ltm4619 Linear Technology Corporation, ltm4619 Datasheet - Page 9

• Manufacturer Part Number: ltm4619
• Description: Dual, 26vin, 4a Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The typical LTM4619 application circuit is shown in
Figure 18. External component selection is primarily
determined by the maximum load current and output
voltage.
Output Voltage Programming
The PWM controller has an internal 0.8V reference voltage.
As shown in the block diagram, a 60.4k internal feedback
resistor R
will default to 0.8V with no feedback resistor. Adding a
resistor R
voltage:
Table 1. V
V
R
Input Capacitors
The LTM4619 module should be connected to a low AC-
impedance DC source. Two 1.5μF input ceramic capacitors
are included inside the module. Additional input capacitors
are needed if a large load 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 capacitor is only needed if the input source im-
pedance is compromised by long inductive leads, traces
or not enough source capacitance.
For a buck converter, the switching duty-cycle can be
estimated as:
OUT
SET
D =
V
(kΩ)
(V)
OUT
V
V
OUT
= 0.8V •
FB
IN
FB
SET
Open
Resistor Table vs Various Output Voltages
0.8
connects V
from V
60.4k + R
121
1.2
FB
R
pin to SGND programs the output
OUT
SET
68.1
1.5
to V
SET
48.7
FB
1.8
pin. The output voltage
28.0
2.5
19.1
3.3
11.5
5
Without considering the inductor current ripple, for each
output, 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 capacitor, polymer capacitor for
bulk input capacitance due to high inductance traces or
leads. One 10μF ceramic input capacitor is typically rated
for 2A of RMS ripple current, so the RMS input current
at the worst case for each output at 4A maximum current
is about 2A. If a low inductance plane is used to power
the device, then two 10μF ceramic capacitors are enough
for both outputs at 4A load and no external input bulk
capacitor is required.
Output Capacitors
The LTM4619 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
low ESR polymer capacitor or ceramic capacitor. The typical
output capacitance range for each output 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. The Linear Technology
μModule Power Design Tool will be provided for stability
analysis. Multiphase operation will reduce effective output
ripple as a function of the number of phases. Application
Note 77 discusses this noise reduction versus output ripple
current cancellation, but the output capacitance should be
considered carefully as a function of stability and transient
response. The Linear Technology μModule Power Design
Tool can calculate the output ripple reduction as the number
of implemented phases increased by N times.
I
CIN(RMS)
OUT
=
can be the low ESR tantalum capacitor, the
I
OUT(MAX)
η
• D • (1− D)
LTM4619
OUT
9
4619f
are