AND8304/D Interfacing to Analog Switches Driving the Control Input of an Analog Switch with 1.8 V or Lower − Is it Safe? http://onsemi.com Introduction Analog switches are everywhere today. Due to their small size and low current consumption, they are popular in portable devices where they are effective in a variety of subsystems including audio and data communications, port connections, and even test. They can be used to facilitate signal routing, allow multiple data types to share an interface connector, or permit temporary access to internal processors during manufacturing. Analog switches are often used to give portable system designers a convenient method of increasing their features or accessibility without duplicating any circuitry. Understanding the key specifications and tradeoffs can make the difference between a temporary fix and a truly optimized solution. NO COM NC Control Input Figure 1. Control Input of Switch For most analog switches out on the market today, the input buffer that lies directly at the interface of the control input is a standard CMOS or TTL inverter. These structures are modeled as seen in Figure 2. The input signal that drives these inverters, is ideally a digital signal that toggles between ground and the VCC applied to the inverter. For an analog switch, the VCC of the inverter is usually the same as the VCC of the switch. Many systems designers using analog switches today, however, want the flexibility to operate the control input voltage, VIN, at a lower voltage than that applied to the switch VCC. The Control Input One aspect of the analog switch that has come under recent attention is the control input voltage range which defines how low the voltage can be that drives the control input pin. The control input, sometimes called select, determines the state of the switch − open/closed or NO/NC − as illustrated in Figure 1. This input is typically driven by a digital signal that toggles between ground and a set DC voltage and often comes from a much larger, integrated chipset, such as a CODEC or baseband processor. In general, these chipsets are operating off of ever−decreasing supply voltages which in turn sets limits on the voltages they are capable of sourcing. At the same time analog switches are not necessarily seeing the same drop in supply voltages. For example, one popular technique is to power the switch directly off the battery, causing it to not only see a potentially high voltage relative to the control input voltage, but also one that varies constantly over time. This can pose a problem for some system designs that employ switches optimized to accept control input voltages that must toggle strictly between ground and the switch VCC. © Semiconductor Components Industries, LLC, 2011 November, 2011 − Rev. 1 VCC Output to Switch Control Input Figure 2. Static CMOS Inverter The first place a designer might look to see whether he or she can safely lower the high level of VIN is the VIH specification of the analog switch datasheet. VIH and VIL 1 Publication Order Number: AND8304/D AND8304/D levels define the minimum high voltage and maximum low voltage the input to the inverter must maintain in order to be considered valid, as illustrated in Figure 3. TTL, VIH = 2.0 V. These specifications gave ample room for compliance when the driving signal toggled between ground and something relatively high, such as 3 V or 5 V. Even if the VCC were lowered the signals driving the control inputs were still coming from generally the same sources and therefore easily able to meet whatever the switch VCC was at the time. The main difference today is that analog switches are now often simultaneously interfacing with very different circuitry blocks, analog and digital. It is not unusual that the signals that drive VCC, VIN, and the data paths are coming from three distinct sources, such as in the example application of Figure 4. This can create a mishmash of signal swings and voltage requirements for the switch. If the VIH level is specified using just the traditional standards and without elaboration, the system designer may find that the switch does not seem to have the range or flexibility needed for the application. VIN toggles between Low and High States and must swing Above VIH and Below VIL to be considered valid VCC VIH VTH VIL GND Figure 3. VIH and VIL Levels with Respect to VCC and Ground Traditionally, VIH was specified fairly simply, often citing standards for either 5 V CMOS, VIH = 0.7 * VCC, or 5 V + Audio Amp Battery of − Portable Device VCC CODEC Figure 4. Example of Various Sources for Voltages and Signal Swings Surrounding an Analog Switch Pushing the Limits 2.0 It may be possible to safely operate a switch with a much lower VIH value than VCC, but to understand what the tradeoffs are, it helps to understand the source of the VIH specification. Both the VIH and the VIL specifications are actually derived values based on what’s known as the threshold voltage, VTH, of the inverter. When ramping up the voltage to the inverter, VTH is defined as the voltage at which the inverter, and subsequently the switch, will toggle. This voltage varies across VCC voltage and temperature and falls in between VIH and VIL. There are two reasons this threshold voltage is not simply specified directly as a minimum voltage switch point. The first is that this threshold is not exact, it varies a little, even for a given VCC and temperature. Figure 5 shows an example of the threshold voltage as it varies across VCC. The voltage falls at slightly different points depending on the direction of approach when ramping VIN: ground to VCC or VCC to ground. When specifying a safe VIH level, the analog switch designer must take into consideration all of these variations and guarantee a value with some safety margin. 1.8 1.6 VIN (V) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 VCC (V) Figure 5. VCC vs. VTH The second reason VTH is not used directly as a VIH or VIL value is that if the system designer were to drive the control input exactly at the threshold voltage the switch would pull a lot of current from the VCC line causing the device to be very leaky − sometimes in the mA range. The farther away http://onsemi.com 2 AND8304/D from the threshold voltage the high and low values of VIN are set, the less leaky the switch will be. This is demonstrated in Figure 6, which shows the leakage current at different VIN values for a handful of different VCC’s. It is evident from this graph that if VIN switches rail−to−rail, then the static leakage current will be very low, regardless of the VCC. But, as the high state voltage of VIN is lowered, the leakage creeps up. For example, for a VCC of 3.3 V, if the high state voltage of VIN is 2.5 V the leakage current will easily be less than 5 mA. But if you lower it down to 1.8 V, the leakage current jumps up to 100 mA. This problem is exacerbated when the application attempts to pair a fixed VIN of, say, 1.8 V with a variable VCC voltage of 2.7 V to 4.2 V, as in the case when a switch is operating directly off a portable battery. Here, the leakage current would vary between < 1 mA and 450 mA depending on the VCC at the moment. Many system designers may think that 450 mA of leakage is too much for any amount of time. But it must be understood that even when swinging rail−to−rail, this leakage current is seen for at least a brief amount of time as the control input voltage is ramped up or down. Most control input signals are digital−type signals, but they are not perfect and there is some amount of rise and fall time associated with each transition. When swinging from ground to VCC, the input signal still has to pass through each VIN level, momentarily exhibiting the ICC leakage associated with each level. When the high state of VIN is lowered with respect to VCC, the leakage will remain at its associated value for as long as VIN is in its high state. This is true for all CMOS input structures. ON Semiconductor’s Solutions ON Semiconductor has developed a new control input buffer designed specifically to interface with low voltage chipsets. The new structure achieves two important goals − it lowers the minimum allowable VIH value that guarantees switching and maintains low leakage for the new expanded range of VIN values. Figure 7 shows leakage curves from ON Semiconductor’s NLAS5223BL. The graph shows the typical leakage current across VIN for three different switch VCC voltages − 2.7 V, 3.3 V, 4.2 V. It is clear that with the new structure the leakage current is significantly reduced for a wider range of VIH values. For example comparing the graphs in Figures 6 and 7, the leakage current for a VCC of 4.2 V and a VIH of 1.8 V is 450 mA for the original structure and 100 mA for the new structure, yielding a significant improvement. 500 2000 1900 450 1800 1700 VCC = 3.3 V 400 1600 1500 1400 350 VCC = 4.2 V VCC = 4.2 V 1300 300 1100 ICC (mA) ICC (mA) 1200 1000 900 250 200 800 VCC = 3.3 V 700 150 600 500 100 400 VCC = 2.7 V 300 50 200 100 0 VCC = 2.7 V 0 0 0.5 1.0 1.5 2.0 1.8 VIN (V) 2.5 3.0 3.5 4.0 0 0.5 1 1.5 VIN (V) 2 2.5 3 3.5 Figure 7. VIN vs. ICC of ON Semiconductor’s NLAS5223BL with Optimized Control Input Figure 6. VIN vs. ICC, Standard Control Input http://onsemi.com 3 4 AND8304/D ON Semiconductor’s portfolio of data and audio switches includes new devices designed to allow low baseband voltages, such as 1.8 V or lower, to drive the control input. The datasheet for each newly released device will include ICC leakage graphs for varying levels of VIN− giving the system designer a more complete picture of the tradeoffs and options available for interfacing to the switch. With this additional information, the designer is one step closer to an analog switch solution that is truly optimized for the application. It is important to remember that even with the lower leakage values, there is still a limit to how low the VIH can safely operate. This goes back to the arguments made previously that the threshold voltage which defines the switch point varies with a number or parameters and a safety margin must be maintained in order to guarantee an effective switch. The VIH values specified in the datasheet take both factors into account. ON Semiconductor’s NLAS5223BL is designed to safely operate with VIH levels down to 1.6 V for VCC levels up to 4.3 V. In this scenario, a battery voltage that varies between 2.7 V and 4.2 V will never induce a leakage above 200 mA, typically much lower, when operated by a control input that toggles between ground and 1.6 V. ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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