LTM8022
13
8022fd
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8022. However, these capacitors
can cause problems if the LTM8022 is plugged into a live
supply (see Linear Technology Application Note 88 for
a complete discussion). The low loss ceramic capacitor
combined with stray inductance, in series with the power
source, forms an underdamped tank circuit. In this case,
the voltage at the V
IN
pin of the LTM8022 can ring to
twice the nominal input voltage, possibly exceeding the
LTM8022’s rating and damaging the part. If the input
supply is poorly controlled or the user will be plug-
ging the LTM8022 into an energized supply, the input
network should be designed to prevent this overshoot.
Figure 5 shows the waveforms that result when an
LTM8022 circuit is connected to a 24V supply through six
feet of 24-gauge twisted pair. The fi rst plot is the response
with a 2.2μF ceramic capacitor at the input. The input voltage
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. In Figure 5b
an aluminum electrolytic capacitor has been added. This
capacitor’s high equivalent series resistance damps the
circuit and eliminates the voltage overshoot. The extra
capacitor improves low frequency ripple fi ltering and can
slightly improve the effi ciency of the circuit, though it is
likely to be the largest component in the circuit. An
alternative solution is shown in Figure 5c. 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 fi ltering. This solution is
smaller and less expensive than the electrolytic capacitor.
For high input voltages its impact on effi ciency is minor,
reducing effi ciency less than one half percent for a 5V
output at full load operating from 24V.
Thermal Considerations
The LTM8022 output current may need to be derated if it
is required to operate in a high ambient temperature or
deliver a large amount of power. The amount of current
derating is dependent upon the input voltage, output
power and ambient temperature. The derating curves
in the Typical Performance Characteristics section can
be used as a guide. These curves were generated by an
LTM8022 mounted to a 33cm
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 resistances
given in the Pin Confi guration diagram may also be used
to estimate the LTM8022 internal temperature. These
thermal coeffi cients 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 LTM8022 to the printed circuit board depends upon
the design of the circuit board. The die temperature of
the LTM8022 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 LTM8022.
The bulk of the heat fl ow out of the LTM8022 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 signifi cant leakage current
(see Typical Performance Characteristics) increasing the
quiescent current of the LTM8022.
APPLICATIONS INFORMATION
