Datasheet LTC3548-2 (Analog Devices) - 9

LTC3548-2
APPLICATIONS INFORMATION
RMS current must be used. The maximum RMS capacitor RMS current handling requirement of the application. current is given by: Aluminum electrolytic, special polymer, ceramic and dry tantalum capacitors are all available in surface mount V (V – V ) I ≈I OUT IN OUT packages. The OS-CON semiconductor dielectric capacitor RMS MAX VIN available from Sanyo has the lowest ESR (size) product where the maximum average output current I of any aluminum electrolytic at a somewhat higher price. MAX equals the peak current minus half the peak-to-peak ripple cur- Special polymer capacitors, such as Sanyo POSCAP, rent, I Panasonic Special Polymer (SP) and Kemet A700, of- MAX = ILIM – ΔIL/2. fer very low ESR, but have a lower capacitance density This formula has a maximum at VIN = 2VOUT, where IRMS than other types. Tantalum capacitors have the highest = IOUT/2. This simple worst-case is commonly used to capacitance density, but they have a larger ESR and it design because even signifi cant deviations do not offer is critical that the capacitors are surge tested for use in much relief. Note that capacitor manufacturer’s ripple cur- switching power supplies. An excellent choice is the AVX rent ratings are often based on only 2000 hours lifetime. TPS series of surface mount tantalums, available in case This makes it advisable to further derate the capacitor, heights ranging from 2mm to 4mm. Aluminum electrolytic or choose a capacitor rated at a higher temperature than capacitors have a signifi cantly larger ESR, and are often required. Several capacitors may also be paralleled to meet used in extremely cost-sensitive applications provided that the size or height requirements of the design. An additional consideration is given to ripple current ratings and long 0.1μF to 1μF ceramic capacitor is also recommended on term reliability. Ceramic capacitors have the lowest ESR VIN for high frequency decoupling, when not using an all and cost, but also have the lowest capacitance density, ceramic capacitor solution. a high voltage and temperature coeffi cient, and exhibit audible piezoelectric effects. In addition, the high Q of
Output Capacitor (COUT) Selection
ceramic capacitors along with trace inductance can lead The selection of COUT is driven by the required ESR to to signifi cant ringing. minimize voltage ripple and load step transients. Typically, In most cases, 0.1μF to 1μF of ceramic capacitors should once the ESR requirement is satisfi ed, the capacitance also be placed close to the LTC3548-2 in parallel with the is adequate for fi ltering. The output ripple (ΔVOUT) is main capacitors for high frequency decoupling. determined by: ⎛ 1 ⎞
Ceramic Input and Output Capacitors
ΔVOUT ≈ ΔI ESR L + 8•f •C ⎝⎜ O OUT ⎠ ⎟ Higher value, lower cost ceramic capacitors are now be- coming available in smaller case sizes. These are tempting where fO = operating frequency, COUT = output capacitance for switching regulator use because of their very low ESR. and ΔIL = ripple current in the inductor. The output ripple Unfortunately, the ESR is so low that it can cause loop is highest at maximum input voltage since ΔIL increases stability problems. Solid tantalum capacitor ESR generates with input voltage. With ΔIL = 0.3 • IOUT(MAX) the output a loop zero at 5kHz to 50kHz that is instrumental in giving ripple will be less than 100mV at maximum VIN and acceptable loop phase margin. Ceramic capacitors remain fO = 2.25MHz with: capacitive to beyond 300kHz and usually resonate with their ESRCOUT < 150mΩ ESL before ESR becomes effective. Also, ceramic caps are prone to temperature effects which requires the designer Once the ESR requirements for COUT have been met, the to check loop stability over the operating temperature RMS current rating generally far exceeds the IRIPPLE(P-P) range. To minimize their large temperature and voltage requirement, except for an all ceramic solution. coeffi cients, only X5R or X7R ceramic capacitors should In surface mount applications, multiple capacitors may be used. A good selection of ceramic capacitors is available have to be paralleled to meet the capacitance, ESR or from Taiyo Yuden, AVX, Kemet, TDK and Murata. 35482fb 9