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IRU1015 Datasheet(PDF) 6 Page - International Rectifier |
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IRU1015 Datasheet(HTML) 6 Page - International Rectifier |
6 / 8 page IRU1015 6 Rev. 1.1 06/29/01 2) With the output capacitance being 1500µF: Where: ∆t = 2µs is the regulator response time To set the output voltage, we need to select R1 and R2: 3) Assuming R1 = 121Ω, 1% Select R2 = 215Ω, 1% Selecting both R1 and R2 resistors to be 1% toler- ance results in the least amount of error introduced by the resistor dividers leaving a ≈ ±2.5% error bud- get for the IRU1015 reference which is well within the initial accuracy of the device. Finally, the input capacitor is selected as follows: 4) Assuming that the input voltage can drop 150mV be- fore the main power supply responds, and that the main power supply response time is ≈ 50ms, then the minimum input capacitance for a 1.2A load step is given by: The ESR should be less than: Where: VDROP L Input voltage drop allowed in step 4 ∆V L Maximum regulator dropout voltage ∆I L Load current step Select a single 1500µF the same type as the output capacitors exceeds our requirements.Figure 6 shows the completed schematic for our example. Figure 5 - Typical regulator response to the fast load current step An example of a regulator design to meet the AMD speci- fication for 486DX4-120MHz is given below. Assume the specification for the processor as shown in Table 1: Type of Vout Imax Max Allowed Processor Nominal Output Tolerance AMD 486DX4 3.45 V 1.2 A ±150 mV Table 1 - GTL+ specification for Pentium Pro The first step is to select the voltage step allowed in the output due to the output capacitor’s ESR: 1) Assuming the regulator’s initial accuracy plus the re- sistor divider tolerance is ≈ ±86mV (±2.5% of 3.45V nominal), then the total step allowed for the ESR and the ESL, is -64mV. Assuming that the ESL drop is -10mV, the remaining ESR step will be -54mV. Therefore the output ca- pacitor ESR must be: The Sanyo MVGX series is a good choice to achieve both price and performance goals. The 6MV1500GX, 1500µF, 6.3V has an ESR of less than 36mΩ typi- cal. Selecting a single capacitor achieves our design goal. The next step is to calculate the drop due to the ca- pacitance discharge and make sure that this drop in voltage is less than the selected ESL drop in the previous step. V ESR V ESL V C T LOAD CURRENT LOAD CURRENT RISE TIME 1015plt1-1.0 CIN = = 400µF 1.2 × 50 0.15 ESR = = 0.167Ω (5 - 3.45 - 1.2 - 0.15) 1.2 ESR = (VIN - VOUT - ∆V - VDROP) ∆I ∆Vc = = = 1.6mV ∆t × ∆I 2 × 1.2 C 1500 ESR ≤ = 45mΩ 54 1.2 R2 =o o o o o - 1p p p p p × 121 =o o o o o - 1p p p p p × 121 = 213Ω VOUT VREF 3.45 1.25 |
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