Datasheet

LTM4649
10
4649f
For more information www.linear.com/LTM4649
APPLICATIONS INFORMATION
The typical LTM4649 application circuit is shown in Fig-
ure17. External component selection is primarily deter-
mined by the maximum load current and output voltage.
Refer to Table 3 for specific external capacitor requirements
for particular applications.
V
IN
to V
OUT
Step-Down Ratios
There are restrictions in the V
IN
to V
OUT
step-down ratio
that can be achieved for a given input voltage. The V
IN
to
V
OUT
minimum dropout is a function of load current and
at very low input voltage and high duty cycle applications
output power may be limited as the internal top power
MOSFET is not rated for 10A operation at higher ambient
temperatures. At very low duty cycles the minimum 110ns
on-time must be maintained. See the Frequency Adjust-
ment section and temperature derating curves.
Output Voltage Programming
The PWM controller has an internal 0.6V ±0.5% refer-
ence voltage. As shown in the Block Diagram, a 10k
0.5% internal feedback resistor connects the V
OUT_LCL
and V
FB
pins together. When the remote sense amplifier
is used, then DIFFOUT is connected to the V
OUT_LCL
pin.
If the remote sense amplifier is not used, then V
OUT_LCL
connects to V
OUT
. The output voltage will default to 0.6V
with no feedback resistor. Adding a resistor R
FB
from V
FB
to ground programs the output voltage:
V
OUT
= 0.6V •
10k + R
FB
R
FB
Table 1. V
FB
Resistor Table vs Various Output Voltages
V
OUT
(V) 0.6 1.0 1.2 1.5 1.8 2.5 3.3
R
FB
(k) OPEN 15 10 6.65 4.99 3.09 2.21
For parallel operation of N LTM4649, the following equa-
tion can be used to solve for R
FB
:
R
FB
=
10k
N
V
OUT
0.6
– 1
In parallel operation the V
FB
pins have an I
FB
current of
20nA maximum each channel. To reduce output voltage
error due to this current, an additional V
OUT_LCL
pin can
be tied to V
OUT
, and an additional R
FB
resistor can be used
to lower the total Thevenin equivalent resistance seen by
this current.
Input Capacitors
The LTM4649 module should be connected to a low AC
impedance DC source. Additional input capacitors are
needed for the RMS input ripple current rating. The I
CIN(RMS)
equation which follows can be used to calculate the input
capacitor requirement. Typically 22µF X7R ceramics are a
good choice with RMS ripple current ratings of ~2A each.
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 bulk 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 ripple current, for each
output, the RMS current of the input capacitor can be
estimated as:
I
CIN(RMS)
=
I
OUT(MAX)
η%
• D • 1− D
( )
In the previous equation, η% is the estimated efficiency of
the power module. The bulk capacitor can be a switcher-rat-
ed electrolytic aluminum capacitor or a Polymer capacitor.
Output Capacitors
The LTM4649 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 capacitors. The typical output
capacitance range is from 200µF to 470µF. Additional output
filtering may be required by the system designer if further
reduction of output ripple or dynamic transient spikes is