Datasheet

LTM4601AHV
11
4601ahvfc
For more information www.linear.com/LTM4601AHV
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 Figures 19 and 20, the 10µF ceramic capacitors are to
-
gether 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 LTM4601AHV devices
in parallel will lower the effective input RMS ripple cur
-
rent due to the interleaving operation of the regulators.
Application 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 LTM4601AHV 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 cur
-
rent to a DC load current of 24A equals ~6A of input RMS
ripple current for the external input capacitors.
Output Capacitors
The LTM4601AHV 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 ripple voltage 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)
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
4601AHV F02
RMS INPUT RIPPLE CURRENT
DC LOAD CURRENT
6-PHASE
4-PHASE
12-PHASE
3-PHASE
2-PHASE
1-PHASE
applicaTions inForMaTion