Rainbow Electronics MAX8715 - Manual (Page 10)

MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
10 ______________________________________________________________________________________
components for a range of standard applications. Table
2 lists component suppliers.
External component value choice is primarily dictated
by the output voltage and the maximum load current,
as well as maximum and minimum input voltages.
Begin by selecting an inductor value. Once L is known,
choose the diode and capacitors.
Inductor Selection
The minimum inductance value, peak current rating, and
series resistance are factors to consider when selecting
the inductor. These factors influence the converter’s effi-
ciency, maximum output load capability, transient-
response time, and output voltage ripple. Physical size
and cost are also important factors to be considered.
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance values minimize the current
ripple and therefore reduce the peak current, which
decreases core losses in the inductor and I
2 R losses in
the entire power path. However, large inductor values
also require more energy storage and more turns of wire,
which increase physical size and can increase I
2 R loss-
es in the inductor. Low inductance values decrease the
physical size but increase the current ripple and peak
current. Finding the best inductor involves choosing the
best compromise between circuit efficiency, inductor
size, and cost.
The equations used here include a constant LIR, which
is the ratio of the inductor peak-to-peak ripple current to
the average DC inductor current at the full load current.
The best trade-off between inductor size and circuit
efficiency for step-up regulators generally has an LIR
between 0.3 and 0.5. However, depending on the AC
characteristics of the inductor core material and the
Table 1. Component Selection
Table 2. Component Suppliers
847-639-6400
561-241-7876
847-956-0666
PHONE
847-639-1469 Coilcraft
561-241-9339 Coiltronics
847-956-0702 Sumida USA
FAX SUPPLIER
803-946-0690
408-986-0424
619-661-6835
847-297-0070
803-626-3123 AVX
408-986-1442 Kemet
619-661-1055 Sanyo
847-699-1194 TOKO
516-435-1110
310-322-3331
516-543-7100
602-303-5454
408-573-4150
847-843-7500
516-864-7630 Zetex
847-843-2798 Nihon
516-435-1824
Central
Semiconductor
310-322-3332
International
Rectifier
602-994-6430 Motorola
408-573-4159 Taiyo Yuden
Inductors
Capacitors
Diodes
V IN (V)
V OUT
(V)
f OSC
(Hz)
L (µH) C OUT (µF)
R COMP
(k Ω )
C COMP
(pF)
C COMP2
(pF)
I OUT(MAX)
(mA)
MAX1790
3.3 12 640k
10 (Sumida
CDRH5D18-100NC)
33 tantalum (AVX
TPSD336020R0200)
120 1200 22 250
3.3 12 1.2M
5.4 (Sumida
CDRH5D18-5R4NC)
33 tantalum (AVX
TPSD336020R0200)
180 650 20 250
3.3 5 640k
5.4 (Sumida
CDRH5D18-5R4NC)
47 tantalum
(6TPA47M)
62 820 56 800
3.3 5 1.2M
2.7 (Sumida
CDRH4D18-2R7)
47 tantalum
(6TPA47M)
91 390 33 800
MAX8715
3.3 9 1.2M
6.8 (Sumida
CLQ4D10-6R8)
3 x 3.3 ceramic
(Taiyo Yuden
LMK325BJ335MD)
82 750 10 150