Applications information, Input capacitor selection, Layout considerations – Rainbow Electronics MAX15032 User Manual
Page 9
discharge equals 50% (proportions could vary), calcu-
late the output capacitance and ESR required for a spec-
ified 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 fur-
ther reduce the ripple. Figure 1 shows a 10
Ω, 2.2µF fil-
ter used to reduce the switching output ripple to
1mV
P-P
with a 20mA output and a ripple voltage of
400µV
P-P
with a 2mA load. The output voltage regula-
tion resistive divider must remain connected to the
diode/output capacitor node.
X7R ceramic capacitors are stable over -40°C to
+125°C temperature range. Where the automotive tem-
perature range is required, use X7R ceramic capaci-
tors. X5R dielectric can be used for -40°C to +85°C
applications.
Input Capacitor Selection
Bypass IN (the input voltage pin) to PGND with a mini-
mum 4.7µF ceramic capacitor. Depending on the sup-
ply source impedance, higher values might be needed.
Make sure that the input capacitor is close enough to
the IC to provide adequate decoupling at IN as well. If
the layout cannot achieve this, add another 0.1µF
ceramic capacitor between IN and PGND in the imme-
diate vicinity of the IC. Bulk aluminum electrolytic
capacitors might be needed to avoid chattering at low
input voltages. In the case of aluminum electrolytic
capacitors, calculate the capacitor value and ESR of
the input capacitor using the following equations:
Applications Information
Layout Considerations
Careful PCB layout is critical to achieve clean and sta-
ble operation. Protect sensitive analog grounds by
using a star ground configuration. Connect GND and
PGND together close to the device at the return terminal
of the output bypass capacitor. Do not connect them
together anywhere else. Keep all PCB traces as short
as possible to reduce stray capacitance, trace resis-
tance, and radiated noise. Ensure that the feedback
connection to FB is short and direct. Route high-speed
switching nodes away from the sensitive analog areas.
Avoid any coupling from LX to FB node by keeping the
FB node away from the LX routing. In addition, decou-
pling LX and FB with a small 22pF capacitor from FB to
GND can be used. Use an internal PCB layer for GND
as an EMI shield to keep radiated noise away from the
device, feedback dividers, and bypass capacitors.
C
F
V
I
V
V
T
I
L
V
V
V
V
ESR m
V
V
V
I
IN
OUT
OUT
IN MIN
IN
S
LPEAK
OPTIMUM
OUT
IN MIN
OUT
IN MIN
IN
IN MIN
OUT
OUT
[
]
.
(
)
[
]
.
_
_
_
_
μ
η
η
=
Ч
Ч
Ч
Ч
Ч
Ч
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
=
Ч
Ч Ч
Ч
−
−
0 5
0 5
Δ
Ω
Δ
Δ
C
F
I
V
T
I
L
V
V
ESR m
V
I
OUT
OUT
OUT
S
LPEAK
OPTIMUM
OUT
IN MIN
OUT
OUT
[
]
.
(
)
[
]
.
_
μ =
Ч
Ч
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
=
Ч
−
−
0 5
0 5
Δ
Ω
Δ
MAX15032
500kHz, 36V Output, 600mW PWM
Step-Up DC-DC Converter
_______________________________________________________________________________________
9
Figure 1. Typical Operating Circuit with RC Filter
MAX15032
IN
V
IN
= 2.9V TO 5.5V
V
OUT
30V
SHDN
PGND
LX
FB
CP
CN
GND
R1
143k
Ω
D1
1A/40V
L1
4.7
μH
R2
6.2k
Ω
C
IN
10
μF
C
OUT
2.2
μF
C
CP
10nF
C
F
2.2
μF
R
F
10
Ω