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NCP5425DBR2G Datasheet(PDF) 18 Page - ON Semiconductor |
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NCP5425DBR2G Datasheet(HTML) 18 Page - ON Semiconductor |
18 / 22 page ![]() NCP5425 http://onsemi.com 18 Given an ESR value of 3.5 m W, the current limit becomes 20 A. If an increased current limit is required, a resistor divider can be added (see Figure 8). Advantages of setting the current limit by using the winding resistance of the inductor (relative to a sense resistor) are higher efficiency and lower heat generation. The tolerance of the inductor ESR must be factored into the design of the current limit. Finally, one or two more components are required for this approach than with resistor sensing. Selecting and Configuring Current Sharing for a 2 Phase Single Output Application When the two controllers are connected as a single output two phase Buck Converter, they are in a Master−Slave configuration. The Slave controller on the right side of Figure 6 tries to follow information provided by the Master controller, on the left. This circuit uses inductor current sensing, in which the parasitic resistances (LSR) of the controllers’ output chokes are used as current sensing elements. On the Slave side (Controller Two), both Error Amplifier inputs are brought to external pins so the reference is available. The RC network in parallel with the output inductor on the Master side (Controller One) generates the reference for the Slave. Current information from the Slave is fed back to the error amplifier’s inverting input. In this configuration, the Slave tries to adjust its current to match the current information fed to its reference input from the Master Controller. If 50−50 current sharing is needed, then Figure 8a is used for both sides to generate the reference and the inverting signals. The values for both sides should be calculated with the following equation: R1 + Lx C1 · Rx ,where, Lx = Inductor value, both controllers should use the same inductor. Rx = Internal resistance of L, from the inductor data sheet. C1 = Select a value such that R1 is less than 15 K W. With the RC time constant selected to equal the Lx/Rx time constant, the voltage across the capacitor will be equal to the voltage drop across the internal resistance of the inductor. For proper sharing, the inductors on both Master and Slave side should be the identical. If a current share ratio other than 50−50 is desired, inductor sense resistor network selection is a three step process: 1. Decide how the total load current will be budgeted between the two controllers. 2. Calculate the value of R1 for the controller with the lesser current share. 3. Calculate the current sense resistor network (2 resistors) for the controller with the greater current share. In the two examples that follow, the inductor sense resistors are designated R1, R2, and R3, as depicted in Figures 12 and 13. Figure 12. 40%/60% Current Sharing Rx Lx Master Switch Node R1 Slave Error Amp R2 Rx Lx C1 DC Output R3 C1 Slave Switch Node Figure 13. 66.7%/33.3% Current Sharing Rx Lx Master Switch Node R2 Slave Error Amp R1 Rx Lx C1 DC Output R3 C1 Slave Switch Node Example 1 Assume we have elected to source 40% of the output current from the master controller, and 60% from the Slave. Figure 12 shows the configuration of the inductor sense networks and Slave error amplifier. The ratio of Slave−to−Master load current is 60%/40%, or 1.5:1. 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 Slave inductor is 1.5 times the drop across the parasitic resistance of the Master 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) |
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