Rainbow Electronics MAX1676 - Manual (Page 9)

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 260k Ω for R6. Use the following equation to
calculate R5:
R5 = R6 [(V OUT / V REF ) - 1]
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 100k Ω 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
MAX1674/MAX1675/MAX1676
High-Efficiency, Low-Supply-Current,
Compact, Step-Up DC-DC Converters
_______________________________________________________________________________________ 9
MAX1674
MAX1675
MAX1676
BATT
(MAX1676)
V IN
LBI
REF
GND
R3
R1
200 Ω
R4
R2
100k
47 µF
22 µH
0.1 µF
LX
LBO
0.1 µF 47 µF
OUTPUT
+3.3V V OUT
LOW-BATTERY
OUTPUT
FB
SHDN
OUT
CLSEL
(MAX1676)
Figure 5. Preset Output Voltage of +3.3V
MAX1674
MAX1675
MAX1676
BATT
(MAX1676)
V
IN
LBI
REF
GND
R6
R5
R3
R1
200 Ω
R4 R2
100k
22 µH
47 µF
0.1 µF
LX
LBO
OUTPUT
2V to 5.5V
FB
SHDN
OUT
CLSEL
(MAX1676)
LOW-
BATTERY
OUTPUT
0.1 µF 47 µF
Figure 7. Setting an Adjustable Output
MAX1674
MAX1675
MAX1676
BATT
(MAX1676)
V IN
LBI
REF
GND
R3
R1
200 Ω
R4 R2
100k
22 µH
47 µF
0.1 µF
LX
LBO
0.1 µF 47 µF
OUTPUT
5.0V
FB
SHDN
OUT
CLSEL
(MAX1676)
LOW-
BATTERY
OUTPUT
Figure 6. Preset Output Voltage of +5V