ltm4615 Linear Technology Corporation, ltm4615 Datasheet - Page 11

• Manufacturer Part Number: ltm4615
• Description: Triple Output, Low Voltage Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

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 specifi c external capacitor
requirements for a particular application.
V
There are restrictions in the maximum V
down ratio than can be achieved for a given input voltage
on the two switching regulators. The LTM4615 is 100%
duty cycle, but the V
a function the load current. A typical 0.5V minimum is
suffi cient.
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 V
The output voltage will default to 0.8V with no feedback
resistor. Adding a resistor R
programs the output voltage:
Table 1. FB Resistor Table vs Various Output Voltages
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.
IN
V
FB
V
OUT
to V
OUT
OUT
=
Open
0.8V
0 8
Step-Down Ratios
.
V
•
4 99
1.2V
10k
.
IN
R
to V
k R
FB
+
5.76k
1.5V
OUT
FB
FB
minimum dropout will be
from the FB pin to GND
OUT
3.92k
1.8V
and FB pins together.
IN
and V
2.37k
2.5V
OUT
1.62k
3.3V
step-
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. 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 fi ltering 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.
D
I
CIN RMS
=
(
V
V
OUT
IN
)
=
I
OUT MAX
η
(
%
)
•
D
• –
(
1
D
)
LTM4615
11
4615f