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.
V
IN
to V
OUT
Step-Down Ratios
There are restrictions in the maximum V
IN
to V
OUT
step-
down ratio that can be achieved for a given input voltage.
Each output of the LTM4616 is capable of 100% duty
cycle, but the V
IN
to V
OUT
minimum drop out is still shown
as a function of its load current. For a 5V input voltage,
both outputs can deliver 8A for any output voltage. For a
3.3V input, all outputs can deliver 8A, except 2.5V
OUT
and
above which is limited to 6A. All outputs derived from a
2.7V input voltage are limited to 5A.
Output Voltage Programming
Each PWM controller has an internal 0.596V reference
voltage. As shown in the Block Diagram, a 10k internal
feedback resistor connects V
OUT
and FB pins together.
The output voltage will default to 0.596V with no feed-
back resistor. Adding a resistor R
FB
from FB pin to GND
programs the output voltage:
V
OUT
=
0.596V •
10k
+
R
FB
R
FB
1.5V
6.65k
1.8V
4.87k
2.5V
3.09k
3.3V
2.21k
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
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
=
V
OUT
V
IN
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
I
CIN(RMS)
=
I
OUT(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 input bulk
capacitor can be a switcher-rated aluminum electrolytic
capacitor or polymer capacitor. 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 C
OUT
are
chosen with low enough effective series resistance (ESR)
to meet the output voltage ripple and transient require-
ments. C
OUT
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 ripple or dynamic transient
spikes is desired. 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. LTpowerCAD is available
4616fe
Table 2. FB Resistor vs Various Output Voltages
V
OUT
R
FB
0.596V
Open
1.2V
10k
For parallel operation of N number of outputs, the below
equation can be used to solve for R
FB
. Tie the FB pins
together for each paralleled output with a single resistor
to ground as determined by:
R
FB
=
10k / N
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
For more information
11
LINEAR_DIMENSIONS [ LINEAR DIMENSIONS SEMICONDUCTOR ]
