Analog Switches with -2V Undershoot Protection

© 2002 Fairchild Semiconductor Corporation AN500788 www.fairchildsemi.com Fairchild Semiconductor Application Note June 2002 Revised July 2002 A N -5044 Analog Sw itches w ith −2V Undershoot Prote ction AN-5044 Analog Switches with −2V Undershoot Protection Abstract System designs are continuously confronted with critical challenges such as impaired signal integrity and voltage and current excursions that inflict damage to integrated cir- cuits. Unwanted voltage and current transients can dam- age electronic circuits, resulting in system malfunctions and costly repairs. This application note will discuss analog switches designed with Fairchild’s Undershoot Hardened Control (UHC) circuitry that provides protection from undershoot transients or negative voltage spikes. Past attempts at solving the problem of undershoot transients will also be explored for relative comparisons. Undershoot Voltage Transients Why should one be concerned about undershoot noise? Historically, undershoot noise in systems was always a concern for some specialized devices such as dynamic RAMs, microprocessors and ASICs that were sensitive to these signal excursions. Typically, solutions for addressing this problem involved using discrete components such as diodes, resistors and capacitors to damp out the noise. The effectiveness of this approach was limited and resulted in high component counts in the application. Figure 1 pro- vides an illustration of an undershoot event. FIGURE 1. Illustration of a Voltage Undershoot Event Reducing system-generated noise is one of the many chal- lenges associated with optimizing system reliability and signal integrity. For example, noise generated from the printed circuit board (PCB) stackup which can manifest itself via the power distribution planes, crosstalk, decou- pling noise, EMI and transmission line reflections. Funda- mentally, system noise is a function of signal characteristics, such as edge rate, voltage swing, and fre- quency of operation. Varying approaches of protecting ICs from damage attributed to unwanted signal excursions and the cost effective design solution will be dictated by the system and application requirements. Fairchild’s analog switches with −2V undershoot protection employ UHC circuitry that monitors the voltage levels of the input and output of the analog switch. This design solution eliminates any discrepancy in voltage levels when the ana- log switch is disabled, thus the undershoot energy will not cause the switch to turn on. Fairchild’s design solution will work well with all −2V undershoot events regardless of the frequency. Without undershoot protection, the switch will turn on and the undershoot energy will be passed on to the ICs downstream, potentially damaging them. Refer to Fig- ure 2 for a reference and schematic screen captures show- ing the effectiveness of the UHC circuitry on the NC7SBU3157. When dealing with these types of design concerns we rec- ommend using Fairchild’s UHC solutions as a cost effective and reliable alternative. UHC is a trademark of Fairchild Semiconductor Corporation. www.fairchildsemi.com 2 A N -5 04 4 Undershoot Voltage Transients (Continued) FIGURE 2. Fairchild’s Analog Switches with UHC Circuitry Comparison Between Design Solutions As previously mentioned, Fairchild’s analog switches are an alternative to the ineffectiveness of other design solu- tions using discrete components when dealing with under- shoot problems in a system. We will explore these different design solutions which are the Schottky Diode Circuit, Fil- ter Circuit and the Unprotected Analog Switch. Data (screen captures) are provided to illustrate the effective- ness (in dealing with undershoot) of each circuitry and compare to the UHC device solution. This is a representa- tion of laboratory test setups and performance may vary with specific application conditions. Schottky Diode Solution This technology was designed for high voltage, low current events such as Electrostatic Discharge (ESD). Schottky diodes will not turn on under low voltage, high current con- ditions. Their turn-on speed of 2-4 nanoseconds is too slow for the Schottky device to clamp the negative transient. Refer to the following screen capture (Figure 3) for a com- parison between the solutions. FIGURE 3. Schottky Diode Solution vs. UHC Solution 3 www.fairchildsemi.com A N -5044 Filter Protection This technology works well in a preset frequency range, but outside the predesigned frequency range this approach does not filter out the negative transients to ground passing the transients to the unprotected IC’s. Refer to Figure 4 for a relative comparison of the effectiveness of the isolation. FIGURE 4. Low Pass Protection Solutions vs. Fairchild’s UHC Solution www.fairchildsemi.com 4 A N -5 04 4 Unprotected Analog Switch When an unprotected analog switch is disabled or has an unselected channel, a negative voltage excursion will for- ward bias the internal NMOSs diode turning the switch on. This will connect the undershoot energy to the unprotected internal circuitry potentially damaging or shortening the life of the IC. Refer to Figure 5 for a relative comparison between solutions. FIGURE 5. Schottky Diode Solution vs. UHC Solution 5 www.fairchildsemi.com A N -5044 Analog Sw itches w ith −2V Undershoot Prote ction Summary Since the complete elimination of system-generated noise is often not possible, controlling and managing the noise is critical for reliable system operation. This application note explored some different approaches for dealing with unwanted noise routinely found on signals in an electronic system. Protecting integrated circuits from harmful voltage transients can be addressed at the system level or at the IC device level. Fairchild Semiconductor offers UHC analog switches as well and other components for cost effective solutions for protection against negative signal excursions, which could potentially, damage devices that are sensitive to these events. Visit Fairchild Semiconductor’s web site at http://fairchildsemi.com for more information on UHC devices. FIGURE 6. Relative Comparison of UHC Against Design Alternatives Fairchild does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and Fairchild reserves the right at any time without notice to change said circuitry and specifications. LIFE SUPPORT POLICY FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be rea- sonably expected to result in a significant injury to the user. 2. A critical component in any component of a life support device or system whose failure to perform can be rea- sonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. www.fairchildsemi.com