LTM4606 Datasheet, PDF (11/24 Pages) Linear Technology – Ultralow EMI 28VIN, 6A DC/DC μModule

LTM4606
APPLICATIONS INFORMATION
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 2 to 3 amps
of RMS ripple current. Refer to your ceramics capacitor
catalog for the RMS current ratings.
To attenuate high frequency noise, extra input capacitors
should be connected to the V_IN pads and placed before
the high frequency inductor to form the ∏ filter. One of
these low ESR ceramic capacitors is recommended to
be placed close to the connection into the system board.
A large bulk 100μF capacitor is only needed if the input
source impedance is compromised by long inductive
leads or traces. Figure 2 shows the conducted EMI
testing results to meet the level 5 of CISPR 25. For differ-
ent applications, input capacitance may be varied to meet
different conducted EMI limits.
80
70
60
50
CIS25QP
40
30
20
10
0
0.15
1
10
30
FREQUENCY (MHz)
4606 F02
Figure 2. Conducted Emission Scan with 12V_IN to 2.5V_OUT at 6A
(3× 10μF Ceramic Capacitors on V_IN pads and 1× 10μF Ceramic
Capacitor on V_D Pads)
Output Capacitors
The LTM4606 is designed for low output voltage ripples.
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, low ESR polymer
capacitor or 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
spike is required. Table 2 shows a matrix of different output
voltages and output capacitors to minimize the voltage
droop and overshoot during a 3A/μs transient. The table
optimizes total equivalent ESR and total bulk capacitance
to maximize transient performance.
Multiphase operation with multiple LTM4606 devices in
parallel will lower the effective output ripple current due
to the phase interleaving operation. Refer to Figure 3
for the normalized output ripple current versus the duty
cycle. Figure 3 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 DIr at zero duty cycle
is ~2.5A for a 12V to 2.5V design. The duty cycle is about
0.21. The 2-phase curve has a ratio of ~0.58 for a duty
cycle of 0.21. This 0.58 ratio of output ripple current to
the inductor ripple current DIr at 2.5A equals ~1.5A of the
output ripple current (ΔI_L).
The output voltage ripple has two components that are
related to the amount of bulk capacitance and effective
series resistance (ESR) of the output bulk capacitance.
The equation is:
ΔV_{OUT(P−P)}
≈
⎛
⎝⎜
8
•
f
ΔI_L
• N • C_OUT
⎞
⎠⎟
+
ESR
•
ΔI_L
Where f is the frequency and N is the number of paral-
leled phases.
Fault Conditions: Current Limit and Overcurrent
Foldback
LTM4606 has a current mode controller, which inher-
ently limits the cycle-by-cycle inductor current not only
in steady-state operation, but also in transient.
To further limit current in the event of an overload condi-
tion, the LTM4606 provides foldback current limiting. If the
output voltage falls by more than 50%, then the maximum
output current is progressively lowered to about one sixth
of its full current limit value.
4606f
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