Datasheet

LTM4627
11
4627fc
For more information www.linear.com/LTM4627
APPLICATIONS INFORMATION
The typical LTM4627 application circuit is shown in Fig-
ure 18. External component selection is primarily deter-
mined by the maximum load current and output voltage.
Refer to T
able 4 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 15A operation at higher ambient
temperatures. At very low duty cycles the minimum 90ns
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 ±1% reference
voltage. As shown in the Block Diagram, a 60.4k internal
feedback resistor connects the V
OUT_LCL
and V
FB
pins
together. When the remote sense amplifier is used, then
DIFF_OUT 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 feed-
back resistor. Adding a resistor R
FB
from V
FB
to ground
programs the output voltage:
V
OUT
= 0.6V •
60.4k + 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 5.0
R
FB
(k) Open 90.9 60.4 40.2 30.1 19.1 13.3 8.25
For a given V
OUT
, R
FB
can be determined by:
R
FB
=
60.4k
V
OUT
0.6V
–
1
For parallel operation of N LTM4627s, the following equa-
tion can be used to solve for R
FB
:
R
FB
=
60.4k /N
V
OUT
0.6V
–1
Tie the V
FB
pins together for each parallel output. The
COMP pins must be tied together also.
Input Capacitors
The LTM4627 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-
rated electrolytic aluminum capacitor or a Polymer capacitor.