EVAL-ADP1829_15 Datasheet, PDF (3/16 Pages) Analog Devices – Evaluation Board for Dual, Interleaved

COMPONENT DESIGN
INPUT CAPACITOR
The input capacitor carries the input ripple current, allowing
the input power source to supply only the dc current. Select the
input bulk capacitor based on its ripple current rating. The two
channels in the ADP1829 operate 180° out of phase, thus
reducing the current rating on the input capacitor.
If the maximum output load currents are about the same, the
input ripple current for both Channel 1 and Channel 2 is less
than half of the higher of the output load currents. The input
capacitor current is approximated as
Iin_Ripple
≈
I_L
2
(1)
where IL is the current though the inductor.
If the load currents of the two channels are significantly different
(the smaller is less than 50% of the larger), in this case, if the
duty cycle D is between 20% and 80%, the input capacitor ripple
current is approximately IL√D(1 − D).
If duty cycle D is less than 20% or greater than 80%, the ripple
current is approximately 0.4IL.
INDUCTOR SELECTION
The choice of inductance determines the ripple current in the
inductor. Less inductance leads to more ripple current, which
increases the output voltage ripple and conduction losses in the
MOSFETs, but allows using smaller inductors and less output
capacitance for a specified peak-to-peak voltage overshoot at
load transient. Generally, choose an inductor value such that the
inductor ripple current is approximately 1/3 of the maximum dc
output current. Use the following equation to calculate the
inductor value:
L = ^{VOUT (1 − D)}
(2)
ΔI_L f_SW
where:
L is the inductor value.
fSW is the switching frequency.
VOUT is the output voltage.
D is the duty cycle.
ΔIL is the inductor ripple current, typically 1/3 of the dc load.
EVAL-ADP1829
OUTPUT CAPACITOR SELECTION
Choose the output capacitor to set the desired output voltage
ripple. The output voltage ripple is a function of the inductor
ripple current and the capacitor impedance at the switching
frequency. The output voltage ripple can be approximated as
ΔV_OUT
=
ΔI _L
⎜⎛
⎜⎝
ESR
+
1
8 f _SW C_OUT
⎟⎞
⎟⎠
(3)
For high ESR capacitors, the ripple is dominated by the ESR,
while for low ESR capacitors, the output ripple is dominated by
the capacitor. ESL of the capacitor also affects the output ripple,
especially the though-hole electrolytic capacitors. In practical
designs, multiple types of capacitors are used. For instance, a
MLCC (multilayer ceramic capacitor) can be paralleled with an
electrolytic capacitor to reduce the ESL and ESR.
Another factor that should be considered is the load-step
transient response on the output, where the output capacitor
supplies the load until the control loop has a chance to ramp the
inductor current. A minimum capacitance at the output is needed
in order to have a fast load-step response and reasonable overshoot
voltage. The minimum capacitance can be calculated as
C_{OUT, min1}
=
ΔI_OUT ² L
2V_OUT ΔV_up
(4)
C_{OUT, min2}
=
2(V_IN
ΔI_OUT ²L
− V_{O UT}) ΔV_down
(5)
where:
ΔIO is the step load.
ΔVup is the output voltage overshoot when the load is
stepped down.
ΔVdown is the output voltage overshoot when the load is
stepped up.
VIN is the input voltage.
COUT,min1 is the minimum capacitance according to the overshoot
voltage ΔVup.
COUT,min2 is the minimum capacitance according to the overshoot
voltage ΔVdown.
Select an output capacitance that is greater than both COUT, min1
and COUT, . min2
Make sure that the ripple current rating of the output capacitors
is greater than the following current:
I _COUT =
ΔI _L ²
12
(6)
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