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ISL6313 Datasheet(PDF) 27 Page - Intersil Corporation |
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ISL6313 Datasheet(HTML) 27 Page - Intersil Corporation |
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27 / 33 page  27 FN6448.0 March 5, 2007 N is the number of phases. It is recommended that the desired overcurrent trip level, IOCP, be chosen so that it’s 30% larger then the maximum load current expected. Due to errors in the inductance or DCR it may be necessary to adjust the value of R1 to match the time constants correctly. The effects of time constant mismatch can be seen in the form of droop overshoot or undershoot during the initial load transient spike, as shown in Figure 21. Follow the steps below to ensure the R-C and inductor L/DCR time constants are matched accurately. 1. Capture a transient event with the oscilloscope set to about L/DCR/2 (sec/div). For example, with L = 1µH and DCR = 1m Ω, set the oscilloscope to 500µs/div. 2. Record ΔV1 and ΔV2 as shown in Figure 21. 3. Select new values, R1(NEW), for the time constant resistor based on the original value, R1(OLD), using Equation 36. 4. Replace R1 with the new value and check to see that the error is corrected. Repeat the procedure if necessary. Loadline Regulation Resistor If loadline regulation is desired, the resistor on the FS pin, RT, should be connected to Ground in order for the internal average sense current to flow out across the loadline regulation resistor, labeled RFB in Figure 7. This resistor’s value sets the desired loadline required for the application. The desired loadline, RLL, can be calculated by Equation 37 where VDROOP is the desired droop voltage at the full load current IFL. . Based on the desired loadline, the loadline regulation resistor, RFB, can be calculated from Equation 38. In Equation 38, RLL is the loadline resistance; N is the number of active channels; DCR is the DCR of the individual output inductors; and RSET is the RSET pin resistor. If no loadline regulation is required, the resistor on the FS pin, RT, should be connected to the VCC pin. To choose the value for RFB in this situation, please refer to “Compensation without load-line regulation” on page 28. IOUT Pin Resistor A copy of the average sense current flows out of the IOUT pin, and a resistor, RIOUT, placed from this pin to ground can be used to set the overcurrent protection trip level. Based on the desired overcurrent trip threshold, IOCP, the IOUT pin resistor, RIOUT, can be calculated from Equation 39. APA Pin Component Selection A 100µA current flows into the APA pin and across RAPA to set the APA trip level. A 1000pF capacitor, CAPA, should also be placed across the RAPA resistor to help with noise immunity. Use Equation 40 below to set RAPA to get the desired APA trip level. An APA trip level of 500mV is recommended for most applications. Compensation The two opposing goals of compensating the voltage regulator are stability and speed. Depending on whether the regulator employs the optional load-line regulation as described in Load-Line Regulation, there are two distinct methods for achieving these goals. COMPENSATION WITH LOAD-LINE REGULATION The load-line regulated converter behaves in a similar manner to a peak current mode controller because the two poles at the output filter L-C resonant frequency split with the introduction of current information into the control loop. The final location of these poles is determined by the system function, the gain of the current signal, and the value of the compensation components, RC and CC. R SET DCR 100 10 6 – × ---------------------------- I OCP N -------------- 400 3 ---------- ⋅⋅ = (EQ. 35) R 1NEW () R 1OLD () V 1 Δ V 2 Δ ---------- ⋅ = (EQ. 36) FIGURE 21. TIME CONSTANT MISMATCH BEHAVIOR ΔV1 VOUT ITRAN ΔV 2 ΔI R LL V DROOP I FL ------------------------- = (EQ. 37) R FB R LL NRSET ⋅⋅ DCR --------------------------------------- 3 400 ---------- ⋅ = (EQ. 38) (EQ. 39) R IOUT R SET N ⋅ DCR I OCP ⋅ -------------------------------- 6 400 ---------- ⋅ = R APA V APA TRIP () 100 10 6 – × --------------------------------- 500mV 100 10 6 – × ----------------------------- 5k Ω == = (EQ. 40) ISL6313 |
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