Options with UHC® Undershoot Protection for Reliable Hot-swap Isolation

Contents
What causes undershoot | UHC® undershoot protection solution | Features | Ideal UHC® applications | UHC® demo

Use this demo to compare the advantages of Fairchild UHC® with other undershoot protection technology.

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Locate datasheets and supporting documents for UHC®

What causes undershoot

An undershoot event-an unintended negative voltage spike on any switch port-can cause an NMOS switch to turn on momentarily. Undershoot is generally associated with transmission lines or bus structures that are un-terminated or under-terminated. Undershoot may cause the bus switch to momentarily turn on allowing corrupted data onto the isolated bus resulting in system faults.

Application notes with additional undershoot protection information

FSTU - Undershoot protected Fairchild Switch family PDF (52 KB)

Bus Switch undershoot protection: Which systems need this protection PDF (146K)

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UHC® undershoot protection solution

To preserve signal integrity and protect against undershoot, switch manufacturers have developed several circuit approaches: Undershoot Hardened Circuitry, Charge Pump, Active Clamp, and Schottky clamp diode. Fairchild's UHC® is the most effective solution in protecting signal integrity.

UHC® prevents negative voltage spikes or undershoot events from transmitting through a disabled switch. Corrupted data is not passed to the other side of the switch or onto the isolated bus.

Solution	Advantages	Disadvantages
Fairchild FSTU series with UHC®	Eliminates relationships between undershoot voltage and VGS and VSS-no amount of undershoot energy will cause switch to turn on.	None
Charge Pump	Pumps up NMOS VT during undershoot event to prevent NMOS turn on.	Increases device ICC to 100µA or more, during normal operation.
Active Clamp	Sources current during an undershoot event to clamp the output.	Fails after -2.8V of undershoot, increases device ICC to 20µA or more, during normal operation.
Schottky Clamp	Diodes Clamps low-energy undershoot to 300mV, which is less than NMOS VT, preventing turn on.	Parasitic bipolar transistor may turn on at around -1.2V causing switch to turn on.

Undershoot Technology	ICC	Propagation Delay	RON	Input Capacitance	Protection
	(VCC = 5.5V)	(VCC = 4.0V)	(VCC = 4.5V)
UHC®	3µA	0.25ns	4 Ohms	3.0pF	A & B ports
Charge Pump	100µA	0.25ns	5 Ohms	3.5pF	A port only
Active Clamp	20µA	0.25ns	3 Ohms	3.0pF	Fails after -2.8V
Schottky Clamp Diode	3µA	0.35ns	4 Ohms	6.0pF	Fails after -1.2V
Unprotected Switch	3µA	0.25ns	4 Ohms	3.0pF	None

Comparison of VOUT of various undershoot protection solutions with an unprotected switch

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Comparison of ICC of various undershoot protection solutions
with an unprotected switch

Features

Fairchild's UHC® technology delivers an undershoot solution with:

Configurable 20-bit switch
Up to 48-bit solutions
25- Ohms series resistor options
Voltage level shifting diode options
Precharge options
0.25ns propagation delay (tPD)
Low ICC 3µA
Zero bounce in flow-through mode
TTL-compatible control inputs
Package options include: BGA, TSSOP, SSOP, SOIC, QSOP, US8, SOT23, SC70

Additional Fairchild Switch information

Using the FST bus switch as a 5V to 3V translator PDF (301K)

Introduction to Fairchild switch products PDF (48 K)

FSTU-Undershoot protected switch family PDF (52 K)

Bus Switch Undershoot Protection: Which systems need this protection and why? PDF (146 K)

Undershoot protected switches

UHC® Demo

This demo contains an interactive simulation demonstrating how Fairchild's Undershoot Hardened Circuitry (UHC®) undershoot protection helps maintain data integrity.

Use this demo to compare the advantages of Fairchild's Undershoot Hardened Circuitry (UHC®) with other undershoot protection technology.

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