This DS9503 is an ESD protection device for 1-Wire® interfaces. In contrast to the DS9502, the DS9503 includes two 5Ω isolation resistors on chip. Although 5Ω are negligible during communication, they represent a high impedance relative to the conducting diode during an ESD event. Thus, the diode absorbs the energy while the resistors further isolate and protect the circuit at the other side of the package. If used with circuits that already have a strong ESD-protection at their I/O port, the ESD protection level is raised to more than 27kV (IEC 801-2 Reference model). In case of abnormal ESD hits beyond its maximum ratings the DS9503 will eventually fail "short" thus preventing further damage.
During normal operation the DS9503 behaves like a Zener diode. When the voltage exceeds the trigger voltage, the I/V characteristic of the device will "snapback" allowing the same or higher amount of current to flow, but at a significantly lower voltage. As long as a minimum current or voltage is maintained, the device will stay in the "snapback mode." If the voltage or the current falls below the holding voltage or holding current, the device will abruptly change to its normal mode and conduct only a small leakage current.