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LTM4615

4615f

APPLICATIONS INFORMATION

Dual Switching Regulator

The typical LTM4615 application circuit is shown in

Figure 12. 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 than can be achieved for a given input voltage

on the two switching regulators. The LTM4615 is 100%

duty cycle, but the VIN to VOUT minimum dropout will be

a function the load current. A typical 0.5V minimum is

sufﬁcient.

Output Voltage Programming

Each regulator channel has an internal 0.8V reference

voltage. As shown in the block diagram, a 4.99k 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 Table vs Various Output Voltages

VOUT

0.8V

1.2V

1.5V

1.8V

2.5V

3.3V

Open

10k

5.76k

3.92k

2.37k

1.62k

Input Capacitors

The LTM4615 module should be connected to a low AC

impedance DC source. One 4.7μF ceramic capacitor is

included inside the module for each regulator channel.

Additional input capacitors are needed if a large load step

is required up to the full 4A level and for RMS ripple cur-

rent requirements. A 47μF bulk capacitor can be used for

more input bulk capacitance. This 47μF 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 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 internal 4.7μF

ceramics on each channel input are typically rated for 1A of

RMS ripple current up to 85°C operation. The worse-case

ripple current for the 4A maximum current is 2A or less.

An additional 10μF or 22μF ceramic capacitor can be used

to supplement the internal capacitor with an additional 1A

to 2A ripple current rating.

Output Capacitors

The LTM4615 switchers are designed for low output volt-

age ripple on each channel. The bulk output capacitors are

chosen with low enough effective series resistance (ESR)

to meet the output voltage ripple and transient require-

ments. The output capacitors can be a low ESR tantalum

capacitor, low ESR polymer capacitor or ceramic capacitor.

The typical output capacitance range is 66μF 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 dif-

ferent output voltages and output capacitors to minimize

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

The table optimizes total equivalent ESR and total bulk

capacitance to maximize transient performance.

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