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RC5061 数据表(PDF) 13 Page - Fairchild Semiconductor |
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RC5061 数据表(HTML) 13 Page - Fairchild Semiconductor |
13 / 18 page PRODUCT SPECIFICATION RC5061 REV. 1.0.0 7/6/00 13 Design Considerations and Component Selection Additional information on design and component selection may be found in Fairchild’s Application Note 57. MOSFET Selection This application requires N-channel Logic Level Enhancement Mode Field Effect Transistors. Desired characteristics are as follows: • Low Static Drain-Source On-Resistance, RDS,ON < 20mΩ (lower is better) • Low gate drive voltage, VGS = 4.5V rated • Power package with low Thermal Resistance • Drain-Source voltage rating > 15V. The on-resistance (RDS,ON) is the primary parameter for MOSFET selection. The on-resistance determines the power dissipation within the MOSFET and therefore significantly affects the efficiency of the DC-DC Converter. For details and a spreadsheet on MOSFET selection, refer to Applica- tions Bulletin AB-8. Inductor Selection Choosing the value of the inductor is a tradeoff between allowable ripple voltage and required transient response. The system designer can choose any value within the allowed minimum to maximum range in order to either minimize ripple or maximize transient performance. The first order equation (close approximation) for minimum inductance is: where: Vin = Input Power Supply Vout = Output Voltage f = DC/DC converter switching frequency ESR = Equivalent series resistance of all output capacitors in parallel Vripple = Maximum peak to peak output ripple voltage budget. The first order equation for maximum allowed inductance is: where: Co = The total output capacitance Ipp = Maximum to minimum load transient current Vtb = The output voltage tolerance budget allocated to load transient Dm = Maximum duty cycle for the DC/DC converter (usually 95%). Some margin should be maintained away from both Lmin and Lmax. Adding margin by increasing L almost always adds expense since all the variables are predetermined by system performance except for CO, which must be increased to increase L. Adding margin by decreasing L can be done by purchasing capacitors with lower ESR. The RC5061 pro- vides significant cost savings for the newer CPU systems that typically run at high supply current. RC5061 Short Circuit Current Characteristics The RC5061 protects against output short circuit on the core supply by turning off both the high-side and low-side MOSFETs and resetting softstart. The short circuit limit is set with the RS resistor, as given by the formula with IDetect ≈ 50µA, ISC is the desired current limit, and RDS,on the high-side MOSFET’s on resistance. Remember to make the RS large enough to include the effects of initial tol- erance and temperature variation on the MOSFET’s RDS,on. Alternately, use of a sense resistor in series with the source of the MOSFET eliminates this source of inaccuracy in the current limit. The value of RS should be less than 8.3KΩ. If a greater value is necessary, a lower RDS,on MOSFET should be used instead. As an example, Figure 4 shows the typical characteristic of the DC-DC converter circuit with an FDB6030L high-side MOSFET (RDS = 20mΩ maximum at 25°C * 1.25 at 75°C = 25m Ω) and a 8.2KΩ R S. Figure 4. RC5061 Short Circuit Characteristic The converter exhibits a normal load regulation characteristic until the voltage across the MOSFET exceeds the internal short circuit threshold of 50µA * 8.2K Ω = 410mV, which occurs at 410mV/25m Ω = 16.4A. (Note that this current limit level can be as high as 410mV/15m Ω = 27A, if the MOSFET has typical RDS,on rather than maximum, and is at 25°C). At this point, the internal comparator trips and signals the con- troller to discharge the softstart capacitor. This causes a drastic reduction in the output voltage as the load regulation collapses into the short circuit control mode. With a 40m Ω output short, Lmin (Vin – Vout) f x Vout Vin x ESR Vripple = Lmax (Vin – Vout) Dm Vtb Ipp2 = 2CO RS ISC *RDS, on IDetect = 0 5 10 15 20 25 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 CPU Output Voltage vs. Output Current |
类似零件编号 - RC5061 |
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类似说明 - RC5061 |
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