MAX8640YELT25T View Datasheet(PDF) - Maxim Integrated
Part Name
Description
Manufacturer
MAX8640YELT25T Datasheet PDF : 13 Pages
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MAX8640Y/MAX8640Z
Tiny 500mA, 4MHz/2MHz Synchronous
Step-Down DC-DC Converters
IN
SHDN
PWM
LOGIC
LX
GND
OUT
0.6V
MAX8640Y
MAX8640Z
Figure 1. Simplified Functional Diagram
MAX8640
IN
ESD
100kΩ
DIODE
VIN
SHDN
4.7nF
GND
Figure 2. Using an R and C circuit to create an always on regu-
lator
Soft-Start
The MAX8640Y/MAX8640Z include internal soft-start
circuitry that eliminates inrush current at startup, reduc-
ing transients on the input source. Soft-start is particu-
larly useful for higher impedance input sources, such
as Li+ and alkaline cells. See the Soft-Start Response
in the Typical Operating Characteristics.
Applications Information
The MAX8640Y/MAX8640Z are optimized for use with a
tiny inductor and small ceramic capacitors. The correct
selection of external components ensures high efficien-
cy, low output ripple, and fast transient response.
Inductor Selection
A 1µH inductor is recommended for use with the
MAX8640Z, and 2.2µH is recommended for the
MAX8640Y. A 1µH inductor is physically smaller but
requires faster switching, resulting in some efficiency
loss. Table 1 lists several recommended inductors.
It is acceptable to use a 1.5µH inductor with either the
MAX8640Y or MAX8640Z, but efficiency and ripple
should be verified. Similarly, it is acceptable to use a
3.3µH inductor with the MAX8640Y, but performance
should be verified.
For optimum voltage positioning of load transients,
choose an inductor with DC series resistance in the
75mΩ to 150mΩ range. For higher efficiency at heavy
loads (above 200mA) or minimal load regulation (but
some transient overshoot), the resistance should be
kept as low as possible. For light-load applications up
to 200mA, higher resistance is acceptable with very lit-
tle impact on performance.
Capacitor Selection
Output Capacitor
The output capacitor, C2, is required to keep the output
voltage ripple small and to ensure regulation loop sta-
bility. C2 must have low impedance at the switching fre-
quency. Ceramic capacitors are recommended due to
their small size and low ESR. Make sure the capacitor
maintains its capacitance over temperature and DC
bias. Capacitors with X5R or X7R temperature charac-
teristics typically perform well. The output capacitance
can be very low; see the Selector Guide for recom-
mended capacitance values. For optimum load-tran-
sient performance and very low output ripple, the output
capacitor value in µF should be equal to or larger than
the inductor value in µH.
6
Maxim Integrated
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