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LTM4606
APPLICATIONS INFORMATION
? D ? ( 1 – D )
I CIN(RMS) =
Without considering the inductor ripple current, the RMS
current of the input capacitor can be estimated as:
I OUT(MAX)
η
In the above equation, η 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 capacitor, or
choose a capacitor rated at a higher temperature than
required. Always contact the capacitor manufacturer for
derating requirements.
In a typical 6A output application, one or two very low
ESR X5R or X7R, 10μF ceramic capacitors are recom-
mended for C1. This decoupling capacitor should be placed
directly adjacent 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 2 to 3 amps
of RMS ripple current. Refer to your ceramics capacitor
catalog for the RMS current ratings.
To attenuate 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 capacitors is recommended to
be placed 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 2 shows the radiated EMI test results to
50
40
30
20
10
0
–10
–20
meet EN55022 Class B. For different applications, input
capacitance may be varied to meet different radiated EMI
limits.
Output Capacitors
The LTM4606 is designed for low output voltage ripple.
The bulk output capacitors defined as C OUT are chosen
with low enough effective series resistance (ESR) to meet
the output voltage ripple and transient requirements. C OUT
can be a low ESR tantalum capacitor, low ESR polymer
capacitor or ceramic capacitor. The typical capacitance is
200μF if all ceramic output capacitors are used. Additional
output filtering may be required by the system designer,
if further reduction of output ripple or dynamic transient
spike is required. Table 2 shows a matrix of different output
voltages and output capacitors to minimize the voltage
droop and overshoot during a 3A/μs transient. The table
optimizes total equivalent ESR and total bulk capacitance
to maximize transient performance.
Multiphase operation with multiple LTM4606 devices in
parallel will lower the effective output ripple current due
to the phase interleaving operation. Refer to Figure 3
for the normalized output ripple current versus the duty
cycle. Figure 3 provides a ratio of peak-to-peak output
ripple current to the inductor ripple current as functions
of duty cycle and the number of paralleled phases. Pick
the corresponding duty cycle and the number of phases
to get the correct output ripple current value. For example,
each phase’s inductor ripple current DIr at zero duty cycle
is ~2.5A for a 12V to 2.5V design. The duty cycle is about
0.21. The 2-phase curve has a ratio of ~0.58 for a duty
cycle of 0.21. This 0.58 ratio of output ripple current to
the inductor ripple current DIr at 2.5A equals ~1.5A of the
output ripple current (?I L ).
The output ripple voltage has two components that are
related to the amount of bulk capacitance and effective
series resistance (ESR) of the output bulk capacitance.
The equation is:
D I L
D V OUT(P ? P) ≈ ? ? + ESR ? D I L
–30
30
226.2 422.4 618.6 814.8 1010
128.1 324.3 520.5 716.7 912.9
FREQUENCY (MHz)
4606 F02
? ?
? 8 ? f ? N ? C OUT ?
Figure 2. Radiated Emission Scan with 12V IN
to 2.5V OUT at 6A (1×100μF X7R Ceramic C OUT )
where f is the frequency and N is the number of paralleled
phases.
4606fd
12
For more information www.linear.com/LTM4606
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