ADP1828LC-EVALZ Analog Devices Inc, ADP1828LC-EVALZ Datasheet - Page 19

• Manufacturer Part Number: ADP1828LC-EVALZ
• Description: BOARD EVALUATION ADP1828LC
• Manufacturer: Analog Devices Inc

Specifications of ADP1828LC-EVALZ

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.8V
• Current - Output: 5A
• Voltage - Input: 5.5 ~ 13.2V
• Regulator Topology: Buck
• Frequency - Switching: 600kHz
• Board Type: Fully Populated
• Utilized Ic / Part: ADP1828
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

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 μF
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:
The input capacitor ripple current is approximately
where:
I
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:
where:
L is the inductor value.
f
V
V
ΔI
dc load current.
SW
L
OUT
IN
is the maximum inductor or load current.
L
is the switching frequency.
is the inductor ripple current, typically 1/3 of the maximum
is the input voltage.
D =
I
L
is the output voltage.
RIPPLE
=
V
f
V
SW
OUT
IN
≈
×
1
I
L
Δ
I
D
L
1 ( D
V
OUT
−
⎡
⎢
⎣
)
1
−
V
V
OUT
IN
⎤
⎥
⎦
Rev. C | Page 19 of 36
(1)
(2)
(3)
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:
where:
ΔV
ΔI
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
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
Make sure that the ripple current rating of the output capacitors
is greater than the maximum inductor ripple current.
Δ
V
L
OUT
OUT
is the inductor ripple current.
ΔV
Δ
is the output ripple voltage.
V
=
OUT
OUT
Δ
I
≅ ΔI
L
≅
8
ESR
C
L
OUT
ESR
Δ
2
I
L
+
f
SW
⎛
⎜
⎜
⎝
8
f
SW
1
C
OUT
⎞
⎟
⎟
⎠
2
+
4 (
f
SW
ESL
ADP1828
)
2
(4)
(5)
(6)