MAX1678EUA Maxim Integrated Products, MAX1678EUA Datasheet - Page 10

• Manufacturer Part Number: MAX1678EUA
• Description: DC/DC Switching Converters
• Manufacturer: Maxim Integrated Products

Specifications of MAX1678EUA

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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 100kΩ to 1MΩ range. R1 is given by:
where V
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:
1-Cell to 2-Cell, Low-Noise,
High-Efficiency, Step-Up DC-DC Converter
Figure 7. 3.3V Standard Application Circuit
10
10 F
INPUT
0.87V TO V
C1
______________________________________________________________________________________
REF
OUT
= 1.23V.
R3
R4
R1 = R2 x
Applications Information
R5
OUT
Output Voltage Selection
Maximum Output Current
and Inductor Selection
BATT
PFI
PFO
SHDN
GND
V
V
47 H, 200mA
OUT
MAX1678
REF
L1
FB
OUT
1
LX
C2
10 F
3.3V
OUT
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
exceed most coil saturation-current ratings by 20% with
no ill effects; however, the maximum recommended I
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
pliers.
Table 1. Suggested Inductors and Suppliers
Figure 8. Adjustable Output Circuit
10 F
Coilcraft
Murata
Sumida
TDK
C1
INPUT
0.87V TO V
PIN
PEAK
I
MAX
OUT MAX
OUT
R3
R4
. Table 1 lists suggested inductors and sup-
(
= 11.2V-µs. It is usually acceptable to
R5
DS1608C-223,
DS1608C-473
LQH4N470K,
LQH3C470K
CD43-220,
CD43-470
NLC453232T-220K,
NLC453232T-470K
)
OUT
I
PEAK
INDUCTOR
M x
BATT
PFI
PFO
SHDN
MAX1678
GND
47 H
1
2
L1
K
MAX
x
L
OUT
K
LX
FB
L
x
[phone]
[phone]
[phone]
[phone]
V
V
BATT
OUT
PHONE
R1
R2
V
TO 5.5V
OUT
C2
PEAK
= 2V