MAX1674 Datasheet, PDF (9/12 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
Selecting the Output Voltage
V_OUT can be set to 3.3V or 5.0V by connecting the FB
pin to GND (5V) or OUT (3.3V) (Figures 5 and 6).
To adjust the output voltage, connect a resistor-divider
from V_OUT to FB to GND (Figure 7). Choose a value
less than 260kΩ for R6. Use the following equation to
calculate R5:
R5 = R6 [(V_OUT / V_REF ) - 1]
V_IN
R1
200Ω
R3
R4
0.1µF
47µF
BATT
(MAX1676)
LBI
LX
OUT
CLSEL
(MAX1676)
FB
SHDN
MAX1674
MAX1675
REF ^MAX1676 _LBO
GND
22µH
0.1µF
V_OUT OUTPUT
+3.3V
47µF
R2
100k
LOW-BATTERY
OUTPUT
where V_REF = +1.3V and V_OUT may range from 2V to
5V. The input bias current of FB has a maximum value
of 50nA which allows large-value resistors (R6 ≤ 260kΩ)
to be used.
Low-Battery Detection
The MAX1674/MAX1675/MAX1676 contain an on-chip
comparator for low-battery detection. If the voltage at
LBI falls below the internal reference voltage (1.30V),
LBO (an open-drain output) sinks current to GND. The
low-battery monitor threshold is set by two resistors, R3
and R4 (Figures 5, 6, and 7). Since the LBI current is
less than 50nA, large resistor values (R4 ≤ 260kΩ) can
be used to minimize loading of the input supply.
Calculate R3 using the following equation:
R3 = R4 [(V_TRIP / V_REF) - 1]
for V_TRIP ≥ 1.3V. V_TRIP is the level where the low-battery
detector output goes low, and V_REF is the internal
1.30V reference. Connect a pull-up resistor of 100kΩ or
greater from LBO to OUT when driving CMOS circuits.
LBO is an open-drain output, and can be pulled as
high as 6V regardless of the voltage at OUT. When LBI
is above the threshold, the LBO output is high imped-
ance. If the low-battery comparator is not used, ground
V_IN
Figure 5. Preset Output Voltage of +3.3V
V_IN
R1
200Ω
R3
R4
0.1µF
47µF
BATT
(MAX1676)
LBI
LX
OUT
CLSEL
(MAX1676)
SHDN
MAX1674
MAX1675 ^LBO
REF ^MAX1676
FB
GND
0.1µF
R2
100k
22µH
OUTPUT
5.0V
47µF
LOW-
BATTERY
OUTPUT
47µF
22µH
R1
200Ω _BATT
(MAX1676)
LX
R3
OUT
SHDN
LBI
CLSEL
(MAX1676)
R4
0.1µF
MAX1674
MAX1675 ^LBO
MAX1676
REF
FB
GND
0.1µF
R2
R5
100k
R6
OUTPUT
2V to 5.5V
47µF
LOW-
BATTERY
OUTPUT
Figure 6. Preset Output Voltage of +5V
Figure 7. Setting an Adjustable Output
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