Diode selection, Output filter capacitor selection, Input capacitor selection – Rainbow Electronics MAX15031 User Manual
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MAX15031
Diode Selection
The MAX15031’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommend-
ed for most applications because of their fast recovery
time and low forward-voltage drop. Ensure that the
diode’s peak current rating is greater than the peak
inductor current. Also the diode reverse-breakdown
voltage must be greater than V
OUT
. The output voltage
of the boost converter.
Output Filter Capacitor Selection
For most applications, use a small output capacitor of
0.1µF or greater. To achieve low output ripple, a capaci-
tor with low ESR, low ESL, and high capacitance value
should be selected. If tantalum or electrolytic capacitors
are used to achieve high capacitance values, always
add a smaller ceramic capacitor in parallel to bypass
the high-frequency components of the diode current.
The higher ESR and ESL of electrolytic capacitors
increase the output ripple and peak-to-peak transient
voltage. Assuming the contribution from the ESR and
capacitor discharge equals 50% (proportions may vary),
calculate the output capacitance and ESR required for a
specified ripple using the following equations:
For very low output ripple applications, the output of the
boost converter can be followed by an RC filter to further
reduce the ripple. Figure 2 shows a 100
Ω (R
F
), 0.1µF
(C
F
) filter used to reduce the switching output ripple to
1mV
P-P
with a 0.1mA load or 2mV
P-P
with a 4mA load.
The output-voltage regulation resistor-divider must remain
connected to the diode and output capacitor node.
Use X7R ceramic capacitors for more stability over the full
temperature range. Use an X5R capacitor for -40°C to
+85°C applications.
Input Capacitor Selection
Bypass PWR to PGND with a 1µF (min) ceramic capaci-
tor and bypass IN to PGND with a 1µF (min) ceramic
capacitor. Depending on the supply source imped-
ance, higher values may be needed. Make sure that the
input capacitors are close enough to the IC to provide
adequate decoupling at IN and PWR as well. If the lay-
out cannot achieve this, add another 0.1µF ceramic
capacitor between IN and PGND (or PWR and PGND)
in the immediate vicinity of the IC. Bulk aluminum elec-
trolytic capacitors may be needed to avoid chattering
at low input voltage. In case of aluminum electrolytic
capacitors, calculate the capacitor value and ESR of
the input capacitor using the following equations:
C
V
x I
x V
x 0.5 x
IN
OUT
OUT
IN_MIN
IN
[
]
µ
η
F
V
=
∆
T
T
I
x L
x V
V
(V
V
S
LPEAK
OPTIMUM
OUT
IN_MIN
OUT
I
−
−
N
N_MIN
IN
IN_M
)
V
x V
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
[ ]
=
ESR m
0.5 x
Ω
∆
x
η
IIN
OUT
V
x I
OUT
C
I
0.5 x
T
I
x L
OUT
OUT
OUT
S
LPEAK
OPTIM
[
]
µF
V
=
−
∆
U
UM
OUT
IN_MIN
(V
V
)
−
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
[ ]
=
ESR m
0.5 x
I
OUT
Ω
∆V
O
OUT
MAX15031
PWR
CNTRL
SHDN
PGND
CP
C
CP
C
OUT1
C
F
0.1
µF
C
PWR
CN
LX
FB
D1
BIAS
SGND
IN
V
IN
= 2.7V TO 5.5V
V
OUT
L1
C
IN
R
2
R
1
R
F
100
Ω
Figure 2. Typical Operating Circuit with RC Filter
80V, 300mW Boost Converter and Current
Monitor for APD Bias Applications
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