Datasheet LTC3607 (Analog Devices) - 10

LTC3607
applicaTions inForMaTion Input Capacitor (CIN) Selection
be less than 100mV at maximum VIN and fO = 2.25MHz In continuous mode, the input current of the converter is a with: ESRCOUT < 150mΩ. square wave with a duty cycle of approximately VOUT/VIN. Once the ESR requirements for COUT have been met, the To prevent large voltage transients, a low equivalent series RMS current rating general y far exceeds the IRIPPLE(P-P) resistance (ESR) input capacitor sized for the maximum requirement, except for an all ceramic solution. RMS current must be used. The maximum RMS capacitor Since the ESR of a ceramic capacitor is so low, the input current is given by: and output capacitor must instead fulfill a charge storage V requirement. During a load step, the output capacitor must I OUT(VIN – VOUT) RMS ≈IOUT(MAX) instantaneously supply the current to support the load VIN until the feedback loop raises the switch current enough where the maximum average output current I to support the load. The time required for the feedback MAX equals the peak current minus half the peak-to-peak ripple cur- loop to respond is dependent on the compensation and the rent, I output capacitor size. Typical y, 3 to 4 cycles are required MAX = ILIM – ΔIL/2. to respond to a load step, but only in the first cycle does This formula has a maximum at VIN = 2VOUT, where IRMS the output drop linearly. The output droop, V = I DROOP, is OUT/2. This simple worst-case is commonly used to usual y about five times the linear drop of the first cycle. design because even significant deviations do not offer Thus, a good place to start is with the output capacitor much relief. Note that capacitor manufacturer’s ripple cur- size of approximately: rent ratings are often based on only 2000 hours lifetime. This makes it advisable to further derate the capacitor, ∆I or choose a capacitor rated at a higher temperature than C OUT OUT ≈ 5 f required. Several capacitors may also be paralleled to meet O • VDROOP the size or height requirements of the design. An additional Though this equation provides a good approximation, more 0.1μF to 1μF ceramic capacitor is also recommended on capacitance may be required depending on the duty cycle VIN for high frequency decoupling, when not using an all and load step requirements. ceramic capacitor solution.
Ceramic Input and Output Capacitors Output Capacitor (COUT) Selection
High value, low cost ceramic capacitors are available in The selection of COUT is driven by the required ESR to small case sizes. Their high ripple current, high voltage minimize voltage ripple and load step transients. Typical y, rating, and low ESR make them ideal for switching regulator once the ESR requirement is satisfied, the capacitance applications. However, due to the self-resonant and high-Q is adequate for filtering. The output ripple (ΔVOUT) is characteristics of some types of ceramic capacitors, care determined by: must be taken when these capacitors are used at the input.  1 V ∆  When a ceramic capacitor is used at the input and the power OUT ≈ ∆IL ESR+   8fO COUT is being supplied through long wires, such as from a wall adapter, a load step at the output can induce ringing at where fO = operating frequency, COUT = output capacitance the VIN pin. At best, this ringing can couple to the output and ΔIL = ripple current in the inductor. The output ripple and be mistaken as loop instability. At worst, the ringing is highest at maximum input voltage since ΔIL increases at the input can be large enough to damage the part. For with input voltage. With ΔIL = 240mA the output ripple will a more detailed discussion, refer to Application Note 88. 3607fb 10 For more information www.linear.com/LTC3607 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts