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CS5303 Datasheet(PDF) 11 Page - ON Semiconductor |
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CS5303 Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 19 page CS5303 http://onsemi.com 11 SWNODE VFB (VOUT) CSA Out CSA Out + VFB Figure 10. Open Loop Operation COMP − Offset T1 T2 Inductive Current Sensing For lossless sensing current can be sensed across the inductor as shown below in Figure 11. In the diagram L is the output inductance and RL is the inherent inductor resistance. To compensate the current sense signal the values of R1 and C1 are chosen so that L/RL = R1 × C1. If this criteria is met the current sense signal will be the same shape as the inductor current, the voltage signal at Cx will represent the instantaneous value of inductor current and the circuit can be analyzed as if a sense resistor of value RL was used as a sense resistor (RS). SWNODE VOUT DACOUT COMP VFB CSREF CS CSA OFFSET PWM- COMP E.A. R1 C1 L RL + + + + + + Figure 11. Lossless Inductive Current Sensing with Enhanced V2 When choosing or designing inductors for use with inductive sensing, tolerances and temperature effects should be considered. Cores with a low permeability material or a large gap will usually have minimal inductance change with temperature and load. Copper magnet wire has a temperature coefficient of 0.39% per °C. The increase in winding resistance at higher temperatures should be considered when setting the ILIM threshold. If a more accurate current sense is required than inductive sensing can provide, current can be sensed through a resistor as shown in Figure 9. Current Sharing Accuracy PCB traces that carry inductor current can be used as part of the current sense resistance depending on where the current sense signal is picked off. For accurate current sharing, the current sense inputs should sense the current at the same point for each phase and the connection to the CSREF should be made so that no phase is favored. (In some cases, especially with inductive sensing, resistance of the pcb can be useful for increasing the current sense resistance.) The total current sense resistance used for calculations must include any pcb trace between the CS inputs and the CSREF input that carries inductor current. Current Sense Amplifier Input Mismatch and the value of the current sense element will determine the accuracy of current sharing between phases. The worst case Current Sense Amplifier Input Mismatch is 5 mV and will typically be within 3 mV. The difference in peak currents between phases will be the CSA Input Mismatch divided by the current sense resistance. If all current sense elements are of equal resistance a 3 mV mismatch with a 2 mΩ sense resistance will produce a 1.5 A difference in current between phases. Operation at > 50% Duty Cycle For operation at duty cycles above 50% Enhanced V2 will exhibit subharmonic oscillation unless a compensation ramp is added to each phase. A circuit like the one on the left side of Figure 12 can be added to each current sense network to implement slope compensation. The value of R1 can be varied to adjust the ramp size. Switch Node CSX CSREF 25 k R1 .01 μF 1 nF 0.1 μF 3 k Gate(L)X Slope Comp Circuit Existing Current Sense Circuit MMBT2222LT1 Figure 12. External Slope Compensation Circuit |
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