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LTM4613
APPLICATIONS INFORMATION
V OUT
? 3 ? R fSET ? 133k ?
? R fSET ? 2 ? 133k ? ?
D = OUT
? D ? ( 1 – D )
I CIN(RMS) =
Foroutputvoltagesmorethan12V,thefrequencycanbe
higher than 600kHz, thus reducing the efficiency signifi-
cantly. Additionally, the minimum off time 400ns normally
limits the operation when the input voltage is close to the
output voltage. Therefore, it is recommended to lower the
frequency in these conditions by connecting a resistor
(R fSET ) from the f SET pin to V IN , as shown in Figure 20.
f =
5 ? 10 ? 11 ?
The load current can affect the frequency due to its con-
stant on-time control. If constant frequency is a necessity,
the PLLIN pin can be used to synchronize the frequency
of the LTM4613 to an external clock subject to minimum
on-time and off-time limits, as shown in Figures 21 to 23.
Input Capacitors
LTM4613 is designed to achieve the low input conducted
EMI noise due to the fast switching of turn-on and turn-
off. Additionally, 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
capacitors, C1-C3, on the V D pins is used to handle most
of the RMS current into the converter, so careful attention
is needed for capacitors C1-C3 selection.
For a buck converter, the switching duty cycle can be
estimated as:
V
V IN
Withoutconsideringtheinductorcurrentripple,theRMS
current of the input capacitor can be estimated as:
I OUT(MAX)
η
In this 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 input capacitor,
or choose a capacitor rated at a higher temperature than
required. Always contact the capacitor manufacturer for
derating requirements.
In a typical 8A output application, three very low ESR,
X5R or X7R, 10μF ceramic capacitors are recommended
for C1-C3. This decoupling capacitance 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 2A of RMS
ripple current. Refer to your ceramics capacitor catalog
for the RMS current ratings.
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 imped-
ance is compromised by long inductive leads or traces.
Output Capacitors
The LTM4613 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 low ESR tantalum capacitor, low ESR polymer ca-
pacitor or ceramic capacitor. The typical capacitance is 4 ×
47μF 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 4A load transient. The table
optimizes total equivalent ESR and total bulk capacitance
to maximize transient performance.
Multiphase operation with multiple LTM4613 devices in
parallel will also lower the effective output ripple current
due to the phase interleaving operation. Refer to Figure 4
for the normalized output ripple current versus the duty
cycle. Figure 4 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 ? I L is ~5.0A for a 36V
4613f
12
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