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PAM2305BJE120 Datasheet(PDF) 10 Page - Power Analog Micoelectronics |
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PAM2305BJE120 Datasheet(HTML) 10 Page - Power Analog Micoelectronics |
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10 / 17 page  10 09/2008 Rev 1.2 , Power Analog Microelectronics Inc www.poweranalog.com Application Information The basic PAM2305 application circuit is shown in Page 1. External component selection is determined by the load requirement, selecting L first and then Cin and Cout. For most applications, the value of the inductor will fall in the range of 1μH to 4.7μH. Its value is chosen based on the desired ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher V or Vout also increases the ripple current as shown in equation 1. A reasonable starting point for setting ripple current is I = 400mA (40% of 1A). (1) The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 1.4A rated inductor should be enough for most applications (1A + 400mA). For better efficiency, choose a low DC-resis tance inductor. In continuous mode, the source current of the top MOSFET is a square wave of duty cycle Vout/Vin. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: This formula has a maximum at V =2Vout, where I =I /2. T his simple worst-case condition is com monly used for design because even significant deviations do not offer much relief. Note that the capacitor manufacturer's 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. Consult the manufac turer if there is any question. The selection of Cout is driven by the required effective series resistance (ESR). Typically, once the ESR requirement for Cout has been met, the RMS current rating generally far exceeds the I (P-P) requirement. The output ripple Vout is determined by: Where f = operating frequency, C =output capacitance and I = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since I increases with input voltage. 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. Using ceramic capacitors can achieve very low output ripple and small circuit size. When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best temperature and voltage charac teristics of all the ceramics for a given value and size. Inductor Selection C and C Selection Using Ceramic Input and Output Capacitors IN L IN RMS O UT RIPPLE OUT L L △ △ IN OUT Δ Δ Thermal consideration Thermal protection limits power dissipation in the PAM2305. When the junction temperature exceeds 150°C, the OTP (Over Temperature Protection) starts the thermal shutdown and turns the pass transistor off. The pass transistor resumes operation after t he junction temperature drops below 120°C. For c ontinuous operation, the j unction temperature should be maintained below 125°C. The power dissipation is defined as: I is the step-down converter quiescent current. The term tsw is used to estimate the full load step-down converter switching losses. Q () 1 2 OUT IN OUT IN RMS OMAX IN VV V C required I I V - éù ëû @ OUT L OUT 1 VI ESR+ 8fC æö @ ç÷ èø VV PAM2305 1A Step-Down DC-DC Converters ()( ) æö D ç÷ èø OUT OUT L IN 1V I= V 1- fL V () () O DSONH IN O DSONL 2 DO SW S O Q IN IN VR + V -V R P=I + t F I +I V V Vo 1.2V 1.5V 1.8V 2.5V 3.3V L 2.2μH 2.2μH 2.2μH 4.7μH 4.7μH |
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