LTM8023EV-PBF LINER [Linear Technology], LTM8023EV-PBF Datasheet - Page 13

• Manufacturer Part Number: LTM8023EV-PBF
• Description: 2A, 36V DC/DC ?Module
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
4. Place the C
5. Connect all of the GND connections to as large a copper
6. Use vias to connect the GND copper area to the boards
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
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 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 6b 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 6c. A 0.7Ω
ground current fl ow directly adjacent or underneath
the LTM8023.
pour or plane area as possible on the top layer. Avoid
breaking the ground connection between the external
components and the LTM8023.
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.
IN
pin of the LTM8023 can ring to twice the
IN
and C
OUT
capacitors such that their
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.
Thermal Considerations
The LTM8023 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, out-
put power and ambient temperature. The derating curves
given in the Typical Performance Characteristics section
can be used as a guide. These curves were generated by a
LTM8023 mounted to a 33cm
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 die temperature of the LTM8023 must be lower than
the maximum rating of 125°C, so care should be taken
in the layout of the circuit to ensure good heat sinking
of the LTM8023. To estimate the junction temperature,
approximate the power dissipation within the LTM8023 by
applying the typical effi ciency stated in this data sheet to
the desired output power, or, if you have an actual module,
by taking a power measurement. Then calculate the tem-
perature rise of the LTM8023 junction above the surface
of the printed circuit board by multiplying the module’s
power dissipation by the thermal resistance. The actual
thermal resistance of the LTM8023 to the printed circuit
board depends upon the layout of the circuit board, but
the thermal resistance given with the Pin Confi guration,
which is based upon a 33cm
be used a guide.
Finally, be aware that at high ambient temperatures the
internal Schottky diode will have signifi cant leakage current
(see Typical Performance Characteristics) increasing the
quiescent current of the LTM8023.
2
2
4-layer FR4 PC board, can
4-layer FR4 printed circuit
LTM8023
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