MAX879C データシートの表示（PDF） - Maxim Integrated
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MAX879C Datasheet PDF : 12 Pages
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5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters
VIN
C1
22µF
L1
22µH
5
2 IN
LX
6
OUT
7
MAX879
SHDN
R1
C3
1
ILIM
AGND
8
FB
PGND
3
4
R2
VOUT
C2
100µF
INPUT
1.0V TO 6.2V
22µF
ON/OFF
2
IN
1 ILIM MAX877
5
LX
7
SHDN
6
OUT
PGND
4
AGND
3
22µH
OUTPUT
5V
100µF
Figure 3. MAX879 Adjustable Voltage
Figure 4. MAX877 Standard Application Circuit
__________Applications Information
Figure 4 shows a MAX877 step-up application circuit.
This circuit starts up and operates with inputs ranging
from 1.0V to 6.2V. Start-up time is a function of the load,
typically less than 5ms. Output current capability is a
function of the input voltage (see Typical Operating
Characteristics).
The converters will regulate down to the output voltage
and seamlessly switch into boost mode as the input
drops below the output voltage. This is especially use-
ful in battery-powered applications, where the battery
voltage may initially exceed the output voltage. To gen-
erate 5V from four alkaline cells in series, the input
ranges from 6.2V to 3.6V. When the battery pack is
fresh, the MAX877 will step down with the active rectifi-
er acting as the switch. As the batteries approach 5V,
or the desired output voltage, the converter’s control
circuitry will ensure a smooth transition into step-up
mode. The converter operates until the batteries are
less than 3V; efficiency is typically 80% with fresh bat-
teries, and is close to 85% at VIN = 4V.
Inductor Selection
The 22µH inductor shown in the Typical Operating
Circuit is sufficient for most MAX877/MAX878/MAX879
designs. Other inductor values ranging from 10µH to
47µH are also suitable. The inductor should have a satu-
ration rating equal to or greater than the peak switch-
current limit, which is 1A without an external current limit
(ILIM connected to IN). It is acceptable to operate the
inductor at 120% of its saturation rating; however, this
may slightly reduce efficiency. For highest efficiency,
use an inductor with a low DC resistance, preferably
under 0.2Ω. Table 1 lists suggested inductor suppliers.
Capacitor Selection
The 100µF, 10V surface-mount tantalum (SMT) output
capacitor shown in the Typical Operating Circuit will
provide a 25mV output ripple or less, stepping up from
3V to 5V at 200mA. Smaller capacitors, down to 10µF,
are acceptable for light loads or in applications that
can tolerate higher output ripple. The input capacitor
may be omitted if the supply has low output impedance
and the input lead length is less than 2 inches (5cm) or
the loads are small.
The primary factor in selecting both the output and input
filter capacitor is low ESR. The ESR of both bypass and
filter capacitors affects efficiency. Optimize performance
by increasing filter capacitors or using specialized low-
ESR capacitors. The smallest low-ESR SMT tantalum
capacitors currently available are Sprague 595D or 695D
series. Sanyo OS-CON organic-semiconductor through-
hole capacitors also exhibit very low ESR, are rated for
the wide temperature range, and are especially suitable
for operation at cold temperatures (below 0°C).
Table 1 lists suggested capacitor suppliers.
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