The thermistor, RPTC, is a small surface mount 0402-size component with a resistance of 4.7k at 115°C. Above 115°C, its resistance rises exponentially with temperature. To prevent the timer network from falsely integrating offsets in the power multiplier, the LTC4380 does not generate timer current at the TMR pin until M2’s drain-to-source voltage reaches 0.7V. With 4A and 0.7V, the MOSFET could dissipate 0.7V • 4A = 2.8W continuously without the TMR network detecting the MOSFET’s temperature rise. The PTC resistor, RPTC, in conjunction with resistors R17–R21 and transistors Q5A, Q5B, Q6A, Q7A, and Q7B shuts down the circuit if MOSFET M2’s case temperature exceeds 115°C.
Do not be dismayed by the number of components in the thermal protection circuit. The overall solution is relatively easy to implement and consists of small components that consume little board area. It is a self-biased circuit that is balanced when RPTC equals R20’s 4.75kΩ value. When the temperature of RPTC, which is placed in close proximity to M2, exceeds 115°C, its resistance grows and causes more current to flow through Q5B than Q5A. Because that results in more current through R17 than R18, Q8A’s base voltage rises and Q8A’s collector pulls the ON pin of the LTC4380 low, turning off M2. At lower temperatures, Q5A’s current is greater than Q5B’s, and Q8A remains off, allowing the ON pin’s internal pull-up to keep the ON pin high. Note that the ON pin current is used as the start-up current of this self-biased circuit through the diode-connected device Q8B.
The ISO 16750-2 and ISO 7637-2 specifications describe the challenging electrical transients that can occur in automotive systems. The LTC4380 low quiescent current surge stopper can be used to protect the onboard electronics from these transients, including both the clamped and unclamped load dump pulses. The circuit presented in this article provides uninterrupted operation when faced with load dump pulses from a modern, clamped alternator. When faced with more extreme unclamped load dump pulses, it shuts off to protect the downstream electronics. The result is a robust solution for ISO 16750-2 and ISO 7637-2 compliance for electronics that draw up to 4 amps of supply current.