Rainbow Electronics MAX1724 - Manual (Page 7)

MAX1722/MAX1723/MAX1724
1.5µA I Q , Step-Up DC-DC Converters
in Thin SOT23-5
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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 paral-
lel 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 battery to the output. V
OUT falls to approxi-
mately 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, con-
nect SHDN to the input.
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
Figure 2. MAX1724 Simplified Functional Diagram
Figure 3. MAX1723 Single-Cell Operation
V OUT = 3.6V
1.2V TO V OUT
D1
10 µ H
10 µF
R2
2.37M
Ω
R1
1.24M
Ω
10 µF
LX
OUT
SHDN
GND FB
MAX1723