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LTM4611IV-PBF Datasheet PDF : 28 Pages
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LTM4611
Applications Information
The typical LTM4611 application circuit is shown in
Figure 21. External component selection is primarily
determined by the maximum load current and output
voltage. Refer to Table 5 for specific external capacitor
requirements for particular applications.
VIN to VOUT Step-Down Ratios
There are restrictions in the VIN to VOUT step-down
ratio that can be achieved for a given input voltage. The
VIN to VOUT minimum dropout is still a function of its load
current at very low input voltages. A dropout voltage of
300mV from input to output of LTM4611 is achievable
at 15A load, but reflected input voltage ripple and noise
should be taken into consideration in such applications.
Additionally, the transient-handling capability of the source
supply feeding LTM4611 can become an important factor
in truly achieving ultralow dropout at high output current.
For example, VIN can sag or overshoot dramatically when
LTM4611 responds to heavy transient step loads on its
output, if insufficient input bypass capacitance is used in
combination with a sluggish source supply.
When VOUT is expected to be within 600mV of VIN, or
when the caliber of the source supply is in question, it
is recommended to evaluate the amount and quality of
input bypass capacitance needed to maintain one’s target
dropout voltage with the source supply that will be used
in the end application. Demo Board DC1588A can be used
for such evaluation.
At very low duty cycles the minimum specified on-time
must be maintained. See the Frequency Adjustment sec-
tion and temperature derating curves.
To prevent overstress to the µpower bias generator, do
not ramp up VIN at a rate exceeding 5V/µs (in practice, it
is difficult to violate this guideline.) There is no restriction
on how rapidly VIN may be discharged.
Output Voltage Programming
The PWM controller has an internal 0.8V ±1.75% reference
voltage over temperature. As shown in the Block Diagram,
a 60.4k internal feedback resistor connects the VOUT_LCL
and VFB pins together. When the remote sense amplifier
is used, then DIFFVOUT is connected to the VOUT_LCL pin.
If the remote sense amplifier is not used, then VOUT_LCL
connects to VOUT. The output voltage will default to 0.8V
with no feedback resistor. Adding a resistor RFB from VFB
to GND programs the output voltage:
VOUT
=
0.8V
•
60.4k +R
R FB
FB
Table 1. VFB Resistor Table vs Various Output Voltages
VOUT
0.8V 1.0V 1.2V 1.5V 1.8V 2.5V 3.3V 5.0V
RFB (kΩ) Open 243 121 68.1 47.5 28.0 19.1 11.5
For parallel operation of N LTM4611s, the following equa-
tion can be used to solve for RFB:
RFB
=
60.4k
VOUT
/N
–1
0.8V
Tie the VFB pins together for each parallel output. The
COMP, TRACK/SS, VOUT_LCL, and RUN pins must also be
tied together as shown in Figures 18 and 19.
For parallel applications, best noise immunity can be
achieved by placing capacitors of value CP from VFB to GND,
and value CFF from VOUT to VFB, local to each µModule.
If space limitations impede realizing this, then placement
of capacitors of value N • CP from VFB to GND, and value
N • CFF from VOUT to the bussed VFB signal, can suffice.
Input Capacitors
The LTM4611 module should be connected to a low
AC impedance DC source. Additional input capacitors
are needed for the RMS input ripple current rating. The
ICIN(RMS) equation which follows can be used to calculate
the input capacitor requirement. Typically 22µF X7R ce-
ramics are a good choice with RMS ripple current ratings
of ~2A each. A 100µF to 150µF 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.
4611f
11
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