1- to 3-Cell, High-Current, Low-Noise,

Step-Up DC-DC Converters with Linear Regulator

14

______________________________________________________________________________________

The low-battery comparator can also be used to moni-

tor the output voltage, as shown in Figure 5.

To set the low-battery threshold to a voltage below the

1.25V reference, insert a resistor divider between REF

and LBN, and connect the battery to the LBP input

through a 10k

Ω current-limiting resistor (Figure 6). The

equation for setting the resistors for the low-battery

threshold is then as follows:

Alternatively, the low-battery comparator can be used

to check the output voltage or to control the load direct-

ly on POUT during start-up (Figure 7). Use the following

equation to set the resistor values:

Reference

The MAX1705/MAX1706 have an internal 1.250V, 1%

bandgap reference. Connect a 0.33µF bypass capaci-

tor to GND within 0.2in. (5mm) of the REF pin. REF can

source up to 50µA of external load current.

_________________ Design Procedure

Setting the Output Voltages

Set the step-up converter output voltage between 2.5V

and 5.5V by connecting a resistor voltage-divider to FB

from OUT to GND, as shown in Figure 8. The resistor

values are then as follows:

1.233V. Since the input bias current into FB is less than

50nA, R2 can have a large value (such as 270k

Ω or

less) without sacrificing accuracy. Connect the resistor

voltage-divider as close to the IC as possible, within

0.2in. (5mm) of the FB pin.

Alternatively, set the step-up converter output to track

the linear regulator by 300mV. To accomplish this, set

TRACK to OUT.

R

R

V

V

POUT

FB

1

2

=









- 1

R

R

V

V

OUT TH

LBP

5

6

,

=









- 1

R

R

V

V

REF

IN TH

5

6

,

=













- 1

MAX1705

MAX1706

LBO

REF

LBN

POUT

GND

R5

R6

0.33

µF

LDO

LBP

MAX1705

MAX1706

LBN

LBO

LBP

POUT

REF

GND

R5

R6

BATTERY

VOLTAGE

R8

10k

0.33

µF

270k

MAX1705

MAX1706

LBP

LBO

LBN

0.33

µF

OUT

POUT

REF

GND

R5

R6

P

C3

0.1

µF

C4

C5

STEP-UP OUTPUT

Figure 5. Using the Low-Battery Comparator to Sense

the Output Voltage

Figure 6. Detecting Battery Voltages Below 1.25V

Figure 7. Using the Low-Battery Comparator for Load Control

During Start-Up