Rainbow Electronics MAX1724 - Manual (Page 6)

MAX1722/MAX1723/MAX1724
1.5µA I Q , Step-Up DC-DC Converters
in Thin SOT23-5
6 _______________________________________________________________________________________
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 efficien-
cy 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 cur-
rent 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 comparator senses that the output has fallen
below the regulation threshold, another cycle begins.
Synchronous Rectification
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 sig-
nificantly 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-
P
N
CONTROL
LOGIC
STARTUP
CIRCUITRY
DRIVER
GND
FB
OUT
LX
CURRENT
LIMIT 1.235V REFERENCE
ERROR
COMPARATOR
ZERO-
CROSSING
DETECTOR
MAX1723
SHDN
Figure 1. MAX1723 Simplified Functional Diagram