Datasheet

LTM8021
11
8021fd
For more information www.linear.com/LTM8021
applicaTions inForMaTion
4. Connect all of the GND connections to as large a
copper pour or plane area as possible on the top layer.
Avoid breaking the ground connection between the
external components and the LTM8021.
Hot-Plugging Safely
The
small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8021. However, these capacitors
can cause problems if the LTM8021 is plugged into a live
supply (see the 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 volt-
age at the V
IN
pin of the LTM8021 can ring to twice the
nominal input voltage, possibly exceeding the LTM8021’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LTM8021 into
an energized supply, the input network should be designed
to prevent this overshoot. Figure 5 shows the waveforms
that result when an LTM8021 circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The first
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
+
LTM8021
4.7µF
V
IN
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
(5a)
(5b)
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
LTM8021
4.7µF0.1µF
0.7Ω
(5c)
+
LTM8021
4.7µF
22µF
AI.EI.
+
V
IN
20V/DIV
I
IN
10A/DIV
20µs/DIV
DANGER
RINGING V
IN
MAY EXCEED
ABSOLUTE MAXIMUM RATING
V
IN
20V/DIV
I
IN
10A/DIV
20µs/DIV
8021 F05
V
IN
20V/DIV
I
IN
10A/DIV
20µs/DIV
Figure 5. Ensures Reliable Operation When the LTM8021 is Connected to a Live Supply