? semiconductor components industries, llc, 2013 march, 2013 ? rev. 1 1 publication order number: AND9108/d 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 f. 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. summary of differences ? no transistor required on stc for fast start ? up ? minimum required decoupling capacitance on vdd external pmos transistor on stc the tss721a requires an external pmos transistor (bss84) in series with the stc capacitor if this capacitor is larger than 50 f in order to meet the maximum start ? up time requirement by the m ? bus standard. the ncn5150 does not require this transistor (q stc ), 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. vdd decoupling capacitor the ncn5150 requires a minimum total decoupling capacitance (c vdd ) of 1 f 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 for more information visit www.m ? bus.com http://onsemi.com application note marking diagrams a = assembly location (w)l = wafer lot yw(w) = year / work week g / = pb-free package ncn 5150 alyw 1 package pictures qfn20 case 485e soic ? 16 case 751b ? 05 ncn5150 awlywwg free datasheet http:///
AND9108/d http://onsemi.com 2 figure 1. general application schematic ? ncn5150 ncn5150 stc ridd gnd c stc ris vdd vio vs pfb rx rxi tx txi c sc r idd c sc c vdd u 1 busl2 vb busl1 mbus r is r bus1 r bus2 tvs 1 figure 2. general application schematic ? tss721a tss721a stc ridd gnd c stc ris vdd vio vs pfb c sc r idd c sc c vdd rx rxi tx txi q stc u 1 busl2 vb busl1 mbus r is r bus1 r bus2 tvs 1 table 1. general application schematic bom differences reference designator tss721a ncn5150 u 1 tss721a ncn5150 c vdd 100 nf 1 f r is 100 100 r idd 30 k (1ul) 13 k (2ul) 30 k (1ul) 13 k (2ul) c sc 220 nf (typ.) 220 nf (typ.) q stc bss84l bss84l (optional) c stc up to 470 f up to 470 f r busl1 , r busl2 220 220 tvs1 1sma40cat3g free datasheet http:///
AND9108/d http://onsemi.com 3 start-up and shut-down 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 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. figure 3. ncn5150 startup (c stc = 220 f, 1 unit load, no stc transistor) (purple = busl1, blue = stc, yellow = vdd, green = pfb) free datasheet http:///
AND9108/d http://onsemi.com 4 figure 4. tss721a startup (c stc = 220 f, 1 unit load, no stc transistor) (purple = busl1, blue = stc, yellow = vdd) figure 5. tss721a startup (c stc = 220 f, 1 unit load, bss84l, no external load) (purple = busl1, blue = stc, yellow = vdd) free datasheet http:///
AND9108/d http://onsemi.com 5 figure 6. tss721a startup (c stc = 220 f, 1 unit load, bss84l, external load of 8 k ) (purple = busl1, blue = stc, yellow = vdd, green = pfb) figure 7. ncn5150 shutdown (c stc = 220 f, 1 unit load, external load of 8 k ) (purple = busl1, blue = stc, yellow = vdd, green = pfb) free datasheet http:///
AND9108/d http://onsemi.com 6 figure 8. tss721a shutdown (c stc = 220 f, 1 unit load, external load of 8 k ) (purple = busl1, blue = stc, yellow = vdd, green = pfb) figure 9. tss721a shutdown (c stc = 220 f, 1 unit load, external load of 8 k disconnected on shutdown) (purple = busl1, blue = stc, yellow = vdd, green = pfb) free datasheet http:///
AND9108/d http://onsemi.com 7 minimum stc capacitor value 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 v stc 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%. 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. figure 10. charge transfer instability (ncn5150, c stc = 2 f, c vdd = 1 f, 1 unit load, no external load) (purple = busl1, blue = stc, green = vdd) however, there is also another factor that is usually stricter in determining the minimum capacitance on c stc . the value of c stc has an influence on the stability of the bus current regulator. for values of c stc 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 c stc capacitor value of 10 f is required. current available for application (during transmit) thanks to the internal operation of the ncn5150, the application has an extra 200 a (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. free datasheet http:///
AND9108/d http://onsemi.com 8 on semiconductor and are registered trademarks of semiconductor co mponents industries, llc (scillc). scillc owns the rights to a numb er of patents, trademarks, copyrights, trade secrets, and other intellectual property. a list ing of scillc?s product/patent coverage may be accessed at ww w.onsemi.com/site/pdf/patent ? marking.pdf. scillc reserves the right to make changes without further notice to any products herein. scillc makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does scillc assume any liability arising out of the application or use of any product or circuit, and s pecifically disclaims any and all liability, including without limitation special, consequential or incidental damages. ?typical? parameters which may be provided in scillc data sheets and/ or specifications can and do vary in different applications and actual performance may vary over time. all operating parame ters, including ?typicals? must be validated for each customer application by customer?s technical experts. scillc does not convey any license under its patent rights nor the right s of others. scillc products are not designed, intended, or a uthorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in whic h the failure of the scillc product could create a situation where personal injury or death may occur. should buyer purchase or us e scillc products for any such unintended or unauthorized appli cation, buyer shall indemnify and hold scillc and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unin tended or unauthorized use, even if such claim alleges that scil lc was negligent regarding the design or manufacture of the part. scillc is an equal opportunity/affirmative action employer. this literature is subject to all applicable copyrig ht laws and is not for resale in any manner. publication ordering information n. american technical support : 800 ? 282 ? 9855 toll free usa/canada europe, middle east and africa technical support: phone: 421 33 790 2910 japan customer focus center phone: 81 ? 3 ? 5817 ? 1050 AND9108/d texas instruments is a trademarks of texas instruments and its subsidiaries. literature fulfillment : literature distribution center for on semiconductor p.o. box 5163, denver, colorado 80217 usa phone : 303 ? 675 ? 2175 or 800 ? 344 ? 3860 toll free usa/canada fax : 303 ? 675 ? 2176 or 800 ? 344 ? 3867 toll free usa/canada email : orderlit@onsemi.com on semiconductor website : www.onsemi.com order literature : http://www.onsemi.com/orderlit for additional information, please contact your local sales representative free datasheet http:///
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