LTM4603/LTM4603-1
12
4603fb
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 6A output application, two 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 LTM4603 devices in
parallel will lower the effective input RMS ripple current 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 func-
tion of the number of phases. The figure provides a ratio
of RMS ripple current to DC load current as a function 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 LTM4603 design provides 10A 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 10A equals ~2.5A of input RMS ripple
current for the external input capacitors.
Output Capacitors
The LTM4603 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 2.5A/µs transient.
The table optimizes total equivalent ESR and total bulk
capacitance to maximize transient performance.
Multiphase operation with multiple LTM4603 devices in
parallel will lower the effective output ripple current due to
the interleaving operation of the regulators. For example,
each LTM4603’s inductor current in a 12V to 2.5V multi-
phase design can be read from the Inductor Ripple Current
vs Duty Cycle graph (Figure 3). The large ripple current
at low duty cycle and high output voltage can be reduced
by adding an external resistor from f
SET
to ground which
increases the frequency. If we choose the duty cycle of
DC = 2.5V/12V = 0.21, the inductor ripple current for 2.5V
output at 21% duty cycle is ~3A in Figure 3.
Figure 2. Normalized Input RMS Ripple Current
vs Duty Cycle for One to Six Modules (Phases)
Figure 3. Inductor Ripple Current vs Duty Cycle
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
4603 F02
RMS INPUT RIPPLE CURRENT
DC LOAD CURRENT
6-PHASE
4-PHASE
3-PHASE
2-PHASE
1-PHASE
DUTY CYCLE (V
OUT
/V
IN
)
0
0
I
L
(A)
1
2
3
4
5
0.2 0.4 0.6 0.8
4603 F03
2.5V OUTPUT
5V OUTPUT
1.8V OUTPUT
1.5V OUTPUT
1.2V OUTPUT
3.3V OUTPUT WITH
82.5k ADDED FROM
V
OUT
TO f
SET
5V OUTPUT WITH
150k ADDED FROM
f
SET
TO GND
