ltm4616 Linear Technology Corporation, ltm4616 Datasheet - Page 10

• Manufacturer Part Number: ltm4616
• Description: Dual 8a Per Channel Low Vin Dc/dc ?module
• Manufacturer: Linear Technology Corporation

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 specifi c external capacitor
requirements for a particular application.
V
There are restrictions in the maximum V
down ratio that can be achieved for a given input voltage.
Each output of the LTM4616 is 100% duty cycle, but the
V
of its load current. For 5V input, all outputs can deliver
8A. For 3.3V input, all outputs can deliver 8A, except 2.5V
which is limited to 6A. All outputs derived from 2.5V are
limited to 6A.
Output Voltage Programming
The PWM controller has an internal 0.596V reference
voltage. As shown in the Block Diagram, a 10kΩ 0.5%
internal feedback resistor connects V
together. The output voltage will default to 0.596V with
no feedback resistor. Adding a resistor R
to GND programs the output voltage:
Table 2. FB Resistor vs Various Output Voltages
For parallel operation of N the below equation can be
used to solve for R
parallel output.
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. Ad-
ditional input capacitors are only needed if a large load
10
IN
IN
V
R
R
V
OUT
to V
to V
FB
OUT
FB
=
OUT
OUT
= 0.596V •
0.596V
0.596
Open
V
10k / N
OUT
minimum drop out is still shown as a function
Step-Down Ratios
− 1
1.2V
FB
10k
: Tying the FB pins together for each
10k + R
R
FB
6.65k
1.5V
FB
4.87k
1.8V
OUT
IN
FB
3.09k
2.5V
to V
and FB pins
from FB pin
OUT
2.21k
3.3V
step-
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:
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
In the above equation, η% is the estimated effi ciency 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 defi ned as C
chosen with low enough effective series resistance (ESR)
to meet the output voltage ripple and transient require-
ments. C
polymer capacitor or ceramic capacitor. The typical output
capacitance range is from 47μF to 220μF . Additional output
fi ltering 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.
I
D =
CIN(RMS)
V
V
OUT
OUT
IN
can be a low ESR tantalum capacitor, low ESR
=
I
OUT(MAX)
η%
• D • 1– D
(
)
OUT
4616f
are