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This is an automotive and industrial DC power-protection circuit—not a household surge protector. The LT4356 controller protects a downstream load by driving an external N-channel MOSFET: it can regulate the output during an overvoltage, limit current during an overload, and shut down if a fault persists. The approach can keep equipment powered through some transients, but its success depends on the MOSFET’s safe operating area, thermal design, and the fault timing.
The original “Sure surge suppression” tip was published in 2007. Its circuit principles remain useful, but its example values are not universal design settings. Analog Devices currently lists the LT4356-1/-2 and LT4356-3 family as recommended for new designs; consult the applicable, current datasheet before selecting parts or setting thresholds. Read the original article · LT4356-1/-2 product information · LT4356-3 product information.
What the circuit is designed to handle
Vehicle and industrial DC rails can encounter very different disturbances, and “surge” does not describe them all. A fast spike from inductive switching may last only microseconds. A load dump or jump-start overvoltage can last much longer. A starter can pull the battery rail down during cold crank; a reversed battery applies negative voltage; and a large downstream capacitor can demand high inrush current at startup. Short circuits and sustained overloads create a separate problem: heat in the protection components.
The 2007 article discussed an automotive load-dump scenario reaching as high as 125 V. Treat that as the scenario cited in that article, not as a universal value for every vehicle or present-day test profile. The required test waveform and severity depend on the vehicle, system specification, and applicable qualification requirements. The original article provides the historical context.
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Why an active surge stopper is different from a TVS
A transient-voltage-suppressor (TVS) diode is a shunt clamp: when voltage rises beyond its breakdown region, it conducts current to limit the excursion. A TVS can be an important part of a protection system, particularly for fast edges, but it does not by itself provide controlled series regulation, sustained current limiting, inrush control, or a timed disconnect. Whether a TVS alone is adequate depends on the transient’s energy and duration, the source impedance, and the ratings of the protected circuit and clamp.
The LT4356 takes a different approach. It controls an external series MOSFET, dropping the excess input voltage rather than relying only on a shunt device to absorb it. If the input rises above the programmed output clamp, the controller can operate the MOSFET in its linear region and hold the protected rail near the selected level. If current exceeds the limit, it regulates current and uses a timer to determine what happens if the fault continues. A fuse, TVS, filtering, and downstream converter protection may still be needed; an active controller is not a universal replacement for them. See the LT4356-1/-2 datasheet.
How the LT4356 power path works
- Input: The vehicle or industrial supply enters the protection stage. A fuse and transient clamp may be placed in the wider system design as required.
- Series pass element: An external N-channel MOSFET carries the load current. In normal operation, the controller drives it on so its conduction drop is low.
- Sensing and control: A sense resistor reports current; a feedback divider sets the output clamp; and a timer capacitor establishes fault timing.
- Protected output: The regulated or passed-through rail feeds the downstream converter or load. Enable or power-good signaling can be used to hold downstream circuitry off until the pass device is adequately enhanced.
The IC is the controller, not the power switch. Its operating-voltage range does not guarantee that a complete circuit can survive every combination of high input voltage, heavy load, and fault duration. The external MOSFET must be chosen for drain-source voltage, current, gate drive, thermal resistance, and especially safe operating area (SOA) while it is dropping voltage in linear operation.
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When the input passes the programmed clamp point, the LT4356 adjusts the MOSFET gate to regulate the output. The 2007 article used 16 V as an example output clamp. That is an example feedback setting—not a fixed LT4356 output and not a suitable value for every load. Choose a clamp below the downstream equipment’s maximum input, allowing for divider tolerance, control accuracy, overshoot, wiring inductance, and other system effects.
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Regulation transfers electrical stress to the pass MOSFET. A first-order estimate of its instantaneous dissipation is PMOSFET ≈ (VIN − VOUT) × ILOAD. For example, a large input-to-output difference at substantial load current can create severe heating even if the MOSFET’s headline continuous-current rating looks generous. Compare the actual voltage, current, pulse duration, and repetition rate with the device’s SOA and transient thermal impedance. Check repeated events as well as a single pulse.
A fault timer limits how long the circuit may remain in a stressful state before shutdown or other fault behavior. The original article described timer thresholds of approximately 1.25 V for a fault warning and 1.35 V for shutdown in its implementation. Treat those as article-era, version-specific figures: verify threshold values and timing behavior in the datasheet for the exact variant and revision being designed around. A “current-limited” circuit is not safe if the pass device is allowed to overheat before the timer acts.
Set current limit and inrush with the external components
The controller senses the voltage across a low-value external resistor. The original article gives an approximate 50 mV sense level, leading to this initial sizing relationship:
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RSENSE ≈ 50 mV / ILIMIT
For its 5 A example, that gives 0.050 V / 5 A = 0.010 Ω, or 10 mΩ. This is a starting calculation, not a substitute for applying the selected datasheet’s threshold tolerance and design guidance. Check the resistor’s pulse and continuous power ratings, connection layout, and temperature rise. The historical article reported 10% worst-case current-limit accuracy for the described implementation; do not assume that figure applies to every version or operating condition without checking the current specifications.
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During a short circuit, the current limit can protect the load and constrain current, but the MOSFET may still dissipate significant power. The timer, current threshold, MOSFET SOA, and expected fault duration must be designed together. Decide whether the system should automatically retry after a fault or remain off until reset; repeated retries can repeatedly heat the MOSFET and drain a battery.
Inrush occurs when the pass device charges downstream bulk capacitance. The LT4356 controls gate drive so the output can rise more gradually, reducing the initial capacitor-charging current. The resulting ramp depends on gate capacitance, the controller’s gate current, downstream capacitance, source impedance, and load behavior. The original article cited about 20 µA of gate-control current in its example. Verify the applicable datasheet, then check startup time, fuse behavior, MOSFET SOA, and whether the downstream electronics tolerate the slower ramp.
Cold crank and reverse battery are separate design problems
Cold crank: passing voltage is not boosting it
Cold crank is an undervoltage condition, not an overvoltage surge. The historical article used approximately 4 V as a severe low-voltage example. A low-loss series protection path avoids the extra drop of a conventional blocking diode, but the LT4356 does not boost the input. If the system must maintain a regulated rail while its input falls below that rail, the downstream power converter must support the required range—for example, with a suitable buck-boost or SEPIC topology. Confirm its startup and operating limits at the minimum input, including the load and any hold-up requirement.
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Reverse battery: check every current path
A series diode can block reversed polarity, but it dissipates power and reduces available voltage during normal operation. A MOSFET-based arrangement can reduce that conduction loss; the original LT4356 discussion describes an additional N-channel MOSFET arrangement for reverse-battery protection. Analog Devices specifies reverse-input protection to −60 V for the LT4356 family. That controller-level specification does not automatically protect every external MOSFET, capacitor, resistor, signal pin, or connected signal path to the same voltage. Check negative transients as well as a static reversed battery, and consider current entering through communication and sensor connections even when the main power input is protected. Confirm the limits in Analog Devices’ product information and the applicable datasheet.
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Choosing among LT4356-1, LT4356-2, and LT4356-3
| Variant | Distinction to consider | Design question |
|---|---|---|
| LT4356-1 | Basic surge-stopper behavior and shutdown handling. | Does the standard fault response meet the system’s restart and standby requirements? |
| LT4356-2 | Retains auxiliary amplifier/reference functions during shutdown, useful for a keep-alive or monitoring function. | Does the system need an auxiliary function while the main pass path is shut down? |
| LT4356-3 | Adds adjustable latch-off fault behavior. | Should a persistent fault require a deliberate reset rather than automatic retry? |
Analog Devices lists the LT4356-1/-2 and LT4356-3 as recommended for new designs. The LT4356-3 product information identifies AEC-Q100 qualification for automotive applications. Qualification, package, ordering code, electrical limits, shutdown current, and exact fault behavior must be checked for the specific part; do not transfer a figure from one variant to another. The 2007 article reported shutdown currents of 5 µA for LT4356-1 and 50 µA for LT4356-2, but those historical figures should not be treated as current guaranteed values without checking the relevant datasheet. LT4356-3 details · LT4356-3 datasheet.
Design workflow: turn the protection requirement into component choices
- Specify the electrical environment. Record nominal and maximum steady-state voltage, minimum cold-crank voltage, transient waveforms and repetition, reverse-battery requirement, load current, startup capacitance, source impedance, and required operating or hold-up time.
- Choose fault behavior and variant. Decide whether faults should retry, latch off, or signal another system-level response. Compare the relevant LT4356 version against that requirement rather than choosing by family name alone.
- Set the clamp voltage. Calculate the feedback divider using the selected datasheet. Keep the set point below the downstream converter’s absolute maximum with margin for tolerances and transient overshoot. The original 16 V example is not a default.
- Select the MOSFET from stress conditions. Check voltage rating, current, gate charge, thermal resistance, SOA at the clamp-current operating point, pulse duration, and repetitive heating. Confirm automotive qualification when the project requires it.
- Calculate the sense resistor. Use the applicable current-sense threshold and tolerances to establish the limit; then check resistor power, pulse rating, layout, and temperature rise.
- Size the timer with SOA analysis. Relate fault timing to MOSFET voltage drop, current, thermal impedance, initial temperature, and repetition. Confirm warning and shutdown behavior for the selected variant.
- Design the surrounding protection. Assess fuse coordination, TVS selection, input filtering, reverse-blocking components, capacitor voltage ratings, and stress on controller pins and signal paths.
- Check undervoltage operation. Verify that the downstream converter starts and works across the protected input range. Add a boost-capable topology if the system needs a regulated output above the cold-crank input.
- Validate inrush and enable timing. Test the maximum expected capacitance and load, output ramp, startup time, power-good or enable sequencing, and fuse behavior.
- Test abnormal and repeated conditions. Exercise load dump, jump start, regulator failure, short circuit, cold crank, reverse battery, repeated transient pulses, power cycling, and the selected retry or latch-off response at temperature extremes.
Use Analog Devices’ current documentation and available demo circuits and LTspice resources as design aids, not as a substitute for validating the finished board and system. The LT4356 product page lists example circuits, including overvoltage-regulator applications.
Where a design can still fail
- High input plus heavy load: Often the hardest MOSFET condition because the device must drop substantial voltage while carrying current. Check SOA and heat, not just continuous-current rating.
- A short circuit at moderate input: Current limiting does not eliminate MOSFET heating. Confirm the timer trips within safe limits.
- Repeated events: A MOSFET or TVS that survives one pulse may fail before cooling after a train of pulses.
- Cold crank followed by a surge: Verify the circuit and downstream converter across the sequence, not only at either voltage extreme in isolation.
- Clamp set too high: Divider tolerance, overshoot, and wiring effects can push the downstream input beyond its safe limit.
- Power enters through another connection: USB, CAN, Ethernet, sensor, or grounded signal wiring may provide a path around the protected power input.
- Retry is inappropriate: Repeated automatic restart may stress hardware or drain the battery; latch-off may instead require a reset or service action.
For some applications, a TVS and fuse are sufficient; for others, a dedicated load-dump protector, ideal-diode controller, or eFuse/hot-swap controller may fit better. Compare actual voltage, timing, current, diagnostics, qualification, and restart requirements. An alternative such as Analog Devices’ LTC4363 is not a drop-in replacement for the LT4356; verify pinout, gate drive, timing, and external components before considering a redesign. See Analog Devices’ related surge-stopper material.
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The LT4356 family is a useful architecture when a design needs a series pass element that can regulate overvoltage and limit current, not merely clamp a brief spike. Its real protection capability is set by the complete circuit—especially the MOSFET, timer, external protection, layout, and validated fault profile. The original 2007 tip remains a helpful starting point; current component limits and system requirements decide the final design.
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