Protection circuit design of MAX2140 internal ESD diode

Overview

Assembly, testing, and troubleshooting sometimes require non-standard operation of the MAX2140 SDARS receiver. One example is hot-swap operation, where the device is connected or disconnected directly from the circuit without turning off power. Hot-swap operations are particularly common in automotive electronics, where modules often need to be reconnected due to the modular design of components, the distance between modules, and the need for multiple systems to work simultaneously.

How Hot-Swapping Operations Cause Diode Failure

Hot-swapping operations can cause transients, including large voltages, inrush currents, ringing, and polarity reversal. Behind these transient processes are physical phenomena such as energy exchange, limited charge/discharge time, and self-excitation.
Figure 1 shows a hot-swap operation of the MAX2140.

During hot plug operation, the cable joint will produce a voltage drop (as shown by the red arrow in the figure). At the same time, the bypass capacitor inside the antenna module is in a short-circuit state. This will cause the MAX2140 electrical ground to be at a higher potential than the antenna module's electrical ground. The internal ESD diode of the MAX2140 is connected to the ground pin 16 of the IC, so this ground potential difference will produce a short-term forward voltage on the diode. This forward voltage spike can exceed the device's absolute maximum ratings, known as electrical overstress (EOS). The forward voltage of the diode is specified from -0.3V to +4.3V (VCC_xx to GND, VINANT to GND, AGCPWM to GND, VOUTANT to GND). Design simulation shows that -1.3V and current of 72mA allow short-term operation.

Design to Prevent ESD Diode Failure

Methods to prevent EOS vary depending on the specific application. What are recommended here are some general design improvement measures:

avoid using too much reactance, such as: energy storage components, bypass capacitors, RF noise suppression inductors, long connection lines, etc.

To bypass surge currents: provide a short direct path to ground for each module; add external diodes in parallel with the internal diodes; place diodes across large coils.

Sequential Power: Powers on in sequence; programmable delay recommended for internal users (Maxim has numerous power sequencing products).

The following design example (Figure 2) shows the MAX2140 with a local loop and the addition of a Schottky diode to bypass the inrush current.

Specific design improvement measures are:

The cable between the MAX2140 receiver and the antenna module has only 0.5 resistance and no inductance.
The antenna module has a 100μF bypass capacitor.
The 5V power supply for the antenna module is provided by the MAX2140 receiver.
The reader may ask what is the maximum current through the capacitor and the maximum voltage drop across the cable during the initial 40μs transient. These values ​​can be obtained by the following expressions:

In this example, the voltage is 6.25V at a current of 12.5A, which is far beyond the allowable transient specifications of the internal ESD diode. Adding an additional Schottky diode can bypass most of the surge current during transients. Schottky diodes can be selected suitable for pulse applications. Based on the design in Figure 2, when using an appropriate capacitor to reduce the maximum inrush current to an acceptable range, using a short length of cable can significantly reduce the impedance and voltage drop.

Conclusion

For problems that arise in non-standard use of devices (such as the hot-swap operation of the MAX2140 receiver), we need to use corresponding methods to solve them. In order to successfully achieve non-standard use of a device, the product must be carefully designed and properly tested with the support and consent of Maxim.