Datasheet

LTM4601/LTM4601-1
12
4601fd
For more information www.linear.com/LTM4601
For a buck converter, the switching duty-cycle can be
estimated as:
D=
V
OUT
V
IN
Without considering the inductor ripple current, the RMS
current of the input capacitor can be estimated as:
I
CIN(RMS)
=
I
OUT(MAX)
η%
• D• 1–D
( )
In the above equation, η% is the estimated efficiency of
the power module. C
IN
can be a switcher-rated electrolytic
aluminum capacitor, OS-CON capacitor or high value ce-
ramic capacitor. 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 Figure 18, the 10µF ceramic capacitors are together
used as a high frequency
input decoupling capacitor. In a
typical 12A output application, three very low ESR, X5R or
X7R, 10µF ceramic capacitors are recommended. These
decoupling capacitors should be placed directly adjacent
to the module input 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.
Multiphase operation with multiple LTM4601 devices in
parallel will lower the effective input RMS ripple current
due to the interleaving operation of the regulators. Appli
-
cation Note 77 provides a detailed explanation. Refer to
Figure
2 for the input capacitor ripple current reduction as
a function of the number of phases. The figure provides
a ratio of RMS ripple current to DC load current as func
-
tion of duty cycle and the number of paralleled phases.
Pick the corresponding duty cycle and the number of phases
to arrive at the correct ripple current value. For example,
the 2-phase parallel LTM4601 design provides 24A at 2.5V
output from a 12V input. The duty cycle is DC = 2.5V/12V
= 0.21. The 2-phase curve has a ratio of ~0.25 for a duty
cycle of 0.21. This 0.25 ratio of RMS ripple current to a
DC load current of 24A equals ~6A of input RMS ripple
current for the external input capacitors.
Output Capacitors
The LTM4601 is designed for low output ripple voltage.
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, a low ESR polymer
capacitor or a 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
spikes is required. Table 2 shows a matrix of different
output voltages and output capacitors to minimize the
voltage droop and overshoot during a 5A/µs transient.
The table optimizes total equivalent ESR and total bulk
capacitance to maximize transient performance.
Figure 2. Normalized Input RMS Ripple Current
vs Duty Cycle for One to Six Modules (Phases)
applicaTions inForMaTion
DUTY CYCLE (V
OUT
/V
IN
)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.9
0.6
0.5
0.4
0.3
0.2
0.1
0
4601 F02
RMS INPUT RIPPLE CURRENT
DC LOAD CURRENT
6-PHASE
4-PHASE
12-PHASE
3-PHASE
2-PHASE
1-PHASE