MAX1674 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – High-Efficiency, Low-Supply-Current, Compact, Step-Up DC-DC Converters

High-Efficiency, Low-Supply-Current,
Compact, Step-Up DC-DC Converters
V_IN
R1
200Ω
R3
0.1µF
47µF
BATT
(MAX1676)
LX
OUT
CLSEL
(MAX1676)
LBI
FB
SHDN
MAX1674
MAX1675
REF ^MAX1676 _LBO
GND
22µH
0.1µF
R2
100k
LOW-
BATTERY
OUTPUT
V_OUT
47µF
R4
Figure 8. Setting Resistor Values for the Low-Battery Indicator
when V_IN < 1.3V
V_TRIP (V_H, V_L)
R3
R4
R7
MAX1674
MAX1675
MAX1676
LBI
OUT
R2
100k
0.1µF
LBO
GND
V_OUT
47µF
( ) VH =
1.3V
_{1 +}

R3 ₊
R7
R3 
R4


( ) VL =
1.3V

^{1 +}

R3 _−
R4
(V_OUT
(1.3V)
− 1.3V) R3
(R2 + R7)



WHERE V_H IS THE UPPER TRIP LEVEL
V_L IS THE LOWER TRIP LEVEL
Figure 9. Adding External Hysteresis to the Low-Battery
Indicator
LBI and LBO. For V_TRIP less than 1.3V, configure the
comparator as shown in Figure 8. Calculate the value of
the external resistors R3 and R4 as follows:
R3 = R4(V_REF - V_TRIP) / (V_OUT - V_REF)
Since the low-battery comparator is noninverting, exter-
nal hysteresis can be added by connecting a resistor
between LBO and LBI as shown in Figure 9. When LBO
is high, the series combination of R2 and R7 source
current into the LBI summing junction.
Applications Information
Inductor Selection
An inductor value of 22µH performs well in most appli-
cations. The MAX1674/MAX1675/MAX1676 will also
work with inductors in the 10µH to 47µH range. Smaller
inductance values typically offer a smaller physical size
for a given series resistance, allowing the smallest
overall circuit dimensions. However, due to higher peak
inductor currents, the output voltage ripple (I_PEAK x
output filter capacitor ESR) also tends to be higher.
Circuits using larger inductance values exhibit higher
output current capability and larger physical dimen-
sions for a given series resistance. The inductor’s incre-
mental saturation current rating should be greater than
the peak switch-current limit, which is 1A for the
MAX1674, 500mA for the MAX1675, and 1A or 0.5A for
the MAX1676. However, it is generally acceptable to
bias the inductor into saturation by as much as 20%,
although this will slightly reduce efficiency. Table 1 lists
suggested components.
The inductor’s DC resistance significantly affects effi-
ciency. See Table 2 for a comparison of inductor speci-
fications. Calculate the maximum output current as
follows:
( ) ^{IOUT MAX}
=
V_IN
V_OUT

^ILIM
–
t_OFF



V_OUT
2
–
x
V_IN
L




η

where I_OUT(MAX) = maximum output current in amps
V_IN = input voltage
L = inductor value in µH
η = efficiency (typically 0.9)
t_OFF = LX switch’s off-time in µs
I_LIM = 0.5A or 1.0A
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