Datasheet

ADP1828
Rev. C | Page 19 of 36
APPLICATION INFORMATION
SELECTING THE INPUT CAPACITOR
The input current to a buck converter is a pulse waveform. It is
zero when the high-side switch is off and approximately equal
to the load current when it is on. The input capacitor carries the
input ripple current, allowing the input power source to supply
only the dc current. The input capacitor needs sufficient ripple
current rating to handle the input ripple as well as an ESR that
is low enough to mitigate input voltage ripple. For the usual
current ranges for these converters, it is good practice to use
two parallel capacitors placed close to the drains of the high-
side switch MOSFETs (one bulk capacitor of sufficiently high
current rating as calculated in Equation 2 along with a 10 F
ceramic capacitor).
Select an input bulk capacitor based on its ripple current rating.
First, determine the duty cycle of the output with the larger load
current:
IN
OUT
V
V
D =
(1)
The input capacitor ripple current is approximately
)1( DDII
LRIPPLE
−≈
(2)
where:
I
L
is the maximum inductor or load current.
D is the duty cycle.
OUTPUT LC FILTER
The output LC filter smoothes the switched voltage at SW, making
the dc output voltage. Choose the output LC filter to achieve the
desired output ripple voltage. Because the output LC filter is
part of the regulator negative-feedback control loop, the choice
of the output LC filter components affects the regulation control
loop stability.
Choose an inductor value such that the inductor ripple current
is approximately 1/3 of the maximum dc output load current.
Using a larger value inductor results in a physical size larger
than required and using a smaller value results in increased
losses in the inductor and/or MOSFET switches.
Choose the inductor value by the following equation:
⎥
⎦
⎤
⎢
⎣
⎡
−
Δ×
=
IN
OUT
OUT
L
SW
V
V
V
If
L 1
1
(3)
where:
L is the inductor value.
f
SW
is the switching frequency.
V
OUT
is the output voltage.
V
IN
is the input voltage.
ΔI
L
is the inductor ripple current, typically 1/3 of the maximum
dc load current.
Choose the output bulk capacitor to set the desired output
voltage ripple. The impedance of the output capacitor at the
switching frequency multiplied by the ripple current gives
the output voltage ripple. The impedance is made up of the
capacitive impedance plus the nonideal parasitic characteristics,
including the equivalent series resistance (ESR) and the equiva-
lent series inductance (ESL). The output voltage ripple can be
approximated with:
2
2
2
)4(
8
1
ESLf
Cf
ESRIV
SW
OUT
SW
L
OUT
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+Δ=Δ (4)
where:
ΔV
OUT
is the output ripple voltage.
ΔI
L
is the inductor ripple current.
ESR is the equivalent series resistance of the output capacitor
(or the parallel combination of ESR of all output capacitors).
ESL is the equivalent series inductance of the output capacitor
(or the parallel combination of ESL of all capacitors).
Note that the factors of 8 and 4 in Equation 4 would normally
be 2π for sinusoidal waveforms, but the ripple current wave-
form in this application is triangular. Parallel combinations
of different types of capacitors, for example, a large aluminum
electrolytic in parallel with MLCCs, may give different results.
Usually the impedance is dominated by ESR at the switching
frequency, as stated in the maximum ESR rating on the capaci-
tor data sheet, so this equation reduces to
∆V
OUT
≅
∆I
L
ESR (5)
Electrolytic capacitors have significant ESL also, on the order
of 5 nH to 20 nH, depending on type, size, and geometry, and
PCB traces contribute some ESR and ESL as well. However,
using the maximum ESR rating from the capacitor data sheet
usually provides some margin such that measuring the ESL is
not usually required.
In the case of output capacitors, the impedance of the ESR and
ESL at the switching frequency are small, for instance, where
the effective output capacitor is a bank of parallel MLCC capa-
citors, the capacitive impedance dominates and the ripple
equation reduces to
SW
OUT
L
OUT
fC
I
V
8
Δ
≅Δ
(6)
Make sure that the ripple current rating of the output capacitors
is greater than the maximum inductor ripple current.