LTM4619EVPBF datasheet(9/24 Pages) LINER

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LTM4619

4619f

The typical LTM4619 application circuit is shown in

Figure 18. External component selection is primarily

determined by the maximum load current and output

voltage.

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 RFB connects VOUT to VFB pin. The output voltage

will default to 0.8V with no feedback resistor. Adding a

resistor RSET from VFB pin to SGND programs the output

voltage:

VOUT = 0.8V •

60.4k +RSET

RSET

Table 1. VFB Resistor Table vs Various Output Voltages

VOUT (V)

0.8

1.2

1.5

1.8

2.5

3.3

RSET (kΩ)

Open

121

68.1

48.7

28.0

19.1

11.5

Input Capacitors

The LTM4619 module should be connected to a low AC-

impedance DC source. Two 1.5μF input ceramic capacitors

are included inside the module. Additional input capacitors

are needed if a large load is required up to the 4A level. A

47μF to 100μF surface mount aluminum electrolytic bulk

capacitor can be used for more input bulk capacitance.

This bulk capacitor is only needed if the input source im-

pedance is compromised by long inductive leads, traces

or not enough source capacitance.

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

estimated as:

VOUT

VIN

Without considering the inductor current ripple, for each

output, 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. The bulk capacitor can be a switcher-rated

electrolytic aluminum capacitor, polymer capacitor for

bulk input capacitance due to high inductance traces or

leads. One 10μF ceramic input capacitor is typically rated

for 2A of RMS ripple current, so the RMS input current

at the worst case for each output at 4A maximum current

is about 2A. If a low inductance plane is used to power

the device, then two 10μF ceramic capacitors are enough

for both outputs at 4A load and no external input bulk

capacitor is required.

Output Capacitors

The LTM4619 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 require-

ments. COUT can be the low ESR tantalum capacitor, the

low ESR polymer capacitor or ceramic capacitor. The typical

output capacitance range for each output is from 47μF to

220μF. Additional output ﬁltering may be required by the

system designer, if further reduction of output ripples or

dynamic transient spikes is required. The Linear Technology

μModule Power Design Tool will be provided for stability

analysis. Multiphase operation will reduce effective output

ripple as a function of the number of phases. Application

Note 77 discusses this noise reduction versus output ripple

current 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 increased by N times.

APPLICATIONS INFORMATION

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