Table 1. inductor vendors – Rainbow Electronics MAX15032 User Manual
Page 8
MAX15032
Determining Peak Inductor Current
If the boost converter remains in the discontinuous
mode of operation, then the approximate peak inductor
current, I
LPEAK
(A), is represented by the formula
below:
where T
S
is the period in µs, V
OUT
is the output voltage in
volts, V
IN_MIN
is the minimum input voltage in volts, I
OUT
is the output current in amperes, L is the inductor value in
µH, and
η is the efficiency of the boost converter.
Determining the Inductor Value
Three key inductor parameters must be specified for
operation with the MAX15032: inductance value (L),
inductor saturation current (I
SAT
), and DC resistance
(DCR). In general, the inductor should have a saturation
current rating greater than the maximum switch peak
current-limit value (I
LIM-LX(MAX)
= 1.7A). DC series
resistance (DCR) should be below 0.1
Ω for reasonable
efficiency. Due to the high switching frequency of the
MAX15032, inductors with a ferrite core or equivalent
are recommended to minimize core losses. Table 1
shows a list of vendors with 4.7µH inductor parts.
Table 1. Inductor Vendors
Use the following formula to calculate the lower bound
of the inductor value at different output voltages and
output currents. This is the minimum inductance value
for discontinuous mode operation for supplying the full
600mW output power:
where V
IN
(V), V
OUT
(V), and I
OUT
(A) are typical val-
ues, T
S
(µs) is the period,
η is the efficiency, and
I
LIM-LX
is the peak LX current (A).
Calculate the optimum value of L (L
OPTIMUM
) to ensure
the full output power without reaching the boundary
between continuous conduction mode (CCM) and DCM
using the following formula:
where:
For a design in which V
IN
= 3.3V, V
OUT
= 30V,
I
OUT
= 20mA,
η = 0.7, and T
S
= 2µs, (L
OPTIMUM
=
4.7µH):
L
MAX
= 10.5µH
and
L
MIN
= 3.3µH
For a worst-case scenario in which V
IN
= 2.9V, V
OUT
=
30V, I
OUT
= 20mA,
η = 0.7, I
LIM-LX(MIN)
= 1A, and T
S
=
1.8µs:
L
MAX
= 9.2µH
and:
L
MIN
= 2.2µH
The choice of 4.7µH is reasonable given the worst-case
scenario above. In general, the higher the inductance,
the lower the switching noise.
Diode Selection
The MAX15032’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 inductor
peak current. Also, the diode reverse breakdown volt-
age must be greater than V
OUT
.
Output Filter Capacitor Selection
For most applications, use a small ceramic surface-mount
output capacitor, 2.2µF or greater. To achieve low output
ripple, a capacitor with low-ESR, low-ESL, and high-
capacitance value should be selected. If tantalum or
electrolytic capacitors are used to achieve high capaci-
tance values, always add a small ceramic in parallel to
bypass the high-frequency components of the diode cur-
rent. The higher ESR and ESL of electrolytic increase both
the output ripple and peak-to-peak transient voltage.
Assuming the contribution from the ESR and capacitor
L
H
V
V
V
Ts
I
V
MAX
IN MIN
OUT
IN MIN
OUT
OUT
[
]
(
)
_
_
μ
η
=
Ч
Ч
Ч
Ч
−
2
2
2
L
L
H
OPTIMUM
MAX
=
[
]
.
μ
2 25
L
H
T
I
V
V
I
MIN
S
OUT
OUT
IN MIN
LIM LX
[
]
(
)
_
μ
η
=
Ч
Ч
Ч
Ч
−
−
2
2
I
T
V
V
I
L
LPEAK
S
OUT
IN MIN
OUT
=
Ч
Ч
Ч
Ч
−
2
(
)
_
η
500kHz, 36V Output, 600mW PWM
Step-Up DC-DC Converter
8
_______________________________________________________________________________________
VENDOR
PHONE
FAX
PART NUMBER
OF 4.7µH
INDUCTOR
TDK
408-437-9585 408-437-9591
SLF7045T-
4R7M2R0-PF
TOKO
847-297-0070 847-699-7864 636CY-4R7M+P3
Coilcraft
800-322-2645 847-639-1469 MOS6020-472MLC