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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,B and C are AVX TAZ series 220uF tantalum capacitors.
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 less expensive solution. Re-compensation of the loop may be required if the output
capacitance is altered. The output contains very narrow voltage spikes because of the
parasitic inductance of C5. Ceramic capacitors C6A and B remove these spikes on the
demo board. In application, trace inductance 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, reverse recovery, 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 a coil 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 equation below shows how to select and inductor value
based on desired ripple current and circuit parameters.
L = D*(Vin – Vout)/(f
sw
* I
pp
)
Given:
D = Duty cycle, approximately Vout/Vin
I
pp
= Peak to peak ripple current, typically 0.2 * Iout DC
f
sw
= Switching frequency in Hz
L = Inductor value in Henries
