LTM4601 LINER [Linear Technology], LTM4601 Datasheet - Page 11

• Manufacturer Part Number: LTM4601
• Description: 12A DC/DC ?Modules with PLL, Output Tracking and Margining
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
For a buck converter, the switching duty-cycle can be
estimated as:
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
In the above equation, η% is the estimated effi ciency of
the power module. C
aluminum capacitor, OS-CON capacitor or high volume
ceramic capacitor. Note the capacitor ripple current rat-
ings 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. Application
Note 77 provides a detailed explanation. Refer to Figure 2
for the input capacitor ripple current requirement as a
function of the number of phases. The fi gure provides a
ratio of RMS ripple current to DC load current as function
of duty cycle and the number of paralleled phases. Pick
D
I
CIN RMS
=
(
V
V
OUT
IN
)
=
I
OUT MAX
η
(
%
U
IN
can be a switcher-rated electrolytic
)
•
U
D
• –
(
1
D
W
)
U
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 voltage ripple.
The bulk output capacitors defi ned as C
with low enough effective series resistance (ESR) to meet
the output voltage ripple and transient requirements. C
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 fi ltering 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 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 Factor for One to Six Modules (Phases)
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
LTM4601/LTM4601-1
0.2
0.3
DUTY FACTOR (V
0.4
1-PHASE
2-PHASE
3-PHASE
4-PHASE
6-PHASE
12-PHASE
0.5
OUT
0.6
/V
IN
0.7
)
0.8
4601 F02
OUT
0.9
are chosen
11
OUT
4601f