Rainbow Electronics MAX1678 - Manual (Page 10)

MAX1678
Applications Information
Output Voltage Selection
The MAX1678 operates with a fixed 3.3V or adjustable
output. To select fixed-voltage operation, connect FB to
GND (Figure 7). For an adjustable output between 2V
and 5.5V, connect FB to a resistor voltage-divider
between OUT and GND (Figure 8). FB regulates to
1.23V.
Since FB leakage is 10nA max, select feedback resistor
R2 in the 100k
Ω
to 1M
Ω
range. R1 is given by:
where V REF = 1.23V.
Maximum Output Current
and Inductor Selection
The MAX1678 is designed to work well with a 47µH
inductor in most low-power applications. 47µH is a suf-
ficiently low value to allow the use of a small surface-
mount coil, but large enough to maintain low ripple. The
Typical Operating Characteristics section shows perfor-
mance curves with several 47µH and 22µH coils. Low
inductance values supply higher output current but
also increase ripple and reduce efficiency. Note that
values below 22µH are not recommended due to
MAX1678 switch limitations. Higher inductor values
reduce peak inductor current (and consequent ripple
and noise) and improve efficiency, but also limit output
current.
The relationship between current and inductor value is
approximately:
where M is an empirical factor that takes into account
losses in the MAX1678 internal switches and in the
inductor resistance. K is the V-µs factor that governs
the inductor charge time. Nominally, M = 0.9 and
K = 8V-µs. M should be further reduced by 0.1 for each
ohm of inductor resistance.
The inductor’s saturation-current rating must exceed
the worst-case peak current limit set by the MAX1678’s
timing algorithm:
where K MAX = 11.2V-µs. It is usually acceptable to
exceed most coil saturation-current ratings by 20% with
no ill effects; however, the maximum recommended I
PEAK
for the MAX1678 internal switches is 550mA, so inductor
values below 22µH are not recommended. For optimum
efficiency, inductor series resistance should be less than
150mV/I
PEAK . Table 1 lists suggested inductors and sup-
pliers.
Table 1. Suggested Inductors and Suppliers
I
K
L
PEAK
MAX =
I M x x
K
L
x
V
V
OUT MAX
BATT
OUT
( )
=
1
2
R1 = R2 x
V
V
OUT
REF
−





 1
1-Cell to 2-Cell, Low-Noise,
High-Efficiency, Step-Up DC-DC Converter
10 ______________________________________________________________________________________
MAX1678
GND FB
BATT
PFI 3.3V
OUT
INPUT
0.87V TO V
OUT
L1
47 µ H, 200mA
PFO
C2
10
µ F
SHDN
LX
OUT
OUT
C1
10 µ F
R3
R4 R5
Figure 7. 3.3V Standard Application Circuit Figure 8. Adjustable Output Circuit
MAX1678
GND
BATT
PFI
V OUT = 2V
TO 5.5V
INPUT
0.87V TO V
OUT
L1
47 µ H
PFO
SHDN
LX
OUT
FB
R1 C2
R2
OUT
C1
10 µ F
R4 R5
R3
LQH4N470K,
LQH3C470K
Murata
NLC453232T-220K,
NLC453232T-470K
TDK
CD43-220,
CD43-470
Sumida
PIN
DS1608C-223,
DS1608C-473
Coilcraft
INDUCTOR
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