LTM8031
14
8031fb
applicaTions inForMaTion
rings as high as 35V and the input current peaks at 20A.
One method of damping the tank circuit is to add another
capacitor with a series resistor to the circuit, as shown
in Figure 4b. A 0.7Ω resistor is added in series with the
input to eliminate the voltage overshoot (it also reduces
the peak input current). A 0.1µF capacitor improves high
frequency filtering. For high input voltages its impact on
efficiency is minor, reducing efficiency less than one-half
percent for a 5V output at full load operating from 24V.
By far the most popular method of controlling overshoot
is shown in Figure 4c, where an aluminum electrolytic
capacitor has been connected to FIN. This capacitor’s high
equivalent series resistance damps the circuit and elimi-
nates the voltage overshoot. The extra capacitor improves
low frequency ripple filtering and can slightly improve the
efficiency of the circuit, though it is likely to be the largest
component in the circuit. Placing the electrolytic capacitor
at the FIN terminals can also improve the LTM8031’s EMI
filtering as well as guard against overshoots caused by
the Q of the integrated filter.
Thermal Considerations
The LTM8031 output current may need to be derated if it is
required to operate in a high ambient temperature or deliver
a large amount of continuous power. The amount of current
derating is dependent upon the input voltage, output power
and ambient temperature. The temperature rise curves
given in the Typical Performance Characteristics section
can be used as a guide. These curves were generated by a
LTM8031 mounted to a 35cm
2
4-layer FR4 printed circuit
board. Boards of other sizes and layer count can exhibit
different thermal behavior, so it is incumbent upon the user
to verify proper operation over the intended system’s line,
load and environmental operating conditions.
The junction-to-air and junction-to-board thermal resis-
tances given in the Pin Configuration diagram may also be
used to estimate the LTM8031 internal temperature. These
thermal coefficients are determined per JESD 51-9 (JEDEC
standard, test boards for area array surface mount package
thermal measurements) through analysis and physical
correlation. Bear in mind that the actual thermal resistance
of the LTM8031 to the printed circuit board depends upon
the design of the circuit board. The die temperature of
the LTM8031 must be lower than the maximum rating of
125°C, so care should be taken in the layout of the circuit
to ensure good heat sinking of the LTM8031.
The bulk of the heat flow out of the LTM8031 is through the
bottom of the module and the LGA pads into the printed
circuit board. Consequently a poor printed circuit board
design can cause excessive heating, resulting in impaired
performance or reliability. Please refer to the PCB Layout
section for printed circuit board design suggestions.
Finally, be aware that at high ambient temperatures the
internal Schottky diode will have significant leakage current
increasing the quiescent current of the LTM8031.
