This application note describes the undershoot protection circuit that prevents bus switches from turning On in response to a negative voltage spike and passing the spike to the isolated side of the device.
Bus switches are commonly used for isolation and data exchange but suffer from a drawback: sensitivity to negative voltage spikes known as undershoot. These transients can cause disabled switches to turn on, leading to data corruption or system faults. Standard clamping diodes often fail to react quickly enough against these events.
The document focuses on the Fairchild Semiconductor FSTU family of bus switches, which feature a patented Undershoot Hardening Circuit (UHC). This circuit prevents the device from passing negative spikes to isolated circuits. The note details specific system conditions where this protection is critical, such as reflected wave designs (e.g., PCI buses) and systems requiring hot insertion or extraction of cards.
Undershoot is caused by negative input and output voltage spikes inherent to the device structure, which can cause a disabled switch to turn on.
Standard clamping diodes are designed primarily for ESD and EOS protection and will not react early enough to protect against data-corrupting undershoot events.
Systems using reflected wave designs (like PCI) or those subjected to hot insertion and extraction frequently experience undershoot events.