Datasheet

LTM8023
14
8023fj
For more information www.linear.com/LTM8023
Figure 5. Layout Showing Suggested External Components, GND
Plane and Thermal Vias
V
OUT
C
IN
V
IN
R
ADJ
R
T
C
OUT
GND
BIAS
AUX
SHARE
RUN/SS
PGOOD
SYNC
8023 F05
APPLICATIONS INFORMATION
PCB Layout
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8023. The LTM8023 is neverthe-
less a switching power supply, and care must be taken to
minimize EMI and ensure proper operation. Even with the
high level of integration, you may fail to achieve specified
operation with a haphazard or poor layout. See Figure 5
for a suggested layout.
4
. Place the C
IN
and C
OUT
capacitors such that their
ground current flow directly adjacent or underneath
the LTM8023.
5
. 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 LTM8023.
6
. Use vias to connect the GND copper area to the boards
internal ground plane. Liberally distribute these GND vias
to provide both a good ground connection and thermal
path to the internal planes of the printed circuit board.
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8023. However, these capacitors
can cause problems if the LTM8023 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, and the volt-
age at the V
IN
pin of the LTM8023 can ring to twice the
nominal input voltage, possibly exceeding the LTM8023’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LTM8023 into
an energized supply, the input network should be designed
to prevent this overshoot. Figure 6 shows the waveforms
that result when an LTM8023 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
the circuit. In Figure 6c 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
Ensure that the grounding and heatsinking are acceptable.
A few rules to keep in mind are:
1
. Place the R
ADJ
and R
T
resistors as close as possible to
their respective pins.
2
. Place the C
IN
capacitor as close as possible to the V
IN
and GND connection of the LTM8023.
3
. Place the C
OUT
capacitor as close as possible to the
V
OUT
and GND connection of the LTM8023.
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