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LTM4604AV

Low Voltage, 4A DC/DC µModule Regulator with Tracking

Analog Devices 

LTM4604A

APPLICATIONS INFORMATION

A typical LTM4604A 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 LTM4604A is 100% duty cycle capable, but the VIN

to VOUT minimum dropout is a function of the load cur-

rent. A typical 0.5V minimum is sufficient (see Typical

Performance Characteristics).

Output Voltage Programming

The PWM controller has an internal 0.8V reference volt-

age. 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 externally

applied feedback resistor. Adding a resistor RFB from the

FB pin to GND programs the output voltage:

VOUT = 0.8V

• 4.99k + RFB

RFB

Table 1. FB Resistor vs Output Voltage

VOUT 0.8V 1V 1.2V 1.5V

RFB Open 20k 10k 5.76k

1.8V

4.02k

2.5V

2.37k

3.3V

1.62k

Input Capacitors

The LTM4604A module should be connected to a low

AC-impedance DC source. Two 10µF ceramic capacitors

are included inside the module. Additional input capaci-

tors 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 esti-

mated as:

D = VOUT

VIN

Without considering the inductor current ripple, the RMS

current of the input capacitor can be estimated as:

ICIN(RMS)

=

IOUT(MAX)

η%

•

D • (1– D)

In the above equation, η% is the estimated efficiency of

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

rated aluminum electrolytic capacitor, OS-CON or

polymer capacitor. 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 LTM4604A 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

spikes 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

LTpowerCAD GUI is available for further optimization.

Fault Conditions: Current Limit and Overcurrent

Foldback

The LTM4604A 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 LTM4604A provides foldback current limiting as

the output voltage falls. The LTM4604A device has over-

temperature shutdown protection that inhibits switching

operation around 150°C.

Rev. D

10

For more information www.analog.com

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