LTM4618EVPBF datasheet(11/24 Pages) LINER

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LTM4618

4618f

APPLICATIONS INFORMATION

Output Capacitors

The LTM4618 is designed for low output voltage ripple

noise. 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, a low ESR

polymer capacitor or ceramic capacitor. The typical output

capacitance range is from 100μF to 300μF. Additional output

ﬁltering may be required by the system designer if further

reduction of output ripple or dynamic transient spikes is

required. Table 4 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 the

total equivalent ESR and total bulk capacitance to optimize

the transient performance. Stability criteria are considered

in the Table 4 matrix, and the Linear Technology μModule

Power Design Tool is available for stability analysis. Mul-

tiphase operation will reduce effective output ripple as a

function of the number of phases. Application Note 77

discusses this noise reduction versus output ripple cur-

rent cancellation, but the output capacitance should be

considered carefully as a function of stability and transient

response. The Linear Technology μModule Power Design

Tool can calculate the output ripple reduction as the number

of implemented phases increases by N times.

Mode Selections and Phase-Locked Loop

The LTM4618 can be enabled to enter high efﬁciency

Burst Mode operation, constant-frequency, pulse-skipping

mode, or forced continuous conduction mode. To select

the forced continuous operation, tie the MODE/PLLIN pin

to ground. To select pulse-skipping mode of operation,

tie the MODE/PLLIN pin to INTVCC. To select Burst Mode

operation, ﬂoat the pin.

A phase-locked loop (PLL) is available on the LTM4618

to synchronize the internal oscillator to an external clock

source that is connected to the MODE/PLLIN pin. The

incoming clock should be applied before the regulator’s

RUN pin is enabled.

Frequency Selection

The switching frequency of the LTM4618’s controller can

be selected using a DC voltage. If the MODE/PLLIN pin

is not being driven by an external clock source, the FREQ

pin can program the controller’s operating frequency

from 250kHz to 780kHz by connecting a resistor divider

as shown in Figure 21. The typical frequency is 750kHz.

But if the minimum on-time is reached, a lower frequency

needs to be set to increase the turn-on time. Otherwise, a

signiﬁcant amount of cycle skipping can occur with cor-

respondingly larger ripple current and voltage ripple.

FREQ PIN VOLTAGE (V)

0.5

1.5

4618 F03

2.5

100

300

400

500

900

800

700

200

600

Figure 3. Relationship Between Switching

Frequency and Voltage at the FREQ Pin

Frequency Synchronization

The MODE/PLLIN pin allows the LTM4618 to be synchro-

nized to an external clock (between 400kHz to 780kHz)

and the internal phase-locked loop allows the LTM4618

to lock onto input clock phase as well. The FREQ pin has

the onboard loop ﬁlter for the PLL. The incoming clock

must be applied before the RUN pin is enabled. For ap-

plications powering the clock source from the LTM4618’s

INTVCC, the RUN pin has to be enabled in order to acti-

vate INTVCC for the clock source. In this situation (see

Figure 22) the TK/SS pin can be used to soft-start the

regulator for 100ms using a ≈ 0.22μF capacitor. This will

allow the regulator to synchronize to the right frequency

before the regulator’s inductor ripple current peaks.

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