LTM4612
13
4612fc
For more information
Figure 4. Radiated Emission Scan with 24V
IN
to
5V
OUT
at 5A (2
×
10µF Ceramic Capacitors on V
IN
Pads and 1
×
10µF Ceramic Capacitor on V
D
Pads)
APPLICATIONS INFORMATION
Input Capacitors
LTM4612 is designed to achieve the low input radiated EMI
noise due to the fast switching of turn-on and turn-off.
In the LTM4612, a high-frequency inductor is integrated
into the input line for noise attenuation. V
D
and V
IN
pins
are available for external input capacitors to form a high
frequency π filter. As shown in Figure 18, the ceramic
capacitor C1 on the V
D
pins is used to handle most of
the RMS current into the converter, so careful attention
is needed for capacitor C1 selection.
For a buck converter, the switching duty cycle can be
estimated as:
D
=
V
OUT
V
IN
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
I
CIN(RMS)
=
I
OUT(MAX)
h
• D • 1– D
(
)
In this equation,
h
is the estimated efficiency of the
power module. Note the capacitor ripple current ratings
are often based on temperature and hours of life. This
makes it advisable to properly derate the input capacitor,
or choose a capacitor rated at a higher temperature than
required. Always contact the capacitor manufacturer for
derating requirements.
In a typical 5A output application, one very low ESR, X5R
or X7R, 10µF ceramic capacitor is recommended for C1.
This decoupling capacitor should be placed directly adja-
cent to the module V
D
pins in the PCB layout to minimize
the trace inductance and high frequency AC noise. Each
10µF ceramic is typically good for 2A to 3A of RMS ripple
current. Refer to your ceramics capacitor catalog for the
RMS current ratings.
FREQUENCY (MHz)
30
dBµV/m
30
70
128.1
226.2
324.3
10
60
20
0
–10
50
40
422.4
520.5
618.6
716.7
814.8
912.9
1010
EN55022 CLASS B LIMIT
4612 F04
To attenuate the high frequency noise, extra input capacitors
should be connected to the V
IN
pads and placed before the
high frequency inductor to form the π filter. One of these
low ESR ceramic input capacitors is recommended to be
close to the connection into the system board. A large
bulk 100µF capacitor is only needed if the input source
impedance is compromised by long inductive leads or
traces. Figure 4 shows the radiated EMI test results to
meet the EN55022 Class B limit. For different applica-
tions, input capacitance may be varied to meet different
radiated EMI limits.
