Datasheet

LTM8021
11
8021fc
APPLICATIONS INFORMATION
C
IN
V
IN
C
OUT
FB
GND V
OUT
RUN/SS
R
ADJ
BIAS
PLANE
8021 F04
Figure 4. Layout Showing Suggested External Components,
GND Plane and Thermal Vias
PCB Layout
Most of the problems associated with the PCB layout
have been alleviated or eliminated by the high level of
integration of the LTM8021. The LTM8021 is nevertheless
a switching power supply, and care must be taken to
minimize EMI and ensure proper operation. Even with the
high level of integration, one may fail to achieve a specifi ed
operation with a haphazard or poor layout. See Figure 4
for a suggested layout.
Ensure that the grounding and heatsinking are acceptable.
A few rules to keep in mind are:
1. Place the C
IN
capacitor as close as possible to the V
IN
and GND connection of the LTM8021.
2. Place the C
OUT
capacitor as close as possible to the
V
OUT
and GND connection of the LTM8021.
3. Place the C
IN
and C
OUT
capacitors such that their ground
currents fl ow directly adjacent to, or underneath the
LTM8021.
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 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.