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ALM2402F-Q1: Overvoltage protection

Part Number: ALM2402F-Q1
Other Parts Discussed in Thread: ALM2403-Q1

My customer wants to use ALM2403-Q1 as a resolver excitation driver, but is also considering ALM2402F for this purpose. From normal use case point of view, there is no problem. But we are concerned about automotive short-circuit scenarios that may destroy the ALM2402F-Q1 more easily than an ALM2403 because of higher supply voltage specification of the latter one.

In case of short-circuit to battery, voltages up to 40 V may occur (truck battery voltages). We are using the protective circuit fom  the application notes, so continuous short-circuit to 40 V and more will not be able to harm the ALM2402F, as the blocking capacitor will not conduct this voltage to the output of the IC (the capacitor is not in the feedback loop for this reason, so no excessive voltage can occur on the - input pin). But at the time when the voltage suddenly rises from about 6 - 7 V to 40 V, the capacitor will conduct for a short time until it is charged. So a transient spike will occur on the ALM2402F output. We do our best by using series resistors and protective diodes to soften the blow. But still we are concerned because the voltage spike may exceed the maximum rating of 18 V.

My question therefore is:

Can you provide a transient or pulse specification on the maximum voltage on the outputs of the ALM2402F? Or can you specify a maximum clamping current to supply (I anticipate that the output pin has internal diode to V+)? Or do I have to rely solely on the 18 V maximum rating?

best regards Martin

  • Martin,

    There is a body diode in ALM2402F-Q1 and ALM2403-Q1 from the output to positive rail capable of carrying 1A dc current but this would not be enough against the shorts to 40V without adding current limiting resistor in series with the output.  Below please find two possible ways to protect the ALM2402F-Q1 and ALM2403-Q1 against the shorts to battery above the absolute maximum rated voltage of the parts - the first one requires an external Schottky diode to bypass the high current through TVS to ground while the second employs the large bypass cap inside the feedback loop in parallel with current limiting resistor, which allow sourcing/sinking high current under normal ac operation without limiting output swing to rail while at the same time limiting sinking current under fault dc conditions.  When the voltage suddenly rises from about 6 - 7 V to 40 V, the capacitor inside the feedback loop will conduct for a short time (in few micro-seconds) very high current BUT the body diode is capable of carrying it without any damage and then the output resistor will limit the fault dc current to acceptable level - see below.

    Please review my app note discussing different protection schemes for both parts against shorts to battery under different conditions: