Datasheet

Detailed Description
The MAX1722/MAX1723/MAX1724 compact, high-effi-
ciency, step-up DC-DC converters are guaranteed to
start up with voltages as low as 0.91V and operate with
an input voltage down to 0.8V. Consuming only 1.5μA of
quiescent current, these devices include a built-in syn-
chronous rectifier that reduces cost by eliminating the
need for an external diode and improves overall efficiency
by minimizing losses in the circuit (see Synchronous
Rectification section). The MAX1722/MAX1724 feature
a clamp circuit that reduces EMI due to inductor ringing.
The MAX1723/MAX1724 feature an active-low shutdown
that reduces quiescent supply current to 0.1μA. The
MAX1722/MAX1723 have an adjustable output voltage,
while the MAX1724 is available with four fixed-output
voltage options (see Selector Guide). Figure 1 is the
MAX1723 simplified functional diagram and Figure 2 is
the MAX1724 simplified functional diagram.
PFM Control Scheme
A forced discontinuous, current-limited, pulse-frequency-
modulation (PFM) control scheme is a key feature of the
MAX1722/MAX1723/MAX1724. This scheme provides
ultra-low quiescent current and high efficiency over a
wide output current range. There is no oscillator; the
inductor current is limited by the 0.5A N-channel current
limit or by the 5μs switch maximum on-time. Following
each on cycle, the inductor current must ramp to zero
before another cycle may start. When the error compara-
tor senses that the output has fallen below the regulation
threshold, another cycle begins.
Synchronous Rectication
The internal synchronous rectifier eliminates the need for
an external Schottky diode, thus reducing cost and board
space. While the inductor discharges, the P-channel
MOSFET turns on and shunts the MOSFET body diode.
As a result, the rectifier voltage drop is significantly
reduced, improving efficiency without the addition of
external components.
Low-Voltage Startup Circuit
The MAX1722/MAX1723/MAX1724 contain a low-volt-
age startup circuit to control DC-DC operation until the
output voltage exceeds 1.5V (typ). The minimum start-
Figure 1. MAX1723 Simplified Functional Diagram
P
N
CONTROL
LOGIC
STARTUP
CIRCUITRY
DRIVER
GND
FB
OUT
LX
CURRENT
LIMIT
1.235V REFERENCE
ERROR
COMPARATOR
ZERO-
CROSSING
DETECTOR
MAX1723
SHDN
MAX1722/MAX1723/
MAX1724
1.5μA I
Q
, Step-Up DC-DC
Converters in TSOT and µDFN
www.maximintegrated.com
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