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AME5251A Datasheet(PDF) 8 Page - Analog Microelectronics |
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AME5251A Datasheet(HTML) 8 Page - Analog Microelectronics |
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8 / 19 page  AME 8 Dual 1A, 1.5MHz Synchronous Step-Down Converter AME5251A Rev.A.04 This formula has a maximum at V IN=2VOUT, where IRMS=I OUT/2. This simple worst-case condition is com- monly used for design because even significant devia- tions do not offer much relief. Note that the capacitor manufacturer ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. The selection of C OUT is determined by the effective series resistance(ESR) that is required to minimize volt- age ripple and load step transients. The output ripple, V OUT, is determined by: Using Ceramic Input and Output Capacitors Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these capacitors are used at the input and output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, V IN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at V IN large enough to damage the part. Output Voltage Programming The output voltage is set by an external resistive di- vider according to the following equation: Where VREF equals to 0.6V typical. The resistive di- vider allows the FB pin to sense a fraction of the output voltage as shown in Figure 1. + ∆ ≅ ∆ OUT L OUT fC ESR I V 8 1 + × = 2 1 1 R R V V REF OUT AME5251 A FB GND R1 R2 0.6V VOUT 5.5V Figure 1: Setting the AME5251A Output Voltage Thermal Considerations In most applications the AME5251A does not dissi- pate much heat due to its high efficiency. But, in appli- cations where the AME5251A is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 160OC, both power switches will be turned off and the SW node will become high impedance. To avoid the AME5251A from exceed- ing the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by: Where PD is the power dissipated by the regulator and θ JA is the thermal resistance from the junction of the die to the ambient temperature. ( )( ) JA R PD T θ = |
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