Aeroflex Colorado Springs Application Note Recommended Bypass Capacitance Values for the VCC Pins of the UT63M147 MIL-STD-1553 Bus Transceiver and SμMMITTM Family Controllers with Integrated 5V Transceivers Table 1: Cross Reference of Applicable Products Product Name Product Revision Internal PIC Number UT63M147 Bus Transceiver Rev G JB01 UT63M147 Bus Transceiver Rev A JB03 UT69151 SμMMIT DXE Rev D MM016 UT69151 SμMMIT DXE Rev C MM023 UT69151 SμMMIT DXE Rev C MM025 UT69151 SμMMIT DXE Rev C MM027 UT69151 SμMMIT XTE Rev G MM019 UT69151 SμMMIT RTE Rev D MM022 1.0 Overview The UT63M147 Transceiver is a complete transmitter and receiver pair for MIL-STD-1553 applications. Each transceiver has dual channels with independent power supply inputs for each channel. The SμMMIT controllers with integrated transceivers also have dedicated power supply inputs for the transceivers. During 1553 transmission, the transient currents on these VCC inputs can be as high as several hundred milliamperes, requiring the designer to use a careful approach to power supply decoupling. This application note looks at the characterization of typical current transients on the VCC inputs and makes recommendations for the proper bypass capacitance. Creation Date: December 2006 Page 1 of 5 Modification Date: January 2007 Aeroflex Colorado Springs Application Note 2.0 Lab Setup In order to characterize the bypass requirements for the VCC pin, it is first necessary to characterize ICC under typical conditions. These conditions are listed in Table 2. Table 2: UT63M147 Test Configuration Parameter Value 5.0V VCC Input Frequency 1 MHz Load Standard Direct-Coupled Load with 35Ω Termination Temperature -40oC, 25oC, 125oC TXIN and TXIN are driven by a 1 MHz signal to simulate the case where the output drivers are being driven constantly. The load termination is a standard direct-coupled configuration as shown in Figure 6 of the UT63M147 data sheet. The laboratory setup is shown in Figure 1. To measure IDD several capacitors are tied close to the input pin to ensure there is adequate charge needed to supply the ac component of the current. An ac-coupled current probe is used to monitor the ac component of IDD. Current Probe 5V - + 0.1uF and 10uF filter capacitors VCC GND UT63M147 Figure 1. Laboratory Setup for Current Characterization. Creation Date: December 2006 Page 2 of 5 Modification Date: January 2007 Aeroflex Colorado Springs Application Note 3.0 Lab Results The ac component of IDD is shown in Figures 2, 3, and 4, corresponding to case temperatures of -40oC, 25oC, and 125oC, respectively. The worst-case transient current is assumed to correspond to the occurrence of the highest peakto-peak current, which occurs at 25oC with a peak-to-peak current of 538mA. The current probe has a scaling factor of 1mA/mV. Figure 2. IDD at Tc = -40oC Figure 3. IDD at Tc = 25oC Creation Date: December 2006 Page 3 of 5 Modification Date: January 2007 Aeroflex Colorado Springs Application Note Figure 4. IDD at Tc = 125oC Creation Date: December 2006 Page 4 of 5 Modification Date: January 2007 Aeroflex Colorado Springs Application Note 4.0 Simulation Results The voltage seen at the supply pin is determined by simulating a circuit using a piece-wise linear model for the current waveform in Figure 3, which corresponds to worst-case transient current. The voltage source is an ideal 5.0V dc source separated from the UT63M147 by a 60cm trace represented by a 680nH inductor and 0.1Ω resistor on both sides of the power supply. This represents a case where the power supply might be located remotely (e.g. where power is supplied via a backplane). A capacitor is placed directly on the power pin whose ESR is determined by assuming a 2.5% dissipation factor, typical for ceramic multi-layer chip capacitors. The circuit is simulated and the ripple voltage on the power pin is measured. The objective is to keep the supply voltage well within the recommended operating range of 4.5V to 5.5V. Table 3 shows the ripple voltage for several different capacitance values. Table 3: Ripple Voltage for VCC Capacitance ESR Ripple Voltage Minimum Voltage Power Dissipation 0.01uF 0.4Ω 2.1V 0.1uF 0.04Ω 134mV 4.9V 3.6mW 1.0uF 0.004Ω 13mV >4.9V 3.6mW The values in the table are approximations only, as the ripple voltage and power dissipation of the capacitor both depend upon the ESR, which depends upon the particular capacitor used. Furthermore, the power supply is assumed to be located remotely from the transceiver, and no ground plane is assumed in the simulation. Therefore, the ripple voltage will be lower in a properly designed system. 5.0 Conclusion From the table it can be observed that a bypass capacitance of 1.0uF is sufficient to keep the supply voltage well within recommended operating conditions. In addition to this, a 0.1uF capacitor should be placed on each pin for higher frequency noise. Since the UT63M147 has dual channels, the bypass capacitors are required on each VCC pin. The SμMMIT controllers should have these bypass capacitors on each VCC pin. Use of these recommended bypass capacitors will reduce only the ripple voltage due to the ac component of the supply current. The designer should use additional filtering as required to reduce the coupling of lower or higher frequency noise and transients within the system due to neighboring components or other factors. Creation Date: December 2006 Page 5 of 5 Modification Date: January 2007