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.
ceramic capacitors are included inside the module. Ad-
ditional input capacitors are only needed if a large load
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
compromisedbylonginductiveleads,tracesornotenough
source capacitance. If low impedance power planes are
used, then this 47µF capacitor is not needed.
V to V
Step-Down Ratios
IN
OUT
There are restrictions in the maximum V to V
step-
IN
OUT
down ratio that can be achieved for a given input voltage.
Each output of the LTM4616 is capable of 100% duty
For a buck converter, the switching duty-cycle can be
estimated as:
cycle, but the V to V
minimum drop out is still shown
IN
OUT
as a function of its load current. For a 5V input voltage,
VOUT
both outputs can deliver 8A for any output voltage. For a
3.3V input, all outputs can deliver 8A, except 2.5V
D =
V
and
IN
OUT
above which is limited to 6A. All outputs derived from a
2.7V input voltage are limited to 5A.
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
Output Voltage Programming
IOUT(MAX)
ICIN(RMS)
=
• D • 1– D
(
)
Each PWM controller has an internal 0.596V reference
voltage. As shown in the Block Diagram, a 10k internal
η%
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 input bulk
capacitor can be a switcher-rated aluminum electrolytic
capacitororpolymercapacitor.Eachinternal10µFceramic
input capacitor is typically rated for 2 amps of RMS ripple
current.
feedback resistor connects V
and FB pins together.
OUT
The output voltage will default to 0.596V with no feed-
back resistor. Adding a resistor R from FB pin to GND
FB
programs the output voltage:
10k + RFB
VOUT = 0.596V •
RFB
Table 2. FB Resistor vs Various Output Voltages
Output Capacitors
V
0.596V
Open
1.2V
10k
1.5V
1.8V
2.5V
3.3V
OUT
The LTM4616 is designed for low output voltage ripple
R
6.65k
4.87k
3.09k
2.21k
FB
noise. The bulk output capacitors defined as C
are
OUT
chosen with low enough effective series resistance (ESR)
For parallel operation of N number of outputs, the below
to meet the output voltage ripple and transient require-
equation can be used to solve for R . Tie the FB pins
FB
ments. C
can be a low ESR tantalum capacitor, low
together for each paralleled output with a single resistor
OUT
ESR polymer capacitor or ceramic capacitor. The typical
outputcapacitancerangeisfrom47µFto220µF.Additional
output filtering may be required by the system designer,
if further reduction of output ripple or dynamic transient
spikesisdesired.Table3showsamatrixofdifferentoutput
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
tooptimizethetransientperformance.Stabilitycriteriaare
to ground as determined by:
10k / N
R
=
FB
V
OUT
− 1
0.596
Input Capacitors
The LTM4616 module should be connected to a low AC
impedance DC source. For each regulator, three 10µF
considered in the Table 3 matrix. LTpowerCAD is available
4616fe
11
For more information www.linear.com/LTM4616
LINEAR_DIMENSIONS [ Linear Dimensions ]
