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LTM4619
APPLICATIONS INFORMATION
V OUT = 0.8V ?
I CIN(RMS) =
I OUT(MAX)
η
R SET =
– 1 ?
? ?
The typical LTM4619 application circuit is shown in
Figure 18. External component selection is primarily deter-
mined by the maximum load current and output voltage.
Output Voltage Programming
The PWM controller has an internal 0.8V reference voltage.
As shown in the block diagram, a 60.4k internal feedback
resistor R FB connects V OUT to V FB pin. The output voltage
will default to 0.8V with no feedback resistor. Adding a
resistor R SET from V FB pin to SGND programs the output
voltage:
60.4k + R SET
R SET
or equivalently
60.4k
? V OUT ?
0.8V ?
Table 1. R SET Resistor Table vs Various Output Voltages
V OUT (V) 0.8 1.2 1.5 1.8 2.5 3.3 5
R SET (k Ω ) Open 121 68.1 48.7 28.0 19.1 11.5
Input Capacitors
The LTM4619 module should be connected to a low AC-
impedance DC source. Two 1.5μF input ceramic capacitors
are included inside the module. Additional input capacitors
are needed if a large load is required up to the 4A level.
A 47μF to 100μF surface mount aluminum electrolytic
capacitor can be used for more input bulk capacitance.
This bulk capacitor is only needed if the input source im-
pedance is compromised by long inductive leads, traces
or not enough source capacitance.
For a buck converter, the switching duty-cycle can be
estimated as:
Without considering the inductor ripple current, for each
output, the RMS current of the input capacitor can be
estimated as:
? D ?(1 ? D)
In the above equation, η is the estimated efficiency of the
power module. The bulk capacitor can be a switcher-rated
aluminum electrolytic capacitor or a polymer capacitor. One
10μF ceramic input capacitor is typically rated for 2A of
RMS ripple current, so the RMS input current at the worst
case for each output at 4A maximum current is about 2A.
If a low inductance plane is used to power the device, then
two 10μF ceramic capacitors are enough for both outputs
at 4A load and no external input bulk capacitor is required.
Output Capacitors
The LTM4619 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, a low
ESR polymer capacitor or ceramic capacitor. The typical
output capacitance range for each output is from 47μF
to 220μF. Additional output filtering may be required by
the system designer. If further reduction of output ripple
or dynamic transient spikes is required, LTpowerCAD is
available for stability analysis. Multiphase operation will
reduce effective output ripple as a function of the num-
ber of phases. Application Note 77 discusses this noise
reduction versus output ripple current cancellation, but
the output capacitance should be considered carefully as
a function of stability and transient response. LTpowerCAD
calculates the output ripple reduction as the number of
implemented phases increased by N times.
D =
V OUT
V IN
4619fb
For more information www.linear.com/LTM4619
9
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