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LTC2927CDDB-TRPBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC2927CDDB-TRPBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LTC2927 8 2927fb RONB 138k RONA 100k ON RAMP MASTER 1.8V 2927 F07 VCC VIN EARLY VIN 3.3V LTC2927 GND SDO FB RUN/SS IN DC/DC FB = 1.235V OUT RAMPBUF TRACK 0.1μF CRAMP 10pF RTB 16.5k RFB 16.5k RFA 35.7k RTA 13k Figure 7. SDO Shutdown Application RONB RONA ON RAMP MASTER SLAVE 2927 F08 VCC VIN EARLY VIN LTC2927 GND FB IN DC/DC FB OUT RAMPBUF TRACK 0.1μF CRAMP RTB RFB RFA RTA Figure 8. Single Supply Application SDO pulls low again when the ON pin is pulled below 1.23V and the RAMP pin is below about 200mV. 3-Step Design Procedure The following 3-step procedure allows one to complete a design for any of the tracking or sequencing profiles shown in Figures 1 to 4. A basic single supply application circuit is shown in Figure 8. 1. Set the ramp rate of the master signal. Solve for the value of CRAMP, the capacitor on the RAMP pin, based on the desired ramp rate (V/s) of the master supply, SM. C I S where I A RAMP RAMP M RAMP =≈ μ 10 1 () 2. Solve for the pair of resistors that provide the desired ramp rate of the slave supply, assuming no delay. Choose a ramp rate for the slave supply, SS. If the slave supply ramps up coincident with the master signal or with a fixed voltage offset, then the ramp rate equals the master supply’s ramp rate. Be sure to use a fast enough ramp rate for the slave supply so that it will finish ramping before the master signal has reached its final supply value. If not, the slave supply will be held below the intended regulation value by the master signal. Use the following formulas to determine the resistor values for the desired ramp rate, where RFB and RFA are the feedback resistors in the slave supply and VFB is the feedback reference voltage of the slave supply: RR S S TB FB M S =• () 2 R V V R V R V R TA TRACK FB FB FB FA TRACK TB ′ = +− () 3 where VTRACK ≈ 0.8V. Note that large ratios of slave ramp rate to master ramp rate, SS/SM, may result in negative values for RTA’. If sufficiently large delay is used in step 3, RTA will be positive, otherwise SS/SM must be reduced. 3. Choose RTA to obtain the desired delay. If no delay is required, such as in coincident and ratio- metric tracking, then simply set RTA = RTA’. If a delay is desired, as in offset tracking and supply sequencing, calculate RTA” to determine the value of RTA where tD is the desired delay in seconds. R VR tS TA TRACK TB DM ″ = • • () 4 RR R TA TA TA = ′″ || ( ) 5 the parallel combination of RTA’ and RTA”. Asnotedinstep2,smalldelaysandlargeratiosofslaveramp rate to master ramp rate (usually only seen in sequencing) may result in solutions with negative values for RTA. In such cases, either the delay must be increased or the ratio of slave ramp rate to master ramp rate must be reduced. APPLICATIO S I FOR ATIO |
Similar Part No. - LTC2927CDDB-TRPBF |
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Similar Description - LTC2927CDDB-TRPBF |
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