Tip of the Week: Sure surge suppression

The typical load-dump profile is shown in Fig. 1. The transient's typical rise time is 5 ns, and then there's an exponential decay over several hundred milliseconds. The amplitude of the transient depends on the alternator's rotational speed and field excitation at the moment of battery disconnection, and, according to the Society of Automotive Engineers (SAE), may be as high as 125 volts.

Steady-state overvoltages caused by a battery jump-start or regulator failure may also occur. Jump-start transients result when a boosted voltage is applied via jumper cables from another vehicle, such as using a 24-volt battery to jump start a 12-volt system. In a worst case scenario the overvoltage condition may exceed the rated battery voltage for up to 5 minutes. Load steps acting on parasitic wiring inductance can also cause microsecond supply spikes. Other automotive transients may be repetitive, such as those caused by the switching of relays and solenoids. Noise from the ignition system and various accessories (power windows, door locks) occur frequently. These transients propagate around the vehicle's wiring system via capacitive or inductive coupling in the wiring cables, or by conductive coupling on a common ground. The key benefit of the LT4356 surge stopper is its ability to ride out these transients. During a transient, the chip acts like a voltage regulator. The user can select suitable feedback resistors to handle transients of 100 volts and higher. An adjustable timer sets the time limit to ensure the device's MOSFET stays within its operating limits before the load is disconnected and the system shuts down. This technique takes better advantage of the external MOSFET's safe operating area (SOA) than would a fixed timer interval.

Figure 2 shows the LT4356's response to a load dump input. The output is regulated to a safe, 16-volt level throughout the event.

Circuit Operation
Figure 3 shows a functional block diagram of the LT4356 surge stopper. Under normal operating conditions it drives the gate of an n-channel MOSFET pass device fully on so that its presence is of no consequence to the load circuitry. The MOSFET is called into duty as a series limiter in case of overvoltage or overcurrent conditions. If the input voltage rises above a regulation point set by the FB divider, the voltage amplifier (VA) drives the MOSFET as a linear regulator, limiting the output voltage to the prescribed value and allowing the load circuitry to continue operating, uninterrupted. To protect the MOSFET and load from short-circuits, the LT4356 also includes current limiting.

When power is first applied, or when the LT4356 is activated by allowing the SHDN line to pull itself high, the external MOSFET is turned on gradually as its gate is slowly driven high. This soft-start provides inrush current limiting to minimize the effects of dynamic loading on the input supply and to reduce thermal fatigue in any upstream fuses. Once the MOSFET is fully on (V_ds <700mV), the enable pin (EN) goes high to activate the load circuitry such as a microprocessor or switching regulator.

During over-voltage or over-current, the current amplifier (IA) or the voltage amplifier (VA) is called into action. In the case of an over-voltage condition, the load circuit continues to operate, with little more than a slight increase in supply voltage (see Fig. 2). The load circuit may continue operating if, in the case of a current overload, sufficient output voltage is available. The timer capacitor ramps up whenever output limiting occurs, regardless of cause.

If the condition persists long enough for the timer pin (TMR) to reach its first voltage threshold of 1.25 volt, the FAULT pin goes low to give early warning to downstream circuitry of impending power loss. Once the 1.35-volt (second fault threshold) is reached, the timer shuts down the MOSFET and waits for a cool-down interval before attempting to restart.

Another feature of the LT4356 is the spare amplifier, which may be used as a power good comparator, input voltage monitor or low dropout linear regulator. For the LT4356-1 version, pulling the SHDN pin low results in a shut down of all functionality. The supply current is reduced to a mere 5 microamps, permitting use in applications where the device is left permanently connected to a battery supply. The auxiliary amplifier and internal reference remain active during shutdown for the LT4356-2 version, with a shutdown current of 50 microamps. You can use the LT4356-2 for applications that require a keep-alive supply voltage for vital functions while the main system has been shut down by configuring the auxiliary amplifier as a low-dropout linear regulator.

In the circuit of Figure 4, an external resistive divider at the feedback (FB) pin is set to limit the output voltage to 16 volts during an overvoltage event. If the input rises above 16 volts, the output will regulate at this level until the fault condition is removed or the timer expires (reaches 1.35 volts after the predetermined timeout condition).

The spare amplifier is configured to monitor the input voltage and indicate undervoltage by means of the AOUT pin. The enable (EN) pin activates the downstream load after the MOSFET is fully on.