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LTC1704B Datasheet(PDF) 21 Page - Linear Technology |
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LTC1704B Datasheet(HTML) 21 Page - Linear Technology |
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21 / 28 page  LTC1704/LTC1704B 21 1704bfa APPLICATIO S I FOR ATIO *1704 modulator gain/phase *2001 Linear Technology *this file written to run with PSpice 9.0 *may require modifications for other SPICE simulators *MOSFETs rfet mod sw 0.02 ;MOSFET rdson *inductor lext sw out1 1u ;inductor value rl out1 outsw 0.005 ;inductor series R *output cap cout outsw out2 1000u ;capacitor value resr out2 0 0.01 ;capacitor ESR *1704 internals emod mod 0 comp 0 5 ;3.3 for 3.3V supply vstim comp 0 0 ac 1 ;ac stimulus .ac dec 100 1k 1meg .probe .end With the gain/phase plot in hand, a loop crossover fre- quency can be chosen. Usually the curves look something like Figure 7. Choose the crossover frequency in the rising or flat parts of the phase curve, beyond the external LC poles. Frequencies between 10kHz and 50kHz usually work well. Note the gain (GAIN, in dB) and phase (PHASE, in degrees) at this point. The desired feedback amplifier gain will be – GAIN to make the loop gain at 0dB at this frequency. Now calculate the needed phase boost, assum- ing 60 ° as a target phase margin: BOOST = – (PHASE + 30 °) If the required BOOST is less than 60 °, a Type 2 loop can be used successfully, saving two external components. BOOST values greater than 60 ° usually require Type 3 loops for satisfactory performance. Finally, choose a convenient resistor value for R1 (10k is usually a good value). Now calculate the remaining values: (K is a constant used in the calculations) f = chosen crossover frequency G = 10(GAIN/20) (this converts GAIN in dB to G in absolute gain) TYPE 2 Loop: K Tan BOOST C fGKR CC K R K fC R VR VV REF OUTSW REF =+ ° = =− () = = − 2 45 2 1 21 12 1 4 21 2 1 2 π π () TYPE 3 Loop: K Tan BOOST C fGR CC K R K fC R R K C fKR R VR VV REF OUTSW REF =+ ° = =− () = = − = = − 2 4 45 2 1 21 12 1 4 21 3 1 1 3 1 23 2 1 π π π () SWITCHING SUPPLY CURRENT LIMIT PROGRAMMING Programming the current limit on the LTC1704 switcher supply is straightforward. The IMAX pin sets the current limit by setting the maximum allowable voltage drop across QB (the bottom MOSFET) before the current limit circuit engages. The voltage across QB is set by its on- resistance and the current flowing in the inductor, which |
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Descripción similar - LTC1704B_15 |
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