ltm8031 Linear Technology Corporation, ltm8031 Datasheet - Page 13

• Manufacturer Part Number: ltm8031
• Description: Ultralow Noise Emc 36v, 1a Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8031. However, these capacitors
can cause problems if the LTM8031 is plugged into a live
or fast rising or falling 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 under-damped tank
circuit, and the voltage at the V
ring to twice the nominal input voltage, possibly exceed-
ing the LTM8031’s rating and damaging the part. A similar
phenomenon can occur inside the LTM8031 module, at the
output of the integrated EMI fi lter, with the same potential
of damaging the part.
If the input supply is poorly controlled or the user will be
plugging the LTM8031 into an energized supply, the input
network should be designed to prevent this overshoot. Fig-
ure 4 shows the waveforms that result when an LTM8031
Figure 3. Layout Showing Suggested External Components,
GND Plane and Thermal Vias
C
OUT
V
GND
OUT
GND
IN
CAPACITOR
OPTIONAL
pin of the LTM8031 can
AUX
FIN
C
IN
BIAS
FIN
SYNC
RUN/SS
V
IN
8031 F03
circuit is connected to a 24V supply through six feet of 24-
gauge twisted pair. The fi rst plot (4a) 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, 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 fi ltering. 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.
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 fi ltering and can slightly improve the
effi ciency 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
fi ltering as well as guard against overshoots caused by
the Q of the integrated fi lter.
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
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 Confi guration diagram may also be
used to estimate the LTM8031 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
2
4-layer FR4 printed circuit
LTM8031
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8031fa