LTM4606V datasheet(11/24 Pages) LINER

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LTM4606

4606f

Figure 2. Conducted Emission Scan with 12VIN to 2.5VOUT at 6A

× 10μF Ceramic Capacitors on VIN pads and 1× 10μF Ceramic

Capacitor on VD Pads)

APPLICATIONS INFORMATION

directly adjacent to the module VD 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 VIN 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.

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 (ΔIL).

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:

fN C

ESR

OUT P P

OUT

()

•• •

•

− ≈

⎛

⎝⎜

⎞

⎠⎟

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.

FREQUENCY (MHz)

0.15

4606 F02

CIS25QP

Output Capacitors

The LTM4606 is designed for low output voltage ripples.

The bulk output capacitors deﬁned as COUT are chosen

with low enough effective series resistance (ESR) to meet

the output voltage ripple and transient requirements. COUT

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

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