Datasheet
MAX1705/MAX1706
1- to 3-Cell, High-Current, Low-Noise,
Step-Up DC-DC Converters with Linear Regulator
14 ______________________________________________________________________________________
LBP is less than 50nA, R6 can be 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 LBP pin. The
inputs have a 0.5V to 1.5V common-mode input range,
and a 16mV input-referred hysteresis.
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 startup (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
FB
, 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
RR
V
V
POUT
FB
12 =
⎛
⎝
⎜
⎞
⎠
⎟
- 1
RR
V
V
OUT TH
LBP
56
,
=
⎛
⎝
⎜
⎞
⎠
⎟
- 1
RR
V
V
REF
INTH
56
,
=
⎛
⎝
⎜
⎞
⎠
⎟
- 1
MAX1705
MAX1706
LBO
REF
LBN
POUT
GND
R5
R6
0.33µF
LDO
LBP
MAX1705
MAX1706
LBN
LBO
LBP
POUT
REF
GND
R5
R6
BATTERY
VOLTAGE
R8
10kΩ
0.33µF
270kΩ
MAX1705
MAX1706
LBP
LBO
LBN
0.33µF
OUT
POUT
REF
GND
R5
R6
P
C3
0.1µF
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 Startup