Datasheet

up voltage is a function of load current (see Typical
Operating Characteristics). This circuit is powered from
the BATT pin for the MAX1722/MAX1724, guaranteeing
startup at input voltages as low as 0.91V. The MAX1723
lacks a BATT pin; therefore, this circuit is powered
through the OUT pin. Adding a Schottky diode in parallel
with the P-channel synchronous rectifier allows for startup
voltages as low as 1.2V for the MAX1723 (Figure 3). The
external Schottky diode is not needed for input voltages
greater than 1.8V. Once started, the output maintains
the load as the battery voltage decreases below the
startup voltage.
Shutdown (MAX1723/MAX1724)
The MAX1723/MAX1724 enter shutdown when the SHDN
pin is driven low. During shutdown, the body diode of the
P-channel MOSFET allows current to flow from the bat-
tery to the output. V
OUT
falls to approximately V
IN
- 0.6V
and LX remains high impedance. Shutdown can be pulled
as high as 6V, regardless of the voltage at BATT or OUT.
For normal operation, connect SHDN to the input.
Figure 2. MAX1724 Simplified Functional Diagram
Figure 3. MAX1723 Single-Cell Operation
P
N
CONTROL
LOGIC
STARTUP
CIRCUITRY
DAMPING
SWITCH
DRIVER
GND
SHDN
R
1
R
2
BATT
LX
OUT
CURRENT
LIMIT
MAX1724
ERROR
COMPARATOR
ZERO-
CROSSING
DETECTOR
1.235V REFERENCE
V
OUT
= 3.6V
1.2V
TO V
OUT
D1
10
µH
10µF
R2
2.37MΩ
R1
1.24MΩ
10µF
LX
OUT
SHDN
GND FB
MAX1723
MAX1722/MAX1723/
MAX1724
1.5μA I
Q
, Step-Up DC-DC
Converters in TSOT and µDFN
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