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C2012X5R0J106M Datasheet(PDF) 11 Page - Richtek Technology Corporation |
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C2012X5R0J106M Datasheet(HTML) 11 Page - Richtek Technology Corporation |
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11 / 14 page  RT8058A 11 DS8058A-02 April 2011 www.richtek.com ⎥⎦ ⎤ ⎢⎣ ⎡ + ≤ OUT L OUT 8fC 1 ESR ΔI ΔV The selection of COUT is determined by the Effective Series Resistance (ESR) that is required to minimize voltage ripple and load step transients, as well as the amount of bulk capacitance that is necessary to ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response as described in a later section. The output ripple, ΔVOUT, is determined by : The output ripple is highest at maximum input voltage since ΔIL increases with input voltage. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important to only use types that have been surge tested for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR but can be used in cost-sensitive applications provided that consideration is given to ripple current ratings and long term reliability. Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic capacitors with trace inductance can also lead to significant ringing. 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, VIN. 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 VIN large enough to damage the part. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ΔILOAD(ESR), where ESR is the effective series resistance of COUT. ΔILOAD also begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. Thermal Considerations For continuous operation, do not exceed absolute maximum operation junction temperature. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow and temperature difference between junction to ambient. The maximum power dissipation can be calculated by following formula : PD(MAX) = ( TJ(MAX) − TA ) / θJA Where TJ(MAX) is the maximum operation junction temperature, TA is the ambient temperature and the θJAis the junction to ambient thermal resistance. For recommended operating conditions specification of RT8058A, The maximum junction temperature is 125 °C. The junction to ambient thermal resistance θJA is layout dependent. For WDFN-10L 3x3 packages, the thermal resistance θJA is 70°C/W on the standard JEDEC 51-7 four layers thermal test board. The maximum power dissipation at TA = 25 °C can be calculated by following formula : PD(MAX) = (125 °C − 25°C) / (70°C/W) = 1.429W for WDFN-10L 3x3 packages The maximum power dissipation depends on operating ambient temperature for fixed TJ(MAX) and thermal resistance θJA. For RT8058A packages, the Figure 2 of derating curves allows the designer to see the effect of rising ambient temperature on the maximum power allowed. |
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