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MAX877 Datasheet(PDF) 7 Page - Maxim Integrated Products

Part No. MAX877
Description  5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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 7 page
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With ILIM shorted to IN, the peak switch current of the
internal NPN power switch is set to 1A. It can be set to
a lower value by connecting a resistor between ILIM
and IN (see
Current Limit section). This enables the use
of physically smaller inductors with lower saturation-
current ratings. At 1A, the switch voltage drop (VSW) is
about 500mV. VSW decreases to about 250mV at 0.1A.
Conventional PWM converters generate constant-
frequency switching noise, while this architecture
produces variable-frequency switching noise. The out-
put ripple is the product of the peak inductor current
and the output capacitor's ESR. Unlike conventional
pulse-skippers, the MAX877/MAX878/MAX879 peak
currents are scaled down at light loads, resulting in
lower output ripple.
Step-Down Mode and Power Dissipation
In battery-powered applications, for example, where
the input voltage exceeds the output voltage, the
MAX877/MAX878/MAX879 behave as “switched” linear
regulators. If the output voltage starts to drop, the
switch turns on and energy is stored in the coil, as in
normal step-up mode. After the switch turns off, the
voltage at LX flies high. The active rectifier turns on
when LX rises above VIN. As in a linear regulator, the
voltage difference between VIN and VOUT appears
across the rectifier (actually a PNP transistor) until the
current goes to zero and the rectifier turns off. At high
VIN to VOUT differentials, the maximum load current is
limited by the maximum allowable power dissipation in
the package (see
Typical Operating Characteristics).
5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters
_______________________________________________________________________________________
7
MAX878
DELAY
TIMER
tOFF
DELAY
TIMER
tON
VREF
0.2025V
SHUTDOWN
CONTROL
SHDN
7
ACTIVE RECTIFIER
RECTIFIER
CONTROL
OUT
PGND
AGND
3
SEL 8
4
6
100
µF
VOUT
22
µF
VIN
22
µH
RLIM
SWITCH
DRIVER
LX
ILIM
IN
21
5
1 : N
Figure 1. MAX878 Block Diagram




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