1-Cell to 3-Cell, High-Power (1A),
Low-Noise, Step-Up DC-DC Converters
MAX1700/MAX1701
0.7V TO 5.5V
POUT
REF
L1
D1
MAX1701
LBO
LBN
MAX1701
CLK/SEL
ONA
ONB
R3
LBP
R4
LX
POUT
10Ω
OUT
0.22µF
VOLTAGE MONITOR
LOW-BATTERY MONITOR
ARBITRARY VOLTAGE MONITOR
R6
10Ω
C3
0.22µF
C4
R5
0.22µF
10k
LBP
ARBITRARY
VOLTAGE
R5
R6
GND
BATTERY
VOLTAGE
POK
LBO
LBN
REF
AO
GND PGND FB AIN
Figure 7. Detecting Battery Voltages Below 1.25V (MAX1701)
OUTPUT
P
Figure 5. Detecting Battery Voltage Above 1.25V
C5
270k
OUT
POUT
LBN
R3
POUT
OUT
LBO
MAX1701
R3
LBP
REF
0.22µF
GND
R4
MAX1701
LBO
LBN
REF
0.22µF
GND
LBP
R4
Figure 8. Using the Low-Battery Comparator for Load Control
During Start-Up
Figure 6. Using the Low-Battery Comparator to Sense the
Output Voltage (MAX1701)
between the positive battery terminal and GND (Figure
5). The resistor values are then calculated as follows:
R3 = R4(V
TH
/V
LBN
-1)
where V
TH
is the desired input voltage trip threshold
and V
LBN
= V
REF
= 1.25V. Since the input bias current
into LBP is less than 20nA, R4 can be a large value
(such as 270kΩ or less) without sacrificing accuracy.
The inputs have a common-mode input range from
0.5V to 1.5V and an input-referred hysteresis of 15mV.
The low-battery comparator can also be used to moni-
tor the output voltage, as shown in Figure 6.
To set the low-battery threshold to a voltage below the
1.25V reference, insert a resistor divider between REF
12
and LBN and connect the battery to the LBP input
through a 10kΩ current-limiting resistor (Figure 7). The
equation for setting the resistors for the low-battery
threshold is then as follows:
R5 = R6(V
REF
/V
LBP
-1)
where V
LBP
is the desired voltage threshold. In Figures
5, 6, and 7, LBO goes low for a low-voltage input. The
low-battery comparator can be used to check the out-
put voltage or to control the load directly on P
OUT
dur-
ing start-up (Figure 8). Use the following equation to set
the resistor values:
R3 = R4(V
OUTTH
/V
LBP
- 1)
where V
OUTTH
is the desired output-voltage trip point
and V
LBP
is connected to the reference or 1.25V.
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
