Fairchild Semiconductor Application Note September 1998 Revised February 2001 GTLP: Single vs. Multiple Output Switching Technical Discussion Abstract Specifications Single Output Switching (SOS) specifications are provided by the supplier as a tool to allow a cursory look at the performance of a device. Actual performance is highly dependent on the application in which it is used. The inclusion of Multiple Output Switching (MOS) specifications gives an additional data point to use when determining the change in relative performance of a device. Derating curves provide a number of additional data points to assist in determining the change in relative performance of a device. The datasheet specifications of propagation delay performance usually have only SOS specifications. Some IC suppliers provide Extended AC Electrical Characteristics that include MOS propagation delay performance and derating curves. This extended data gives a useful comparison to standard SOS specifications in a controlled test environment. This application note provides a description of SOS and MOS specifications that can be used to integrate new devices into a design. These tools must be used with caution when calculating parameters such as timing budgets in actual applications. The testing conditions used in setting standard specifications for a datasheet are most likely different than the actual loading and conditioning of the devices in an application. Definitions Single Output Switching Single Output Switching (SOS) describes the single bit propagation delay performance of a device. The tested propagation delay performance is statistically processed into a standardized specification that is used in datasheets to provide a way to compare similar products from competing suppliers. Multiple Output Switching Multiple Output Switching (MOS) describes the multiple bit propagation delay performance of a device. The word “multiple” usually refers to 8, 16 or 32 bits, depending on the total number of data bits that the device has. It can also refer to any other combination of multiple switching data bits, but is always two or more. The MOS measurement conditions usually mimic the SOS conditions except for multiple bit switching. There is not a standardized methodology for specifying performance, making it somewhat difficult to compare similar products from competing suppliers. The test measuring conditions or testing environment for SOS and MOS rely on controlled parameters such as test loads, trace lengths and impedances, and frequency of operation. While the measurement conditions seem far removed from an actual application, they are currently the best standardized test conditions that are available to describe device propagation delay performance. Understanding Specifications Datasheet specifications (SOS, MOS or any other parameter) try to best describe the device performance in nearrealistic applications. Currently the SOS and MOS propagation delay performance is measured using 30pF/25Ω or 50pF/500Ω lumped loads for the Gunning Transceiver Logic Plus (GTLP) family of products. The configuration of the TTL load can be modelled as shown in Figure 1. The output pull-up value of 6V is used for 3.3V VCC operation. When testing SOS propagation delay performance, each of the single bit paths are measured separately with the test load. Each single bit is remeasured over the range of operating VCC and temperature. The statistical minimum, maximum, and mean of the sample of single bit paths are then used to calculate the databook specification. When testing MOS propagation delay performance all bit paths are simultaneously switched in phase with the test sense probe being moved to measure each output. The load capacitance can be varied for additional extended AC electrical characteristics. Typically, derating curves of propagation delay versus load, use lumped load capacitances of 10 pF, 30 pF, 50 pF, 100 pF and at times 250 pF. FIGURE 1. AC Test Circuit © 2001 Fairchild Semiconductor Corporation AN500190 www.fairchildsemi.com AN-5002 GTLP: Single vs. Multiple Output Switching Technical Discussion AN-5002 AN-5002 How is MOS Used Common Mistakes When available, MOS specifications are used to determine relative deltas in propagation delay performance of the device. Because MOS testing uses the AC test circuit of Figure 1, the user must be careful in using the propagation delay values for timing budget analysis. The actual propagation delay performance will depend on the type and distribution of the load the device is driving. There are some common mistakes when interpreting SOS propagation delay specifications. The most common mistake is assuming that the specification guarantees maximum propagation delay if all outputs were simultaneously switching. MOS derating curves explain the degradation beyond the specified SOS propagation delay. The other common mistake is to assume the SOS propagation delay maximum specification guarantees performance across all loading conditions. There are often datasheet derating curves for the change in propagation delay over capacitive load. The test load in all cases is lumped versus distributed. The usefulness of MOS data applies more to applications that may be synchronous in nature when more than one output is switching simultaneously. Synchronous switching, especially in-phase synchronous switching, is generally considered the worst case application from the driving device point of view and is consequently the setup used for MOS testing. Data Specifications Format Table 1 and Table 2 are examples of datasheet specifications. Table 1 gives the maximum and minimum specifications of SOS propagation delay over the industrial/ commercial temperature range, VCC range, and standard loading. Table 2 gives MOS specifications of propagation delay with the same testing conditions as SOS but with all outputs switching. TABLE 1. AC Electrical Characteristics TA = −40°C to +85°C, CL = 30 pF, RL = 25Ω Symbol VCC = 3.3V ± 0.15V Parameter Units VCCQ = 5.0V ± 0.25V Min Max tPLH (A to B) Propagation Delay 1.0 6.5 ns tPHL Propagation Delay 1.0 8.2 ns TABLE 2. Extended AC Electrical Characteristics TA = −40°C to +85°C, Symbol Parameter (A to B) 18 Outputs Switching CL = 30 pF, RL = 25Ω VCC = 3.3V ± 0.15V Units VCCQ = 5.0V ± 0.25V Min Max tPLH Propagation Delay 1.0 8.8 ns tPHL Propagation Delay 1.0 9.7 ns www.fairchildsemi.com 2 Derating curves provide device performance data beyond standard datasheet specifications. These curves are often provided when a new family of products are introduced and can be used for all the functions in the family. The MOS derating curves describe the change in propagation delay of a device as the number of switching outputs for that device increases. Examples of tPLH and tPHL derating curves include a table of the statistical mean @ 3.3V/5.0V, 30 pF along with the corresponding plot of the data. TABLE 3. Mean Propagation Delays (tPLH) Outputs Mean Propagation Delay Switching (A to B) LH GTLP16612 1 4.1 5 4.71 9 5.37 13 5.93 18 6.44 FIGURE 2. Derating Curve (tPLH) TABLE 4. Mean Propagation Delays (tPHL) Outputs Mean Propagation Delay HL GTLP16612 1 5.02 5 5.47 9 5.89 13 6.23 18 6.54 FIGURE 3. Derating Curve (tPHL) Conclusion Single Output Switching (SOS) specifications are provided by the supplier as a tool for the user to allow a cursory look into the performance of a device. Actual performance is highly dependent on the application in which it is used. The inclusion of Multiple Output Switching (MOS) specifications give the user an additional data point to use when determining the change in relative performance of a device. Derating curves provide a number of additional data points to assist the user in determining the change in relative performance of a device. With a clearer understanding of SOS and MOS specifications it is possible to be better prepared to integrate new devices into a design. These tools must be used with caution when calculating parameters, such as timing budgets, in actual applications. The testing conditions used in setting standard specifications for a datasheet are most likely different than the actual loading and conditioning of the devices in an application. The use of standard specifications, such as SOS, make the selection of which family to use an easier task. Before testing in the actual application, using MOS and derating curves offers more detailed information available to make the selection process easier. 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: 2. A critical component in any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. 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 reasonably expected to result in a significant injury to the user. www.fairchildsemi.com 3 www.fairchildsemi.com AN-5002 GTLP: Single vs. Multiple Output Switching Technical Discussion Derating Curves