Datasheet

LTM8032
15
8032fg
For more information www.linear.com/LTM8032
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8032. However, these capacitors
can cause problems if the LTM8032 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
IN
pin of the LTM8032 can
ring to twice the nominal input voltage, possibly exceeding
the LTM8032’s rating and damaging the part. A similar
phenomenon can occur inside the LTM8032 module, at the
output of the integrated EMI filter, with the same potential
of damaging the part.
If the input supply is poorly controlled or the user will be
plugging the LTM8032 into an energized supply, the input
network should be designed to prevent this overshoot. Fig
-
ure 5 shows the waveforms that result when an LTM8032
circuit is connected to a 24V supply through six feet of
24-gauge twisted pair. The first plot (5a) is the response
applicaTions inForMaTion
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. An
alternative solution is shown in Figure 5b. 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 ef
-
ficiency 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 5c, where an
aluminum electrolytic capacitor has been connected to FIN.
This capacitor’s high equivalent series resistance damps
the circuit and eliminates 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. Figure 5c
shows the capacitor added to the V
IN
terminals, but placing
the electrolytic capacitor at the FIN terminals can improve
the LTM8032’s EMI filtering as well as guard
against
overshoots caused by the Q of the integrated filter.