12V circuits? If the power and ground lines were inadvertently reversed, would the circuits survive? Does your application reside in a harsh environment, where the input supply can ring very high or even below ground? Even if these events are unlikely, it only takes one to destroy a circuit board.

What can you do to protect your sensitive circuits from voltages that are too high, too low, or even negative? To block negative supply voltages, system designers traditionally place a power diode in series with the supply. However, this diode takes up valuable board space and dissipates a significant amount of power at high load currents.

Another common solution is to place a high voltage P-channel MOSFET in series with the supply. The P-channel MOSFET dissipates less power than the series diode, but the MOSFET, and the circuitry required to drive it, drives up costs.

One drawback to both of these solutions is that they sacrifice low supply operation, especially the series diode. Also, neither protects against voltages that are too high—protection that requires more circuitry, including a high voltage window comparator and charge pump.

Undervoltage, Overvoltage And Reverse-supply Protection

The LTC4365 is a unique solution that elegantly and robustly protects sensitive circuits from unpredictably high or negative supply voltages. The LTC4365 blocks positive voltages as high as 60V and negative voltages as low as –40V. Only voltages in the safe operating supply range are passed along to the load. The only external active component required is a dual N-channel MOSFET connected between the unpredictable supply and the sensitive load.

Figure 1 shows a complete application. A resistive divider sets the overvoltage (OV) and undervoltage (UV) trip points for connecting/disconnecting the load from VIN. If the input supply wanders outside this voltage window, the LTC4365 quickly disconnects the load from the supply.

Figure 1. Complete 12V automotive undervoltage, overvoltage and reverse-supply protection circuit.

The dual N-channel MOSFET blocks both positive and negative voltages at VIN. The LTC4365 provides 8.4V of enhancement to the gate of the external MOSFET during normal operation. The valid operating range of the LTC4365 is as low as 2.5V and as high as 34V—the OV–UV window can be anywhere in this range. No protective clamps at VIN are needed for most applications, further simplifying board design.

Accurate and Fast Overvoltage and Undervoltage Protection

Two accurate (±1.5%) comparators in the LTC4365 monitor for overvoltage (OV) and undervoltage (UV) conditions at VIN. If the input supply rises above the OV or below the UV thresholds, respectively, the gate of the external MOSFET is quickly turned off. The external resistive divider allows a user to select an input supply range that is compatible with the load at VOUT. Furthermore, the UV and OV inputs have very low leakage currents (typically < 1nA at 100°C), allowing for large values in the external resistive divider.

Figure 2 shows the how the circuit of Figure 1 reacts as VIN slowly ramps from –30V to 30V. The UV and OV thresholds are set to 3.5V and 18V, respectively. VOUT tracks VIN when the supply is inside the 3.5V–18V window. Outside of this window, the LTC4365 turns off the N-Channel MOSFET, disconnecting VOUT from VIN, even when VIN is negative.

Figure 2. Load protection as VIN is swept from –30V to 30V.

Novel Reverse Supply Protection

The LTC4365 employs a novel negative supply protection circuit. When the LTC4365 senses a negative voltage at VIN, it quickly connects the GATE pin to VIN. There is no diode drop between the GATE and VIN voltages. With the gate of the external N-channel MOSFET at the most negative potential (VIN), there is minimal leakage from VOUT to the negative voltage at VIN.

Figure 3 shows what happens when VIN is hot plugged to –20V. VIN, VOUT and GATE start out at ground just before the connection is made. Due to the parasitic inductance of the VIN and GATE connections, the voltage at VIN and GATE pins ring significantly below –20V. The external MOSFET must have a breakdown voltage that survives this overshoot.

Figure 3. Hot swapping VIN to –20V.

The speed of the LTC4365 reverse protection circuits is evident by how closely the GATE pin follows VIN during the negative transients. The two waveforms are almost indistinguishable on the scale shown. Note that no additional external circuits are needed to provide reverse protection.

AC Blocking

The LTC4365 has a recovery delay timer that filters noise at VIN and helps prevent chatter at VOUT. After either an OV or UV fault (or when VIN goes negative) has occurred, the input supply must return to the desired operating voltage window for at least 36ms in order to turn the external MOSFET back on.

Going out of and then back into fault in less than 36ms keeps the MOSFET off.

Figure 4 shows the LTC4365 blocking an AC line voltage of 40V to –40V. The GATE pin follows VIN during the negative portions, but remains at ground when VIN goes positive. Note that VOUT remains undisturbed.

Figure 4. 36ms recovery timer blocks 28V, 60Hz AC line voltage.

High Voltage Transients During Fault Condition

Figure 5 shows a test circuit designed to produce transients during an overvoltage condition. The nominal input supply is 24V with an overvoltage threshold of 30V. Figure 6 shows the waveforms during an overvoltage condition at VIN. These transients depend on the parasitic inductances on the VIN and GATE pins. The circuits survived the transients without damage, even though the optional power clamp (D1) was not used during the experiments.

Figure 5. OV fault with large VIN inductance.

Figure 6. Transients during OV fault when no TranZorb (TVS) is used.

Select Between Two Supplies

With the part in shutdown, the VIN and VOUT pins can be driven by two different power supplies at different voltages. The LTC4365 automatically drives the GATE pin below the lower of the two supplies, thus preventing current from flowing in either direction through the external MOSFET. The application of Figure 7 uses two LTC4365s to select between two power supplies. Care should be taken to ensure that only one of the two LTC4365s is enabled at any given time.

Figure 7. Selecting between one of two supplies.

Reverse VIN Hot Swap When VOUT Is Powered

LTC4365 protects against negative VIN connections even when VOUT is driven by a separate supply. With the LTC4365 in shutdown and VOUT powered to 20V, Figure 8 shows the waveforms when VIN is hot swapped to –20V. As long as the breakdown voltage of the external MOSFET is not exceeded (60V), the 20V supply at VOUT is not affected by the reverse polarity connection at VIN.

Figure 8. Negative VIN hot swap with VOUT powered.

Conclusion

The LTC4365 controller protects sensitive circuits from overvoltage, undervoltage and reverse supply connections. The supply voltage is passed to the output only if it is qualified by the user adjustable UV and OV trip thresholds. Any voltage outside this window is blocked, up to 60V and down to –40V.

The LTC4365’s novel architecture results in a rugged, small solution size with minimal external components, and it is available in tiny 8-pin 3mm × 2mm DFN and TSOT-23 packages. No reverse voltage blocking diode in series with the supply is needed; the LTC4365 performs this function automatically with back-to-back external MOSFETS. The LTC4365 has a wide 2.5V to 34V operating range and consumes only 10µA during shutdown.

作者

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Victor Fleury

Victor Fleury is a senior design engineer in the Mixed Signal Products Group at Analog Devices. He has specialized in high voltage and high speed products during his 18 years in the Industry. Victor holds a BSEE from MIT and an MSEE from Northeastern University.