Rainbow-electronics MAX1706 Manual de usuario descargar pdf (Pagina 14)

MAX1705/MAX1706

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 10kΩ 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:

where V

OUT,TH

is the desired output voltage trip point

and V

LBP

is connected to the reference or 1.25V.

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:

where V

, the step-up regulator feedback setpoint, is

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

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

POUT

1 2 =













- 1

R R

OUTTH

LBP

5 6













- 1

R R

REF

INTH

5 6













- 1

MAX1705

MAX1706

LBO

REF

LBN

POUT

GND

0.33µF

LDO

LBP

MAX1705

MAX1706

LBN

LBO

LBP

POUT

REF

GND

BATTERY

VOLTAGE

10k

0.33µF

270k

MAX1705

MAX1706

LBP

LBO

LBN

0.33µF

OUT

POUT

REF

GND

0.1µF

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

1 2 ... 9 10 11 12 13 14 15 16 17 18 19 20

MAX1705/MAX1706 1

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Rainbow-electronics MAX1706 Manual de usuario Pagina 14

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