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LTM4614
APPLICATIONS INFORMATION
V OUT
V IN
? D ? ( 1 – D )
I CIN(RMS) =
4.99k + R FB
V OUT = 0.8V ?
DualSwitchingRegulator
A typical LTM4614 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 specific external capacitor requirements for a
particular application.
V IN to V OUT Step-Down Ratios
There are restrictions in the maximum V IN and V OUT step-
down ratio than can be achieved for a given input voltage
on the two switching regulators. The LTM4614 is 100%
duty cycle capable, but the V IN to V OUT minimum dropout
will be a function the load current. A typical 0.5V minimum
is sufficient. See Typical Performance Characteristics.
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 OUT and FB pins together.
The output voltage will default to 0.8V with no externally
applied feedback resistor. Adding a resistor R FB from the
FB pin to GND programs the output voltage:
R FB
Table 1. FB Resistor Table vs Various Output Voltages
V OUT 0.8V 1.0V 1.2V 1.5V 1.8V 2.5V 3.3V
R FB Open 20k 10k 5.76k 3.92k 2.37k 1.62k
Input Capacitors
The LTM4614 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.
For a buck converter, the switching duty cycle can be
estimated as:
D =
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
I OUT(MAX)
η %
In the above equation, η % is the estimated efficiency of
the power module. The bulk capacitor can be a switcher-
rated aluminum electrolytic OS-CON or polymer capacitor.
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 worst-
case ripple current for the 4A maximum current is 2A or
less. An additional 10μF or 22μF local ceramic capacitor
can be used to supplement the internal capacitor with an
additional 1A to 2A ripple current rating. See Figure 11
for recommended PCB layout.
Output Capacitors
The LTM4614 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
requirements. The output capacitors can be low ESR tan-
talum capacitors, low ESR polymer capacitors or ceramic
capacitors. The typical output capacitance range is 66μF
to 100μF. Additional output filtering may be required by
the system designer if further reduction of output ripple
or dynamic transient spikes is required. Table 4 shows a
matrix of different 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.
See Figure 11 for recommended PCB layout.
4614fb
8
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