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NCP5425DBR2G Datasheet(PDF) 19 Page - ON Semiconductor |
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NCP5425DBR2G Datasheet(HTML) 19 Page - ON Semiconductor |
19 / 22 page ![]() NCP5425 http://onsemi.com 19 The values of R2 and R3 can be found by solving two simultaneous equations: R2 + R3 2 R1 + (R2 * R3) (R2 ) R3) Solving for R2 and R3 yields: R2 + 1.5R1 R3 + 3R1 Example 2 Assume we have elected to source 66.7% output current from the Master controller, and 33.3% from the Slave. Figure 13 shows the configuration of the inductor sense networks and Slave error amplifier for this case. The ratio of Master−to−Slave load current is equal to 66.7%/33.3%, or 2:1. Therefore R2 and R3 must be chosen to satisfy two conditions: A parallel equivalent resistance equal to R1, and, A ratio such that the drop across the parasitic resistance of the Master inductor is 2 times the drop across the parasitic resistance of the Slave inductor when the inputs to the Slave error amplifier are equal (assumes the inductors are identical). The optimum value of R1 is described by the equation: R1 + Lx (C1 * Rx) The values of R2 and R3 can be found by solving two simultaneous equations: R2 + R3 R1 + (R2 * R3) (R2 ) R3) Solving for R2 and R3 yields: R2 + 2R1 R3 + 2R1 Note that the Mode pin must be floating for a two−phase, single output design. This disables the internal Error Amplifier Reference clamp, and increases its common mode range. No Load Zero Balance To improve current matching, a low pass filter can be inserted between the Master controller inductor sensing RC network and the Slave controller Vref2 input pin (see Figure 14). This will attenuate the amplitude of the out−of−phase ripple current signal superimposed on the DC current signal, providing a smoother Slave Error Amplifier reference input. Vin Q1 L1 R1 C1 R3 R3 R4 C3 + − Vfb1 Q2 Master Error Amp Internal 0.8 V Ref. L2 R2 C2 + − Vfb2 Vref2 RF Low Pass Filter Slave Error Amp Q3 Q4 Figure 14. Addition of a Low Pass Filter to the Current Sense Reference Input Vout CF With the value of RF set to approximately two times the value of R1, Cf can be calculated as follows: Cf + 1 (2p ·f· RF), where : f = operating frequency of the controller When a filter is added, the response delay introduced by the RC time constant must be considered. Configuring a Dual Output Application To configure the NCP5425 for a dual output application: • The Mode pin must be grounded • An external voltage reference must be provided for Controller 2, via the Vref2 pin |
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