LTM4616(RevA) 查看數據表（PDF） - Linear Technology

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LTM4616
(Rev.:RevA)

Dual 8A per Channel Low VIN DC/DC µModule

Linear Technology 

LTM4616

APPLICATIONS INFORMATION

The typical LTM4616 application circuit is shown in

Figure 18. External component selection is primarily

determined by the maximum load current and output

voltage. Refer to Table 3 for specific external capacitor

requirements for a particular application.

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.

Each output of the LTM4616 is 100% duty cycle, but the

VIN to VOUT minimum drop out is still shown as a function

of its load current. For 5V input, all outputs can deliver 8A.

For 3.3V input, all outputs can deliver 8A, except 2.5VOUT

which is limited to 6A. All outputs derived from 2.7V input

are limited to 5A.

Output Voltage Programming

The PWM controller has an internal 0.596V reference

voltage. As shown in the Block Diagram, a 10k 0.5%

internal feedback resistor connects VOUT and FB pins

together. The output voltage will default to 0.596V with

no feedback resistor. Adding a resistor RFB from FB pin

to GND programs the output voltage:

VOUT

=

0.596V

•

10k + RFB

RFB

Table 2. FB Resistor vs Various Output Voltages

VOUT 0.596V 1.2V

RFB Open 10k

1.5V

6.65k

1.8V

4.87k

2.5V

3.09k

3.3V

2.21k

For parallel operation of N the below equation can be

used to solve for RFB. Tying the FB pins together for each

parallel output:

RFB =

10k / N

VOUT − 1

0.596

Input Capacitors

The LTM4616 module should be connected to a low AC

impedance DC source. For each regulator, three 10μF

ceramic capacitors are included inside the module.

Additional input capacitors are only needed if a large load

10

step 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 input

capacitor is only needed if the input source impedance is

compromised by long inductive 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:

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 so the RMS input current at the worst case

for 8A maximum current is about 4A. The bulk capacitor

can be a switcher-rated electrolytic aluminum capacitor,

polymer capacitor for bulk input capacitance. Each internal

10μF ceramic input capacitor is typically rated for 2 amps

of RMS ripple current.

Output Capacitors

The LTM4616 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 requirements.

COUT can be a low ESR tantalum capacitor, low ESR

polymer capacitor or ceramic capacitor. The typical output

capacitance range 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. Table 3 shows a matrix of different output voltages

and output capacitors to minimize the voltage droop and

overshoot during a 3A/μs transient. The table optimizes

total equivalent ESR and total bulk capacitance to optimize

the transient performance. Stability criteria are considered

in the Table 3 matrix, and the Linear Technology μModule

Power Design Tool will be provided for stability analysis.

4616fa

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