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LM3101M Arkusz danych(PDF) 10 Page - National Semiconductor (TI) |
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LM3101M Arkusz danych(HTML) 10 Page - National Semiconductor (TI) |
10 / 20 page Functional Description (Continued) To operate under current mode or charge mode control insert a resistor RMC between the MC pin and ground and connect the CMI pin to the Current Mode Sense Circuit (see Figure 4 ) At the MC pin a voltage proportional to the oscil- lator ramp voltage develops (see Figure 3 ) The voltage ramp applied to RMC generates a current ramp which is duplicated on the CMI pin due to the current mirror The current ramp which flows out of the CMI pin to the resistor RF is the compensation ramp needed to stabilize convert- ers operating at duty cycles above 50% The slope of the compensating ramp can be scaled by RF which is connect- ed between the CMI pin and the terminating resistor RS of the current sense transformer In all practical cases RF will be much greater than RS For both control modes the cur- rent ramp provides slope compensation according to the equation D ICOMP Dt e 24 103 (RT RMC) m A ms X So with RT equal 25 kX and RMC equal 6 kX the compen- sation ramp slope is D ICOMP Dt e 24 103 (25 kX 6kX) e 160 mA ms The resistor RF together with the capacitor CF serves as an RC filter for the leading edge spike of the current sense waveform (the spike is caused by the output rectifier reverse recovery andor the winding capacitance of the power transformer) CHARGE MODE CONTROL Under charge mode control the current sense transformer drives a capacitor C1 that integrates the sensed switch current on a cycle-by-cycle basis Figure 5 shows the inte- grating current sense circuitry and the simplified details of the associated internal circuitry of the LM3101 Transistor Q1 discharges the integrating capacitor C1 once every switch cycleduring the switch off-time (Q1 can provide up to 20 mA of discharge current) Charge mode control yields the fastest average current control loop TLH11436 – 10 TLH11436 – 11 FIGURE 5 Charge Mode Operation 10 |
Podobny numer części - LM3101M |
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Podobny opis - LM3101M |
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