AND9108/D M-BUS Design Changes Between TSS721A and NCN5150 NCN5150 is a drop in replacement of the TSS721A for all designs using a VDD capacitor larger than 1 mF. The NCN5150 M−BUS transceiver (in SOIC version) is pin−to−pin compatible with the TSS721A from Texas Instruments™ and can replace this component with no changes to the PCB layout on a typical implementation. Some minor differences between both parts are detailed in this document. http://onsemi.com APPLICATION NOTE Summary of Differences • No transistor required on STC for fast start−up • Minimum required decoupling capacitance on VDD External PMOS Transistor on STC PACKAGE PICTURES The TSS721A requires an external PMOS transistor (BSS84) in series with the STC capacitor if this capacitor is larger than 50 mF in order to meet the maximum start−up time requirement by the M−BUS standard. The NCN5150 does not require this transistor (QSTC), and allows the STC capacitor to be connected directly to the STC pin. Optionally, for existing PCB layouts where no possibility exists to bypass the transistor footprint, the transistor can be kept in place without any problem. QFN20 CASE 485E VDD Decoupling Capacitor The NCN5150 requires a minimum total decoupling capacitance (CVDD) of 1 mF on the output of the 3.3 V regulator to remain stable. Typical designs with the TSS721A will use a 100 nF capacitor. Related Standards: European Standard EN 13757−2 EN 1434−3 © Semiconductor Components Industries, LLC, 2013 March, 2013 − Rev. 1 MARKING DIAGRAMS 1 NCN 5150 ALYW G A (W)L YW(W) G/G For more information visit www.m−bus.com 1 SOIC−16 CASE 751B−05 NCN5150 AWLYWWG = Assembly Location = Wafer Lot = Year / Work Week = Pb-Free Package Publication Order Number: AND9108/D AND9108/D VS VIO VDD RBUS1 CVDD BUSL2 U1 NCN5150 TXI TX RX RXI PFb mC TVS1 VB MBUS BUSL1 RBUS2 RIS SC RIS GND RIDD STC RIDD CSC CSTC Figure 1. General Application Schematic – NCN5150 VS VIO VDD RBUS1 CVDD TSS721A TXI TX RX RXI PFb mC BUSL2 U1 TVS1 VB MBUS BUSL1 RBUS2 RIS SC RIS GND RIDD CSC STC RIDD QSTC Figure 2. General Application Schematic – TSS721A Table 1. GENERAL APPLICATION SCHEMATIC BOM DIFFERENCES Reference Designator TSS721A NCN5150 U1 TSS721A NCN5150 CVDD 100 nF 1 mF RIS 100 W 100 W RIDD 30 kW (1UL) 13 kW (2UL) 30 kW (1UL) 13 kW (2UL) CSC 220 nF (typ.) 220 nF (typ.) QSTC BSS84L BSS84L (optional) CSTC up to 470 mF up to 470 mF RBUSL1, RBUSL2 220 W 220 W TVS1 1SMA40CAT3G http://onsemi.com 2 CSTC AND9108/D Start-up and Shut-down cause the PF pin to rise above the recommended voltage rating of the microcontroller it is connected to. During start-up of the NCN5150 no unwanted step is present on VDD and all of the IO pins are turned on only when VDD is turned. Shutdown of both components is shown in figures 7, 8 and 9. We can see the PFb pin (green) go from logic high to low. This happens immediately after the bus voltage collapses. The VDD is disconnected later. All these figures were taken with an external load on VDD, because otherwise the VDD capacitor would retain its charge for a long time. However, when the load on VDD is removed after the voltage has collapsed (as is the case for digital circuits) the TSS721A will charge back to ∼300 mV, which can be undesirable for certain circuits. The NCN5150 does not have this behavior. Shown in figures 3, 4, 5 and 6 is the start-up and shutdown of the NCN5150 next to the corresponding waveforms captured from the TSS721A (both with and without STC transistor). We can see clearly that the startup of the TSS721A is much longer without the STC transistor, and will not comply with the M-BUS standard defined maximum start-up time of 3 s for the same STC capacitor value. The solution in this case is to add an extra PMOS in series with the STC capacitor. While this technique reduces the startup to similar time as the NCN5150 at the cost of an extra component, it also causes a 700 mV step on VDD if no load is present on VDD. If a sufficient external load is present, this step will not occur, however in this case a strange effect can be seen on PF, where this pin rises along with the STC voltage until the VDD regulator is turned on, as shown in figure 6. This can Figure 3. NCN5150 Startup (CSTC = 220 mF, 1 Unit Load, no STC transistor) (purple = BUSL1, blue = STC, yellow = VDD, green = PFb) http://onsemi.com 3 AND9108/D Figure 4. TSS721A Startup (CSTC = 220 mF, 1 Unit Load, no STC transistor) (purple = BUSL1, blue = STC, yellow = VDD) Figure 5. TSS721A Startup (CSTC = 220 mF, 1 Unit Load, BSS84L, no external load) (purple = BUSL1, blue = STC, yellow = VDD) http://onsemi.com 4 AND9108/D Figure 6. TSS721A Startup (CSTC = 220 mF, 1 Unit Load, BSS84L, external load of 8 kW) (purple = BUSL1, blue = STC, yellow = VDD, green = PFb) Figure 7. NCN5150 Shutdown (CSTC = 220 mF, 1 Unit Load, external load of 8 kW) (purple = BUSL1, blue = STC, yellow = VDD, green = PFb) http://onsemi.com 5 AND9108/D Figure 8. TSS721A Shutdown (CSTC = 220 mF, 1 Unit Load, external load of 8 kW) (purple = BUSL1, blue = STC, yellow = VDD, green = PFb) Figure 9. TSS721A Shutdown (CSTC = 220 mF, 1 Unit Load, external load of 8 kW disconnected on shutdown) (purple = BUSL1, blue = STC, yellow = VDD, green = PFb) http://onsemi.com 6 AND9108/D Minimum STC capacitor value Therefore, for practical purposes, a ratio of at least 6 times between the nominal values is recommended. The effect of choosing a lower STC capacitor value at startup is shown in figure 10. You can see the STC voltage dropping when charge is transferred to the VDD capacitor. In this case, the value of the STC capacitor is low enough for the STC voltage to drop below the disable threshold of the VDD regulator, resulting in the VDD capacitor being charged in steps. This can have potential bad effects when the microcontroller already starts up at a lower voltage. In that case, the microcontroller will be powered solely from the small charge on the VDD capacitor, and the VDD voltage will collapse, resulting in a cycle of microcontroller reboots. Choosing a low STC capacitor value will also severely reduce the shutdown time. For applications where the external circuit is not powered by the bus, it can make sense to choose a low value capacitor for STC. The lower limit on this capacitor is determined by two factors that apply equally to the TSS721A and the NCN5150. The first lower limit is determined by the charge transfer to the VDD capacitor when the device is starting up. To achieve a clean startup, the voltage drop incurred by this charge transfer should not cause the STC voltage to drop below the turn-off threshold. For both parts, this limits the STC voltage drop DVSTC to 1.3 V. This leads us to a minimum ratio between the capacitor values on VDD and STC of: This equation does not take into account the tolerances on capacitor value, which can be up to −80% or +20%. Figure 10. Charge Transfer Instability (NCN5150, CSTC = 2 mF, CVDD = 1 mF, 1 Unit Load, no external load) (purple = BUSL1, blue = STC, green = VDD) Current available for application (during transmit) However, there is also another factor that is usually stricter in determining the minimum capacitance on CSTC. The value of CSTC has an influence on the stability of the bus current regulator. For values of CSTC that are too small, the bus current regulator will become unstable and an oscillation will become visible on the bus current. To ensure stable operation under all environmental conditions and sample variation, a minimum CSTC capacitor value of 10 mF is required. Thanks to the internal operation of the NCN5150, the application has an extra 200 mA (typical) available to be drawn from VDD or STC when the transceiver is sending a space. This extra current is on top of the expanded current budget offered by the NCN5150 due to its reduced power consumption. This may ease the design of an isolated application. On the TSS721A, the current available is constant. http://onsemi.com 7 AND9108/D Texas Instruments is a trademarks of Texas Instruments and its subsidiaries. ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. SCILLC reserves the right to make changes without further notice to any products herein. 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