AN017
7
Input/Output Capacitors
The input capacitor C2B is a AVX TAZ series 4.7µF tantalum capacitor and it was chosen
due to its low ESR, and effective low frequency filtering; see BOM for specific part number.
The input ripple current for a buck converter is high, typically I
OUT
/2. Tantalum capacitors
become resistive at higher frequencies, requiring careful ripple-rating selection to prevent
excessive heating. Measure the capacitor case rise above ambient in the worst case thermal
environment of the application, and if it exceeds 10°C, increase the voltage rating or lower
the ESR rating. Ceramic capacitors’ ESL (effective series inductance) tends to dominate
their ESR, making them less susceptible to ripple-induced heating. Ceramic capacitors filter
high frequencies well, and C1A and B were chosen for that purpose.
The output capacitors C5A and B are AVX TAZ series 47uF tantalum capacitors; see BOM
for specific part number. AVX TAZ series capacitors were chosen to provide a design
starting point using high reliability MIL-PFR-55365/4 qualified capacitors. Ceramic
capacitance is not recommended as the main output capacitor, since loop stability relies on a
resistive characteristic at higher frequencies to form a zero. At switching frequencies, ripple
voltage is more a function of ESR than of absolute capacitance value. If lower output ripple
voltage is required, reduce the ESR by choosing a different capacitor or placing more
capacitors in parallel. For very low ripple, an additional LC filter in the output may be a
more suitable solution. Re-compensation of the loop may be required if the output
capacitance is altered. The output contains very narrow voltage spikes caused by the
parasitic inductance of C5. Ceramic capacitors C6A and B remove these spikes on the demo
board. In application, trace impedance and local bypass capacitors will perform this
function.
Catch Diode CR1 and L1
Use diodes designed for switching applications, with adequate current rating and fast turn-
on times, such as Schottky or ultrafast diodes. The parameters of interest are forward
voltage, maximum reverse voltage, reverse leakage current, average operating current, and
peak current. Lower forward voltage yields higher circuit efficiency and lowers power
dissipation in the diode. The reverse voltage rating must be greater than the input voltage.
Average diode current is always less than output current, but under a shorted output
condition, diode current can equal the switch current limit. If the application must withstand
this condition, the diode must be rated for maximum switch current. There are a number of
tradeoffs to consider when selecting an inductor for your application. The inductance value
determines the peak to peak ripple current under various operating conditions. A common
starting point for the peak to peak current ripple is 20% of the load current. The following
equation determines an inductor value based on desired ripple current and circuit
parameters.
