LTM8020_15 Datasheet, PDF (9/16 Pages) Linear Technology – 200mA, 36V DC/DC Module Regulator

LTM8020
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 LTM8020. The LTM8020 is never-the-
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 3
for a suggested layout.
Ensure that the grounding and heat sinking 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 LTM8020.
2. Place the C_OUT capacitor as close as possible to the
V_OUT and GND connection of the LTM8020.
3. Place the C_IN and C_OUT capacitors such that their
ground current flows directly adjacent or underneath
the LTM8020.
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 LTM8020.
5. The copper pours also serve as the heat sink for the
LTM8020. Place several vias in the GND plane to act as
heat pipes to other layers of the printed circuit board.
V_IN
SHDN
C_IN
BIAS
V_OUT
C_OUT
ADJ
COPPER
R_ADJ
GND
VIAs TO GND PLANE
8020 F03
Figure 3. Layout Showing Suggested External
Components, GND Plane and Thermal Vias
Positive-to-Negative Voltage Regulation
The LTM8020 can generate a negative output by tying the
V_OUT pads to system ground and connecting GND as shown
in the Typical Applications section. In this configuration,
SHDN must be level shifted or referenced to GND, and the
available output current may be reduced.
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8020. However, these capacitors
can cause problems if the LTM8020 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 under damped tank circuit, and the volt-
age at the V_IN pin of the LTM8020 can ring to twice the
nominal input voltage, possibly exceeding the LTM8020’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LTM8020 into
an energized supply, the input network should be designed
to prevent this overshoot. Figure 4 shows the waveforms
that result when an LTM8020 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 4b 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 filtering and can slightly improve the efficiency of the
circuit, though it is likely to be the largest component in the
circuit. An alternative solution is shown in Figure 4c. A 1Ω
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. This
solution is smaller and less expensive than the electrolytic
capacitor. For high input voltages its impact on efficiency
is minor, reducing efficiency less than one-half percent for
a 5V output at full load operating from 24V.
8020fd
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