MAX1705 Maxim, MAX1705 Datasheet - Page 14

• Manufacturer Part Number: MAX1705
• Description: 1- to 3-Cell / High-Current / Low-Noise / Step-Up DC-DC Converters with Linear Regulator
• Manufacturer: Maxim

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
and V
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.
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
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.
1- to 3-Cell, High-Current, Low-Noise,
Step-Up DC-DC Converters with Linear Regulator
14
_________________ Design Procedure
______________________________________________________________________________________
LBP
OUT,TH
FB
is connected to the reference or 1.25V.
, the step-up regulator feedback setpoint, is
R
R
R
5
5
1
is the desired output voltage trip point
R
R
R
2
Setting the Output Voltages
6
6
V
V
V
V
POUT
OUT TH
V
IN TH
V
REF
FB
LBP
,
,
- 1
- 1
- 1
Reference
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
BATTERY
VOLTAGE
C5
0.33 F
P
10k
R8
270k
POUT
LBO
LBP
POUT
LBO
LBN
REF
MAX1705
MAX1706
MAX1705
MAX1706
0.33 F
GND
STEP-UP OUTPUT
C4
0.1 F
GND
C3
REF
LBN
LDO
LBP
LBO
LBP
REF
OUT
MAX1705
MAX1706
GND
R5
R6
POUT
R5
R6
LBN
0.33 F
R6
R5