Datasheet LTC3407-3 (Analog Devices) - 9

LTC3407-3
APPLICATIONS INFORMATION Output Capacitor (COUT) Selection
ceramic capacitors along with trace inductance can lead The selection of C to signifi cant ringing. OUT is driven by the required ESR to 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 LTC3407-3 in parallel with the is adequate for fi ltering. The output ripple (ΔVOUT) is main capacitors for high frequency decoupling. determined by: 1 VOUT IL ES R + VIN 8f O COUT 10μF 100k RUN2 VIN RUN1 where f = operating frequency, COUT = output capacitance MODE/SYNC POR RESET and ΔIL = ripple current in the inductor. The output ripple L2 LTC3407-3 μH L1 μH is highest at maximum input voltage since ΔIL increases VOUT2 SW2 SW1 VOUT1 with input voltage. With ΔIL = 0.3 • ILIM the output ripple V V OUT2 OUT1 will be less than 100mV at maximum V COUT2 COUT1 IN and fO = 2.25MHz GND with: 34073 F02 ESRCOUT < 150mΩ Once the ESR requirements for C
Figure 2. LTC3407-3 General Schematic
OUT have been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement, except for an all ceramic solution.
Ceramic Input and Output Capacitors
In surface mount applications, multiple capacitors may Higher value, lower cost ceramic capacitors are now be- have to be paralleled to meet the capacitance, ESR or coming available in smaller case sizes. These are tempting RMS current handling requirement of the application. for switching regulator use because of their very low ESR. Aluminum electrolytic, special polymer, ceramic and dry Unfortunately, the ESR is so low that it can cause loop tantulum capacitors are all available in surface mount stability problems. Solid tantalum capacitor ESR generates packages. The OS-CON semiconductor dielectric capacitor a loop “zero” at 5kHz to 50kHz that is instrumental in giving available from Sanyo has the lowest ESR(size) product acceptable loop phase margin. Ceramic capacitors remain of any aluminum electrolytic at a somewhat higher price. capacitive to beyond 300kHz and usually resonate with their Special polymer capacitors, such as Sanyo POSCAP, ESL before ESR becomes effective. Also, ceramic caps are Panasonic Special Polymer (SP), and Kemet A700, of- prone to temperature effects which requires the designer fer very low ESR, but have a lower capacitance density to check loop stability over the operating temperature than other types. Tantalum capacitors have the highest range. To minimize their large temperature and voltage capacitance density, but they have a larger ESR and it coeffi cients, only X5R or X7R ceramic capacitors should is critical that the capacitors are surge tested for use in be used. A good selection of ceramic capacitors is available switching power supplies. An excellent choice is the AVX from Taiyo Yuden, AVX, Kemet, TDK, and Murata. TPS series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. Aluminum electrolytic Great care must be taken when using only ceramic input capacitors have a signifi cantly larger ESR, and are often and output capacitors. When a ceramic capacitor is used used in extremely cost-sensitive applications provided that at the input and the power is being supplied through long consideration is given to ripple current ratings and long wires, such as from a wall adapter, a load step at the output term reliability. Ceramic capacitors have the lowest ESR can induce ringing at the VIN pin. At best, this ringing can and cost, but also have the lowest capacitance density, couple to the output and be mistaken as loop instability. a high voltage and temperature coeffi cient, and exhibit At worst, the ringing at the input can be large enough to audible piezoelectric effects. In addition, the high Q of damage the part. 34073fb 9