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LTM4604

4604f

APPLICATIONS INFORMATION

A typical LTM4604 application circuit is shown in Figure 15.

External component selection is primarily determined by

the maximum load current and output voltage. Refer to

Table 4 for speciﬁc external capacitor requirements for a

particular application.

VIN to VOUT Step-Down Ratios

There are restrictions in the maximum VIN and VOUT step-

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

The LTM4604 is 100% duty cycle, but the VIN to VOUT

minimum dropout is a function of the load current. A typi-

cal 0.5V minimum is sufﬁcient (see Typical Performance

Characteristics).

Output Voltage Programming

The PWM controller has an internal 0.8V reference voltage.

As shown in the Block Diagram, a 4.99k, 0.5% internal

feedback resistor connects the VOUT and FB pins together.

The output voltage will default to 0.8V with no feedback

resistor. Adding a resistor RFB from the FB pin to GND

programs the output voltage:

OUT

499

.•

Table 1. FB Resistor vs Output Voltage

VOUT

0.8V

1.2V

1.5V

1.8V

2.5V

3.3V

RFB

Open

10k

5.76k

4.02k

2.37k

1.62k

Input Capacitors

The LTM4604 module should be connected to a low ac-

impedance DC source. Two 10μF ceramic capacitors are

included inside the module. Additional input capacitors

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

full 4A level. An input 47μF bulk capacitor is only needed

if the input source impedance is compromised by long

inductive leads or traces.

For a buck converter, the switching duty cycle can be

estimated as:

OUT

Without considering the inductor current ripple, the RMS

current of the input capacitor can be estimated as:

CIN RMS

OUT MAX

()

••

–

()

In the above equation,

η% is the estimated efﬁciency of

the power module. The bulk capacitor can be a switcher-

rated electrolytic aluminum capacitor, OS-CON capacitor

for bulk input capacitance due to high inductance traces

or leads. If a low inductance plane is used to power the

device, then no input capacitance is required. The two

internal 10μF ceramics are typically rated for 2A to 3A of

RMS ripple current. The worst-case ripple current for the

4A maximum current is 2A or less.

Output Capacitors

The LTM4604 is designed for low output voltage ripple.

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 an X5R/X7R ceramic capacitor. The typical

output capacitance range is 22μF to 100μF. Additional

output ﬁltering may be required by the system designer

if further reduction of output ripple or dynamic transient

spike is required. Table 4 shows a matrix of different

output voltages and output capacitors to minimize the

voltage droop and overshoot during a 2A/μs transient.

The table optimizes the total equivalent ESR and total

bulk capacitance to maximize transient performance. The

Linear Technology μModule Power Design Tool can be for

further optimization.

Fault Conditions: Current Limit and Overcurrent

Foldback

The LTM4604 has current mode control, 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 LTM4604 provides foldback current limiting as

the output voltage falls. The LTM4604 device has over-

temperature shutdown protection that inhibits switching

operation around 150°C.

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