The LTC4381 from Analog Devices is an integrated solution for low quiescent current surge stopper applications that protect loads from high voltage transients. Overvoltage protection is provided by clamping the gate voltage of an internal 9mΩ N-channel MOSFET to limit the output voltage to a safe value during overvoltage events such as load dump in automotive applications or surges on Industrial systems. The MOSFET safe operating area is production tested and guaranteed for the stresses during high voltage transients.
Fixed output clamp voltages can be selected for 12V and 24V/28V systems and all other voltages up to 80V are accommodated by the adjustable clamp versions. Overcurrent protection is also provided, an internal multiplier generates a TMR pin current proportional to VDS and ID, so that operating time in both overcurrent and overvoltage conditions is limited in accordance with MOSFET stress.
The GATE pin can drive back-to-back MOSFETs for reverse input protection, eliminating the voltage drop and dissipation of a Schottky diode solution, whilst the devices low 6μA operating current make it suitable for use in always-on and battery powered applications.
Typical Application Circuit
During normal operation, a 20μA charge pump (see Block Diagram) drives MOSFET M1 fully on, providing a low impedance path from input to the load. The MOSFET gate is clamped to ground by a Zener stack, if the input voltage rises to the point where the output approaches the gate clamp, the output is effectively limited to one threshold voltage (typically 3V) below the gate clamp and the input surge is blocked from reaching the load.
Two output clamping voltages to ground are available: 28.5V for use in 12V systems, and 47V for use in 24V and 28V systems, the clamping voltage is selectable using the SEL pin. Besides the output to ground clamp, the output is also limited to 10.5V above the VCC pin. There is no GATE clamp to ground for the LTC4381-3 and LTC4381-4 versions and the output is only limited to 10.5V above the voltage at the VCC pin. A Zener diode clamp connected from the VCC pin to ground thus clamps the voltages at both the VCC and SRC pins during overvoltage events. Output load current is limited by a current limit amplifier (IA), using a sense resistor in series with the MOSFET source to monitor the current. The current limit threshold is 50mV, rising to 62mV when the output is less than 1.5V. MOSFET stress is monitored by a timer, whose current is a function of MOSFET’s VDS as well as ID. VDS is monitored by RDRN at the DRN pin, while ID is monitored by sensing the voltage drop across RSNS. The timer allows the load to continue functioning during short transient events while protecting the MOSFET from being damaged by a sustained overvoltage, such as load dump in vehicles, or an output overload or short circuit.
A multiplier sets the timer period depending on the power dissipation in the MOSFET. Higher power dissipation corresponds to a shorter timer period, helping to keep the MOSFET within its safe operating area (SOA). The timer responds to stresses at start-up and during voltage and current limiting. TMR pin current is integrated on timing capacitor CTMR and if TMR charges to 1.215V, the MOSFET is turned off. At this point, the LTC4381-1 and LTC4381-3 latch off, and can be reset by cycling power or by pulling the ON pin low for at least 100μs. For the LTC4381-2 and LTC4381-4, the TMR pin enters a cool down phase, allowing time for the MOSFET temperature to equalize with its surroundings before automatically restarting. The TMR pin slowly charges up and down in between 3.4V and 1.215V for 15 times and discharges to ground at the last cycle. When the TMR pin has reached the 100mV threshold, the MOSFET is turned back on. The cool down interval can be curtailed by pulling the ON pin low for at least 10ms/μF of CTMR. In addition to resetting the timer, the ON pin is used for on/off control and for undervoltage detection. The ON pin threshold is 1.05V. The open drain FLT pin pulls low whenever the timer is faulted off and goes high again when reset by a power cycle, by pulling the ON pin low for at least 100μs or in the case of the LTC4381-2 and LTC4381-4, when the TMR pin discharges to 100mV.
Functional Block Diagram
The LTC4381 low quiescent current surge stopper offers robust performance, the device clamps the output voltage permitting the load to operate safely through 100V transients and load dump surges such as ISO 16750-2 Test A. Current limiting protects the input supply from output overload and short-circuits. In the presence of a sustained input overvoltage or output overload, the LTC4381 shuts off after a delay to protect the internal power MOSFET. Below graph demonstrates clamping performance with a 12V 0.5A output load when subjected to a 100V input surge.
Surge Stopping Performance
The LTC4381 devices are supplied in a compact 7 x 5mm DFN32 package and are available in a choice of 3 operating temperature ranges, 0°C to + 70°C (suffix C), -40°C to + 85°C (suffix I) and -40°C to + 125°C (suffix H).
The DC2713A circuit-board enables developers to fully evaluate the performance and features of the LTC4381. The circuit-board is available in four assembly options, DC2713A-A/DC2713A-B employ a jumper to set the output clamp voltage to either 28.5V or 47V, to suit 12V or 24V/28V systems and the DC2713A-C/DC2713A-D clamp the output to 66.5V for 48V applications. DC2713A-A/DC2713A-C latch off after a fault, whereas DC2713A-B/DC2713A-D automatically restart after a cool down delay. The availability of the circuit-boards with different configurations allow developers to evaluate a range of different scenario’s particular to their specific requirements.
The LTC4381 low quiescent current surge stopper is suitable for applications which require protection against input or output overload conditions such as Automotive 12V, 24V and 48V Systems, Avionics, Industrial Surge Protection, Hot Swap/Live Insertion, High Side Switch for Battery Powered Systems and Automotive Load Dump Protection.
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