Overvoltage-protection circuits often protect electronic devices from power-supply transients, such as a rise from plugging in batteries or an external power adapter. Although these devices traditionally find use as hysteretic switching controllers, you can reconfigure the LM3485 (Figure 1) to provide a robust overvoltage-protection circuit.

By selecting the feedback resistors using the formula V_IN=1.252(R₁+R₂)/R₂, you can program the IC to trip off at any level from 4.5 to 35V. In Figure 1, R₁ and R₂ turn off the PFET when V_IN exceeds 13.8V. You can calculate the hysteresis using the formula V_HYS=0.01(R₁+R₂)/R₂ (Figure 2). In this example, the expression calculates a hysteresis of 110 mV. The accompanying oscilloscope plot of V_OUT versus V_IN shows the sample circuit with a hysteresis of roughly 800 mV (2V/division, 0V level at lowest line, 500 nsec/division). Why isn't it 110 mV as calculated? We measured the turn-off propagation delay of the sample circuit at approximately 450 nsec, almost one complete time division, whereas the turn-on propagation delay was only 70 nsec (all measured using a 40Ω load). By taking these propagation delays into account, the scope plot approaches the calculated hysteresis and 13.8V trip level. However, compare these times with competing ICs with larger typical propagation delays of 500- to 6000-nsec turn-off times and 1800- to 7000-µsec turn-on times. The improved propagation delay is a result of the LM3485's driver, which can sink 320 mA and source 440 mA, as opposed to other overvoltage-protection circuits, which sink only approximately 60 mA.

The LM3485 also has an adjustable overcurrent-protection feature. In the sample circuit, when the current exceeds 1.1A, the LM3485 turns off the FET. After 9 µsec, the LM3485 turns back on the FET and begins sensing the current again through the FET's on-resistance. For more precise current sensing, add an external current-sense resistor between the FET and V_IN and then move the I_SENSE line of the LM3485 over to the source of the FET. The sample circuit in Figure 1 is derived from the standard LM3485 evaluation board. You can easily modify this board to create an overvoltage-protection circuit by removing a few components—the inductor, the diode, and the C_FF capacitor—by moving the feedback line from V_OUT to V_IN, and by selecting suitable resistor values.

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