Selecting the Output Voltage

pin to GND (5V) or OUT (3.3V) (Figures 5 and 6).

To adjust the output voltage, connect a resistor-divider

less than 260k

Ω for R6. Use the following equation to

calculate R5:

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:

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

High-Efficiency, Low-Supply-Current,

Compact, Step-Up DC-DC Converters

_______________________________________________________________________________________

9

MAX1674

MAX1675

MAX1676

BATT

(MAX1676)

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

LOW-BATTERY

OUTPUT

FB

SHDN

OUT

CLSEL

(MAX1676)

Figure 5. Preset Output Voltage of +3.3V

MAX1674

MAX1675

MAX1676

BATT

(MAX1676)

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)

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