Rainbow Electronics MAX1708 - Manual (Page 9)

MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
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_______________Detailed Description
The MAX1708 step-up converter offers high efficiency
and high integration for high-power applications. It
operates with an input voltage as low as 0.7V and is
suitable for single- to 3-cell battery inputs, as well as
2.5V or 3.3V regulated supply inputs. The output volt-
age is preset to +3.3V or +5.0V or can be adjusted with
external resistors for voltages between +2.5V to +5.5V.
The MAX1708 internal N-channel MOSFET switch is
rated for 5A (RMS value) and can deliver loads to 2A,
depending on input and output voltages. For flexibility,
the current limit and soft-start rate are independently
programmable.
A 600kHz switching frequency allows for a small induc-
tor to be used. The switching frequency is also syn-
chronizable to an external clock ranging from 350kHz
to 1MHz.
ONA,
ONB
The logic levels at ONA and ONB turn the MAX1708 on
or off. When ONA = 1 or ONB = 0, the device is on.
When ONA = 0 and ONB = 1, the device is off (Table
1). Logic high on-control can be implemented by con-
necting ONB high and using ONA for shutdown.
Implement inverted single-line on/off control by ground-
ing ONA and toggling ONB. Implement momentary
pushbutton on/off as described in the Applications
Information section. Both inputs have approximately
0.15V of hysteresis.
Switching Frequency
The MAX1708 switches at the fixed-frequency internal
oscillator rate (600kHz) or can be synchronized to an
external clock. Connect CLK to OUT for internal clock
operation. Apply a clock signal to CLK to synchronize
to an external clock. The MAX1708 will synchronize to a
new external clock rate in two cycles and will take
approximately 40µs to revert to its internal clock fre-
quency once the external clock pulses stop and CLK is
driven high. Table 2 summarizes oscillator operation.
Operation
The MAX1708 switches at a constant frequency
(600kHz) and modulates the MOSFET switch pulse
width to control the power transferred per cycle and
regulate the voltage across the load. In low-noise appli-
cations, the fundamental and the harmonics generated
by the fixed switching frequency are easily filtered out.
Figure 2 shows the simplified functional diagram for the
MAX1708. Figure 3 shows the simplified PWM con-
Figure 1. Standard Operating Circuit
KEEP TRACES
SHORT AND WIDE
ON/OFF
CONTROL
MAX1708
L1
2.2 µH
C1
150
µF
V IN
GND
R1
C3
C4
0.22 µF
GND
REF
SS/LIM
PGND PGND PGND
C5
0.1
µF
FB
GND
OUT
3.3/5
CLK
D1
LX LX LX
R2
2
Ω
C2
150
µF
V OUT
+5V
GND
ONA
ONB
KEEP TRACES
SHORT AND WIDE