Datasheet LTC3407A (Analog Devices) - 9

LTC3407A
APPLICATIONS INFORMATION
This formula has a maximum at VIN = 2VOUT, where IRMS capacitance density. However, they also have a larger = IOUT/2. This simple worst-case is commonly used to ESR and it is critical that they are surge tested for use in design because even signifi cant deviations do not offer switching power supplies. An excellent choice is the AVX much relief. Note that capacitor manufacturer’s ripple cur- TPS series of surface mount tantalums, available in case rent ratings are often based on only 2000 hours lifetime. heights ranging from 2mm to 4mm. Aluminum electrolytic This makes it advisable to further derate the capacitor, capacitors have a signifi cantly larger ESR, and are often or choose a capacitor rated at a higher temperature than used in extremely cost-sensitive applications provided that required. Several capacitors may also be paralleled to meet consideration is given to ripple current ratings and long the size or height requirements of the design. An additional term reliability. Ceramic capacitors have the lowest ESR 0.1μF to 1μF ceramic capacitor is also recommended on and cost, but also have the lowest capacitance density, VIN for high frequency decoupling, when not using an all a high voltage and temperature coeffi cient, and exhibit ceramic capacitor solution. audible piezoelectric effects. In addition, the high Q of ceramic capacitors along with trace inductance can lead
Output Capacitor (COUT) Selection
to signifi cant ringing. Other capacitor types include the The selection of COUT is driven by the required ESR to Panasonic Special Polymer (SP) capacitors. 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 LTC3407A in parallel with the is adequate for fi ltering. The output ripple (ΔVOUT) is main capacitors for high frequency decoupling. determined by:
Ceramic Input and Output Capacitors
VOUT IL ESR+ 1 8f Higher value, lower cost ceramic capacitors are now be- O COUT 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 gener- with input voltage. With ΔIL = 0.3 • ILIM the output ripple ates a loop “zero” at 5kHz to 50kHz that is instrumental in will be less than 100mV at maximum VIN and fO = 1.5MHz giving acceptable loop phase margin. Ceramic capacitors with: remain capacitive to beyond 300kHz and usually resonate ESR with their ESL before ESR becomes effective. Also, ceramic COUT < 150mΩ caps are prone to temperature effects which requires the Once the ESR requirements for COUT have been met, the designer to check loop stability over the operating tem- RMS current rating generally far exceeds the IRIPPLE(P-P) perature range. To minimize their large temperature and requirement, except for an all ceramic solution. voltage coeffi cients, only X5R or X7R ceramic capacitors In surface mount applications, multiple capacitors may should be used. A good selection of ceramic capacitors have to be paralleled to meet the capacitance, ESR or is available from Taiyo Yuden, TDK, and Murata. RMS current handling requirement of the application. Great care must be taken when using only ceramic input Aluminum electrolytic, special polymer, ceramic and dry and output capacitors. When a ceramic capacitor is used tantulum capacitors are all available in surface mount at the input and the power is being supplied through long packages. The OS-CON semiconductor dielectric capacitor wires, such as from a wall adapter, a load step at the output available from Sanyo has the lowest ESR(size) product can induce ringing at the VIN pin. At best, this ringing can of any aluminum electrolytic at a somewhat higher price. couple to the output and be mistaken as loop instability. Special polymer capacitors, such as Sanyo POSCAP, of- fer very low ESR, but have a lower capacitance density At worst, the ringing at the input can be large enough to than other types. Tantalum capacitors have the highest damage the part. 3407afa 9