Datasheet

EVAL-ADP1829
Rev. 0 | Page 3 of 16
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
2
L
Ripple
I
Iin ≈
(1)
where I
L
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 I
L
√D(1 − D).
If duty cycle D is less than 20% or greater than 80%, the ripple
current is approximately 0.4I
L
.
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:
SW
L
UT
O
fI
DV
L
Δ
−
=
)1(
(2)
where:
L is the inductor value.
f
SW
is the switching frequency.
V
OUT
is the output voltage.
D is the duty cycle.
I
L
is the inductor ripple current, typically 1/3 of the dc load.
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
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+Δ=Δ
OUT
SW
L
OUT
Cf
ESRIV
8
1
(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
up
UT
O
OUT
VV
LI
C
Δ
Δ
=
2
2
min1OUT,
(4)
down
UTO
IN
OUT
VVV
LI
C
Δ−
Δ
=
)(2
2
min2OUT,
(5)
where:
I
O
is the step load.
V
up
is the output voltage overshoot when the load is
stepped down.
V
down
is the output voltage overshoot when the load is
stepped up.
V
IN
is the input voltage.
C
OUT,min1
is the minimum capacitance according to the overshoot
voltage V
up
.
C
OUT,min2
is the minimum capacitance according to the overshoot
voltage V
down
.
Select an output capacitance that is greater than both C
OUT, min1
and C
OUT, min2
.
Make sure that the ripple current rating of the output capacitors
is greater than the following current:
12
2
L
COUT
I
I
Δ
=
(6)