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ISL6730CFUZ Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6730CFUZ Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 19 page  ISL6730A, ISL6730B, ISL6730C, ISL6730D 11 FN8258.1 August 8, 2013 A high value RCS renders more accurate current sensing. It is recommended to use the RCS to render 120mV peak voltage at the maximum line voltage during full load condition. Where η is the efficiency of the converter at the maximum line input with full load. Since the RCS sees the average input current, high value RCS generates high power dissipation on the RCS. Use a reasonable RCS according to the resistor power rating. The worst-case power dissipation occurs at the input low line when input current is at its maximum. Power dissipation by the resistor is: where: IRMSMAX is the maximum input RMS current at the minimum input line voltage, VRMSmin. Select the RSEN according to the peak current limit requirement. The resistor is sized for an overload current 25% more than the peak inductor peak current. Negative Input Capacitor Generation (Patent Pending) The patent pending negative capacitor generation capability of the ISL6730A, ISL6730B, ISL6730C, ISL6730D allows the capacitor CF2 to be moved from before the bridge rectifier (Figure 12) to after the bridge rectifier (Figure 13). Thus, a smaller lower cost CF2 can be used. The change in topology reduces the size of the EMI filter. Furthermore, CF1 can be increased thus decreasing the size of LF (Figure 13). For applications where the output power is above 500W, the negative capacitance helps to improve the power factor dramatically. Please refer to Table 2 for the recommended filtering capacitor to be placed after the bridge rectifier, CF1. Additional CF1 may be used to accommodate the use of small boost inductor or to eliminate the differential mode filter inductor as long as the equipment meets the power factor or goal. The equivalent negative capacitor is a function of the input voltage divider ratio, KBO, the current sensing gain and current compensation error integration gain. Adjusting the negative Ceq can be achieved by adjusting the current compensation network. Frequency Modulation The ISL6730A, ISL6730B, ISL6730C, ISL6730D can further reduce EMI filter size by lowering the differential noise power density. The reduction is achieved by switching frequency modulation. The frequency varies with the VIN pin. The switching frequency reaches the peak value when the VIN pin voltage is 2V as shown in Figure 6. The peak value of ISL6730A/C is 124kHz, and the ISL6730B/D is 62kHz. Output Voltage Regulation The output voltage is sensed through a resistor divider. The middle point of the resistor divider is fed to the FB pin. The resistor divider ratio sets the output voltage. The transconductance error amplifier generates a current in proportion to the difference between the FB pin and the 2.5V internal reference. The PFC is stabilized by the compensation network that is connected from the COMP pin to the ground. The voltage of the COMP sets the input average power by determining the amplitude of the current reference. To keep the FIGURE 11. INDUCTOR CURRENT SENSING SCHEME Q1 COUT VOUT L CF1 VI RCS ISEN RSEN CURRENT MIRROR 2:1 I CS 0.5 I OC > I CS R CS 120mV V RMSMAX η ⋅⋅ 2P Omax ⋅ ------------------------------------------------------------- > (EQ. 4) P RCS I RMSMAX ()2 R CS ⋅ = (EQ. 5) FIGURE 12. TYPICAL PFC INPUT FILTER CIRCUIT FIGURE 13. LOW COST PFC INPUT FILTER CIRCUIT TABLE 2. CF1 Po < 100W 100W < Po < 500W Po > 500W Typical C(µF)/100W 0.68 0.33 0.22 CF1 CF2 VLINE CF3 EMI CHOKE Lm BRIDGE RECTIFIER LF CF1 CF2 VLINE CF3 EMI CHOKE Lm BRIDGE RECTIFIER LF |
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