Rainbow Electronics MAX8640Z - Manual (Page 7)

MAX8640Y/MAX8640Z
500mA, 4MHz/2MHz Synchronous
Step-Down DC-DC Converters in SC70
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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
frequency. Ceramic capacitors are recommended due
to their small size and low ESR. Make sure the capaci-
tor 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 larg-
er than the inductor value in µH.
Input Capacitor
The input capacitor, C1, reduces the current peaks
drawn from the battery or input power source and
reduces switching noise in the IC. The impedance of
C1 at the switching frequency should be kept very low.
Ceramic capacitors are recommended due to their
small size and low ESR. Make sure the capacitor main-
tains its capacitance over temperature and DC bias.
Capacitors with X5R or X7R temperature characteris-
tics typically perform well. Due to the MAX8640Y/
MAX8640Z soft-start, the input capacitance can be
very low. For optimum noise immunity and low input rip-
ple, choose a capacitor value in µF that is equal to or
larger than the inductor’s value in µH.
PC Board Layout and Routing
High switching frequencies and large peak currents
make PC board layout a very important part of design.
Good design minimizes excessive EMI on the feedback
paths and voltage gradients in the ground plane, both
of which can result in instability or regulation errors.
Connect the inductor, input capacitor, and output
capacitor as close together as possible, and keep their
traces short, direct, and wide. Connect the two GND
pins under the IC and directly to the grounds of the
input and output capacitors. Keep noisy traces, such
as the LX node, as short as possible. Refer to the
MAX8640Z evaluation kit for an example PC board lay-
out and routing scheme.
Selector Guide
RECOMMENDED COMPONENTS
PART
OUTPUT
VOLTAGE (V)
FREQUENCY
(MHz) L1 (µH) C2 (µF)
TOP MARK
MAX8640YEXT08 0.8 1.2 2.2 10 ACQ
MAX8640YEXT11 1.1 1.7 2.2 4.7 ACR
MAX8640YEXT12 1.2 1.8 2.2 4.7 ACS
MAX8640YEXT13 1.3 1.9 2.2 4.7 ACG
MAX8640YEXT15 1.5 2.0 2.2 4.7 ADD
MAX8640YEXT16 1.6 2.0 2.2 4.7 ADB
MAX8640YEXT18 1.8 2.0 2.2 4.7 ACI
MAX8640YEXT19 1.9 2.0 2.2 4.7 ACH
MAX8640YEXT25 2.5 1.7 2.2 4.7 ACJ
MAX8640ZEXT08 0.8 2.4 1 4.7 ACL
MAX8640ZEXT11 1.1 3.4 1 2.2 ACM
MAX8640ZEXT12 1.2 3.6 1 2.2 ACN
MAX8640ZEXT13 1.3 3.7 1 2.2 ACO
MAX8640ZEXT15 1.5 3.9 1 2.2 ACP
MAX8640ZEXT18 1.8 4.0 1 2.2 ACU