Rainbow Electronics MAX8649 User Manual
Page 30
MAX8649
1.8A Step-Down Regulator with Differential
Remote Sense in 2mm x 2mm WLP
30
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Input Capacitor Selection
The input capacitor in a step-down DC-DC regulator
reduces current peaks drawn from the battery or other
input power source and reduces switching noise in the
controller. A 10µF ceramic capacitor in parallel with a
0.1µF ceramic capacitor is recommended for most appli-
cations. The impedance of the input capacitor at the
switching frequency should be less than that of the input
source so that high-frequency switching currents do not
pass through the input source. The input capacitor must
meet the input ripple-current requirement imposed by
the step-down regulator. Ceramic capacitors are pre-
ferred due to their resilience to power-up surge currents.
Choose the input capacitor so that the temperature rises
due to input ripple current do not exceed approximately
+10°C. For a step-down DC-DC regulator, the maximum
input ripple current is 1/2 of the output. This maximum
input ripple current occurs when the step-down regulator
operates at 50% duty factor (V
IN
= 2 x V
OUT
). Refer to
the MAX8649 Evaluation Kit data sheet for specific input
capacitor recommendations.
Output Capacitor Selection
The step-down DC-DC regulator output capacitor
keeps output ripple small and ensures control-loop
stability. A 10µF ceramic capacitor in parallel with a
0.1µF ceramic capacitor is recommended for most
applications. The output capacitor must also have low
impedance at the switching frequency. Ceramic, poly-
mer, and tantalum capacitors are suitable, with ceramic
exhibiting the lowest ESR and lowest high-frequency
impedance.
Output ripple due to capacitance (neglecting ESR) is
approximately:
Additional ripple due to capacitor ESR is:
Refer to the MAX8649 Evaluation Kit data sheet for spe-
cific output capacitor recommendations.
Power Dissipation
The MAX8649 has a thermal-shutdown feature that pro-
tects the IC from damage when the die temperature
exceeds +160°C. See the
Thermal-Overload Protection
section for more information. To prevent thermal over-
load and allow the maximum load current on each reg-
ulator, it is important to ensure that the heat generated
by the MAX8649 can be dissipated into the PCB.
When properly mounted on a multilayer PCB, the junc-
tion-to-ambient thermal resistance (
θ
JA
) is typically
76°C/W.
PCB Layout
Due to fast switching waveforms and high current
paths, careful PCB layout is required to achieve optimal
performance. Minimize trace lengths between the IC
and the inductor, the input capacitor, and the output
capacitor; keep these traces short, direct, and wide.
The ground connections of C
IN
and C
OUT
should be as
close together as possible and connected to PGND.
Connect AGND and PGND directly to the ground plane.
The MAX8649 evaluation kit illustrates an example PCB
layout and routing scheme.
V
ESR
I
ESR
RIPPLE
L PEAK
( )
=
×
(
)
V
I
f
C
RIPPLE
L PEAK
OSC
OUT
=
Ч
Ч
(
)
2
π