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LTM4615
APPLICATIONS INFORMATION
V OUT = 0.8V ?
V OUT
I CIN(RMS) =
? D ? ( 1 – D )
R FB =
V OUT
? 1
DualSwitchingRegulator
The typical LTM4615 application circuit is shown in Fig-
ure 12. External component selection is primarily deter-
mined 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 to V OUT step-
down ratio that can be achieved for a given input voltage
on the two switching regulators. The LTM4615 is 100%
duty cycle, but the V IN to V OUT minimum dropout will be
a function the load current. A typical 0.5V minimum is
sufficient.
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 feedback
resistor. Adding a resistor R FB from the FB pin to GND
programs the output voltage:
4.99k + R FB
R FB
or equivalently,
4.99k
0.8V
Table 1. FB Resistor Table vs Various Output Voltages
V OUT 0.8V 1.2V 1.5V 1.8V 2.5V 3.3V
FB Open 10k 5.76k 3.92k 2.37k 1.62k
Input Capacitors
The LTM4615 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
current requirements. A 47μF bulk capacitor can be used
for more input capacitance. This 47μF capacitor is only
needed if the input source impedance is compromised by
long inductive leads or traces. The bulk capacitor can be
a switcher-rated aluminum electrolytic OS-CON capacitor.
For a buck converter, the switching duty cycle can be
estimated as:
D =
V IN
Without considering the inductor ripple current, 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. 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 worse-case ripple current for the 4A maximum current
is 2A or less. An additional 10μF or 22μF ceramic capacitor
can be used to supplement the internal capacitor with an
additional 1A to 2A ripple current rating.
Output Capacitors
The LTM4615 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 a low ESR
tantalum capacitor, low ESR polymer capacitor or ceramic
capacitor. 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.
11
For more information www.linear.com/LTM4615
4615fb
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