5 Summary

This application report discussed several schemes that may be employed at the PCB level under various circuit conditions in order to prevent ALM2402F-Q1 and ALM2403-Q1 from over-voltage and/or over-current related damage. Since the amplifiers used in excitation of primary coil in resolver applications require to be powered from sub-regulated supply voltage in order to minimize power dissipation inside the package, this may lead to fault condition damaging the part. For that reason, amplifiers used in the automotive resolver applications must be protected with the external circuitry against a possible damage resulting from accidental shorting of the amplifier output to a battery or any other voltage source higher than the supply voltage, Vcc, used to power the op amps.

Hence, depending on the exact details of the application potential fault conditions, there are a variety of ways to prevent damage to the part:

• In case of battery voltage being within the part’s absolute maximum rated voltage, using a simple blocking diode allows Vsup being pulled up by Vbat without damage
• With Vbat just a few volts above the absolute maximum rated voltage of the part, adding blocking diode, TVS and a small output resistor to limit the current through internal body diode to a safe level prevents damage
• For a battery voltage much higher than the part’s absolute maximum rated voltage, using a small output resistor while diverting the high current through the external Schottky and TVS diodes may solve the potential problem but it is a bulky and costly solution
• In case of Vbat much higher than the absolute maximum rated voltage of the part, one may also use a large output resistor to limit the current through body diode to a safe level but this requires a large increase in the supply voltage that during resolver normal operation results in the loss of power efficiency
• Therefore, with a battery voltage much higher than part’s absolute maximum rated voltage, an alternative approach is to add a large series output resistor inside the feedback loop to limit the current through body diode under fault condition but bypass it with a large non-polarized capacitor, as shown in Figure 4-1 and Figure 4-2, which eliminates the voltage drop across the output resistor under normal operation and thus maintains high power efficiency due to lower required supply voltage

All of the above schemes must be carefully considered in terms of their effectiveness, cost, board space and power efficiency before optimal solution may be chosen based on the potential fault conditions specific to the application. Even though this application note addresses only a single supply conditions found in most automotive resolver circuits, a similar protection schemes may be implemented in a dual supply applications against shorts to voltages below the negative rail. Likewise, similar schemes may be used more broadly to protect other analog IC’s from various fault conditions caused by accidental shorting of its output to voltages beyond supplies used to power it.