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Application ReportSLVA470–November 2011TPS62130/40/50Sequencing and TrackingTahar Allag /Chris Glaser Battery Power ApplicationsABSTRACTThe TPS6213x/4x/5x is a family of synchronous buck DC-DC converters with a wide operating input voltage range from 3V to 17V and adjustable output voltage of 0.9V to 6V.Because of the wide input and output voltage ranges,they are ideal for use in systems which contain multiple output voltage rails.This application note describes how to use the EN,PG,and SS/TR pins in tracking and sequencing applications in such systems which contain any combination of the TPS6213x,TPS6214x,or TPS6215x devices.Contents 1Device Description ..........................................................................................................11.1Soft Start and Tracking (SS/TR).................................................................................21.2Enable (EN).........................................................................................................21.3Power Good (PG).. (22)Sequential Start Up Using EN and PG (33)Ratiometric Start Up Using Connected SS/TR Pins (44)Ratiometric and Simultaneous Start Up ..................................................................................54.1V OUT 1Leading V OUT 2...............................................................................................74.2V OUT 2Leading V OUT 1...............................................................................................74.3Simultaneous Start Up ............................................................................................85Conclusion and References ...............................................................................................95.1Conclusion ..........................................................................................................95.2References ..........................................................................................................9List of Figures 1Soft Start Ramp Up Time (22)Sequential Start Up Using the EN and PG Pins (33)Sequential Start Up Using the EN and PG Pins (Results of the Circuit of Figure 2) (34)Ratiometric Start Up Using the SS/TR Pins (45)The Ratiometric Start Up (Result of the Circuit in Figure 4) (56)Ratiometric and Simultaneous Start Up Sequence (67)Ratiometric Start Up with V OUT 1Leading V OUT 2 (78)Ratiometric Start Up with V OUT 2Leading V OUT 1.........................................................................89Simultaneous Start Up (9)1Device DescriptionThe TPS6213x/4x/5x devices are easy to use synchronous step down DC-DC converters suited for applications that requires tracking and sequencing.They have a built in power good function to indicate the status of the device,a soft start circuit to control the output voltage slope during start up,and an enable function for controlling the turn on of the device.Each of these functions is useful for tracking and sequencing.1SLVA470–November 2011TPS62130/40/50Sequencing and Tracking Submit Documentation Feedback Copyright ©2011,Texas Instruments IncorporatedT Time 1.8 V SS V OUTRegulation Level 0.8 VVfb V (SS/TR) 1.25 V 2.00 V/div 1.00 V/div 1.00 V/divDevice Description 1.1Soft Start and Tracking (SS/TR)The rate in which the output voltage rises up to the full operational level during the start up phase iscontrolled through the SS/TR pin.A capacitance (C SS )is connected between the SS/TR pin and the IC ground;and the size of the capacitor determines the soft start ramp up time (T SS )per Equation 1.Equation 2shows the relationship between the SS/TR pin voltage and FB pin voltage.T SS =C SS ×(V SS/TR (MAX)/I SS )[sec](1)V FB =0.64×V SS/TR (2)V SS/TR (MAX)is the maximum SS/TR pin voltage of 1.25V that relates to the maximum FB pin voltage of0.8V.Though the SS/TR pin voltage may increase beyond 1.25V,the FB pin voltage stops tracking the increasing SS/TR pin voltage at this point and remains at its final value of 0.8V.The voltage at the FB pin corresponds to the output voltage.I SS is the SS/TR pin ’s internal source current which equals I SS =2.5µA.By sourcing a constant current onto the capacitor C SS ,the device linearly ramps up the voltage on theSS/TR pin,which corresponds to the voltage on the FB pin and thus the output voltage.Figure 1shows the typical start up ramp for a device set to 1.8V output.Figure 1.Soft Start Ramp Up Time1.2Enable (EN)The EN pin of the TPS6213x/4x/5x controls the turn on of the device.Setting the enable pin to logic high starts up the device.Conversely,the device is shutdown when the enable pin is logic low.The highthreshold voltage of this pin is 0.9V and the low voltage threshold is 0.3V.Any voltage between the two levels results in an undefined state of the logic input,EN.1.3Power Good (PG)The power good output is used to indicate that the output voltage has reached regulation.It floats high when the output voltage reaches its appropriate level and is pulled low by the device when the device is enabled and the output voltage is below the regulated level.The PG pin is an open drain output thatrequires an external pull up resistor.This pin can be used for enabling other devices in the system when the output voltage reaches the desired level.2TPS62130/40/50Sequencing and TrackingSLVA470–November 2011Submit Documentation FeedbackCopyright ©2011,Texas Instruments IncorporatedV EN PGV 1OUT V 2OUT Sequential Start Up Using EN and PG 2Sequential Start Up Using EN and PGThe sequential start up method uses the EN and PG functions to turn on a second device after the firstdevice has reached regulation.Figure 2shows two TPS6213x/4x/5x devices in a sequential system set up.The PG pin of the first device is used to enable the second device.When the first output voltagereaches its appropriate level after getting enabled,the PG pin turns high.As a consequence,the second device is enabled.Note that both SS/TR pins are connected independently,so the soft start times can be different for each device.Figure 2.Sequential Start Up Using the EN and PG PinsFigure 3shows the result of the sequential start up circuit drawn in Figure 2.The enable pin of the firstdevice turns high to enable the first converter and its output voltage,V OUT 1,starts rising up.After its soft start time,the output voltage reaches 3.3V,which is its regulation voltage.The power good goes high and enables the second device.Once the second converter gets enabled,its output voltage rises up to its full operation level of 1.8V after its programmed soft start time.This illustrates a typical sequential start upusing two TPS6213x/4x/5x devices.Figure 3.Sequential Start Up Using the EN and PG Pins (Results of the Circuit of Figure 2)3SLVA470–November 2011TPS62130/40/50Sequencing and Tracking Submit Documentation Feedback Copyright ©2011,Texas Instruments IncorporatedRatiometric Start Up Using Connected SS/TR Pins 3Ratiometric Start Up Using Connected SS/TR PinsIn the ratiometric start up sequence,the EN and SS/TR pins of both devices are connected together,as shown in Figure 4.A single soft start capacitor is used.This configuration forces both devices to start up at the same time when their common enable pin turns high.Also,since the SS/TR pins are connected together,the soft start time (T SS )is the same for both devices and both devices reach regulation at the same time.Note that the soft start time,as calculated from Equation 1,will be half of what it should bebecause both devices ’SS/TR pin current sources are feeding onto a single soft start capacitor.To achieve the desired soft start time with Equation 1,double the I SS value to account for the second current source.Figure 4.Ratiometric Start Up Using the SS/TR PinsIn Figure 4,both output voltages start once the enable pin goes high and both voltages reach regulation at the same time.Since the output voltages are not the same,but the soft start time is equal,their slopes are different.4TPS62130/40/50Sequencing and TrackingSLVA470–November 2011Submit Documentation FeedbackCopyright ©2011,Texas Instruments IncorporatedENV1OUTV2OUT Ratiometric and Simultaneous Start UpFigure5.The Ratiometric Start Up(Result of the Circuit in Figure4)4Ratiometric and Simultaneous Start UpIn ratiometric and simultaneous start up,the enable pins of both converters are connected together;thus, both devices start at the same time.The SS/TR pin of the first device is connected to ground through itsCSS capacitor.The CSScapacitor value defines the soft start ramp up time as expressed in Equation1.However,the SS/TR pin in the second device is connected to the output voltage of the first converterthrough two resistors,R1and R2,as shown in Figure6.5 SLVA470–November2011TPS62130/40/50Sequencing and Tracking Submit Documentation FeedbackCopyright©2011,Texas Instruments IncorporatedRatiometric and Simultaneous Start Up Figure6.Ratiometric and Simultaneous Start Up SequenceIn this configuration,since the SS/TR pin of the second converter is connected to the output voltage of thefirst converter,the VOUT 2start up slope is related to the start up slope of VOUT1.Also,the VOUT2start upslope depends on the values of R1and R2.Equation2and the following equations can be used to control the slope of VOUT2.VSS/TR 2=VOUT1×R2/(R1+R2)+ISS×R2(3)VOUT 2=[VOUT1×R2/(R1+R2)+ISS×R2]×0.64×[(R3+R4)/R4(4)Equation2provides the relationship between the voltages of the FB pin and the SS/TR pin.VFB variesfrom0V to0.8V;thus V(SS/TR)varies from0V to1.25V.The relationship between the SS/TR pin voltageand the output voltage of the first converter is expressed in Equation3.This equation is obtained by thevoltage divider from VOUT 1summed with the internal SS/TR pin current source.The current source is small(2.5µA),so it can be neglected if the value of R2is smaller than10KΩ.Note that an error of2%inEquation3is produced if R2×ISS is neglected with R2=10KΩ.Equation4gives the relationship betweenVOUT 2in terms of VOUT1.R3and R4are the feed back resistors of the second converter as shown inFigure6.They are used to set the output voltage of VOUT 2when it is in regulation.VOUT1and VOUT2are theoutput voltages of each converter at some point in time during start up.They will be used in the following sections to achieve different ratiometric sequencing configurations.With the configuration of Figure6,three situations are possible:VOUT 1ramps up faster than VOUT2,VOUT2ramps up faster than VOUT 1,and VOUT1ramps up at the same rate as VOUT2.The next sections explainhow to calculate the values of R1and R2for the desired ramp up relationship.R3and R4are sized in the same way as described by the data sheet to set the output voltage.6TPS62130/40/50Sequencing and Tracking SLVA470–November2011Submit Documentation FeedbackCopyright©2011,Texas Instruments IncorporatedV 1OUT V 2OUT Ratiometric and Simultaneous Start Up4.1V OUT 1Leading V OUT 2In this configuration,the output voltage of the first converter leads the output voltage of the secondconverter.In Figure 7,V OUT 1and V OUT 2are regulated to 3.3V and 1.8V respectively.Also both voltages reach their regulated levels at the same time.The V FB voltage should reach 0.8V for full regulation,and from Equation 2,V SS/TR should equal 1.25V.This should happen when V OUT 1reaches 3.3V.Choosing R2as 1K Ωand with I SS =2.5µA,R1is found from Equation 3:1.25V =3.3V ×1K Ω/(R1+1K Ω)+2.5µA ×1K Ω.(5)R1is found to be equal to 1.65K Ω.Figure 7.Ratiometric Start Up with V OUT 1Leading V OUT 2Also,Equation 4can be used instead to calculate the values of R1and R2for a design where V OUT 1and V OUT 2do not need to reach regulation at the same time.The next section shows an example.4.2V OUT 2Leading V OUT 1With this configuration,the output voltage of the second converter leads the output voltage of the firstconverter.In Figure 8,V OUT 1and V OUT 2are regulated to 3.3V and 1.8V respectively.However,thesecond output voltage reaches its regulation voltage first.R1and R2are designed in such a way the first output voltage reaches only 1.4V as V OUT 2gets to 1.8V as shown in Figure 8.As described in theprevious section,R2is first fixed and Equation 4is used to calculate R1.The values of R3and R4arefixed to 150K Ωand 187K Ωrespectively and R2is chosen as 2K Ω.1.8V =[1.4V x 2K Ω/(R1+2K Ω)+2.5µA x 2K Ω]x 0.64x [(150K Ω+187K Ω)/150K Ω](6)R1is found to be equal to 240Ω.7SLVA470–November 2011TPS62130/40/50Sequencing and Tracking Submit Documentation Feedback Copyright ©2011,Texas Instruments IncorporatedV1OUTV2OUTRatiometric and Simultaneous Start Up Figure8.Ratiometric Start Up with VOUT 2Leading VOUT14.3Simultaneous Start UpIn the simultaneous start up,the slopes of both output voltages are the same.Thus,both voltages reach regulation in two different times.Equation3calculates the required R1and R2values.Note that in thismethod,when VSS/TR equals1.25V the first converter is at an output voltage equal to VOUT 2’s regulationvoltage.For this example,VOUT 2=1.8V when VOUT1=1.8V.If R2is equal to1KΩ,Equation3is used asfollows:1.25V=1.8V×1KΩ/(R1+1KΩ)+2.5µA×1KΩ.(7)The R1value is found to be432Ω.Figure9shows the result of simultaneous start up.8TPS62130/40/50Sequencing and Tracking SLVA470–November2011Submit Documentation FeedbackCopyright©2011,Texas Instruments Incorporated Conclusion and ReferencesV1OUTV2OUTFigure9.Simultaneous Start Up5Conclusion and References5.1ConclusionThis application note described three different tracking and sequencing methods:sequential,ratiometric, and simultaneous.Sequential start up using the EN and PG pins starts two or more devices in someorder,one device after another.The ramp up slope of each device is independent.In ratiometric start up with connected SS/TR pins,all devices turn on at the same time and have the same ramp up time.With simultaneous start up,two devices start up at the same rate.With small adjustments to resistor values, one device can ramp up at a faster or slower rate compared to the other device.All of these options are available with the TPS6213x/4x/5x family of devices.5.2References1.TPS62130Datasheet(SLVSAG7)2.TPS62140Datasheet(SLVSAJ0)3.TPS62150Datasheet(SLVSAL5)4.TPS54320Datasheet(SLVS982)9 SLVA470–November2011TPS62130/40/50Sequencing and Tracking Submit Documentation FeedbackCopyright©2011,Texas Instruments IncorporatedIMPORTANT NOTICETexas Instruments Incorporated and its subsidiaries(TI)reserve the right to make corrections,modifications,enhancements,improvements, and other changes to its products and services at any time and to discontinue any product or service without notice.Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete.All products are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment.TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard warranty.Testing and other quality control techniques 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