LTM4618EVPBF datasheet(10/24 Pages) LINER

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LTM4618

4618f

The typical LTM4618 application circuit is shown in

Figure 21. External component selection is primarily

determined by the maximum load current and output

voltage.

VIN to VOUT Step-Down Ratios

There are restrictions in the maximum VIN to VOUT step-

down ratio that can be achieved for a given input voltage.

One of the restrictions is the minimum on-time tON(MIN),

which is the smallest time duration that the LTM4618 can

operate. Make sure that the operating on-time is larger than

the minimum on-time as shown in the equation below. See

the Thermal Considerations and Output Current Derating

sections in this data sheet for the current restrictions.

tON(MIN) is approximately 90ns, guardband to 110ns.

tON(MIN) <

VOUT

VIN •ƒ

Output Voltage Programming

The PWM controller has an internal 0.8V reference voltage.

As shown in the Block Diagram, a 60.4k internal feedback

resistor connects VOUT to the VFB pin. Adding a resistor RFB

from the VFB pin to SGND programs the output voltage:

VOUT = 0.8V •

60.4k +RFB

RFB

Table 1. VFB Resistor Table vs Various Output Voltages

VOUT (V)

0.8

1.2

1.5

1.8

2.5

3.3

RFB (kΩ)

Open

243

121

69.8

48.7

28.7

19.1

11.5

APPLICATIONS INFORMATION

Input Capacitors

The LTM4618 module should be connected to a low AC-

impedance DC source. One 1.5μF input ceramic capacitor

is included inside the module. Additional input capacitors

are only needed if a large load step is required up to

the 6A level. A 47μF to 100μF surface mount aluminum

electrolytic bulk capacitor can be used for more input bulk

capacitance. This bulk input capacitor is only needed if

the input source impedance is compromised by long in-

ductive leads, traces or not enough source capacitance.

If low impedance power planes are used, then this 47μF

capacitor is not needed.

For a buck converter, the switching duty-cycle can be

estimated as:

VOUT

VIN

Without considering the inductor current ripple, the RMS

current of the input capacitor can be estimated as:

ICIN(RMS) =

IOUT(MAX)

•D •(1–D)

In the above equation,

η is the estimated efﬁciency of

the power module. One 10μF ceramic input capacitor is

typically rated for 2A of RMS ripple current, so the RMS

input current at the worst case 6A maximum current is

about 3A. If a low inductance plane is used to power the

device, then two 10μF ceramic capacitors are enough for

the output at 6A load and no external input bulk capacitor

is required. The input RMS ripple current can be cancelled

by paralleling multiple LTM4618 power modules out of

phase, allowing the use of fewer input capacitors. Ap-

plication Note 77 explains the details.

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