lt3980 Linear Technology Corporation, lt3980 Datasheet - Page 17

• Manufacturer Part Number: lt3980
• Description: 58v, 2a, 2.4mhz Step-down Switching Regulator With 85?a Quiescent Current
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
and the V
when the input voltage is present and that protects against
a shorted or reversed input.
Figure 8. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output. It Also Protects the Circuit from
a Reversed Input. The LT3980 Runs Only When the Input Is Present
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 9 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents fl ow in the LT3980’s V
diode (D1) and the input capacitor (C1). The loop formed
by these components should be as small as possible. These
components, along with the inductor and output capacitor,
should be placed on the same side of the circuit board,
and their connections should be made on that layer. Place
a local, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and V
traces will shield them from the SW and BOOST nodes.
The Exposed Pad on the bottom of the package must be
soldered to ground so that the pad acts as a heat sink. To
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT3980 to additional ground planes within the circuit
board and on the bottom side.
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3980 circuits. However, these capaci-
V
IN
MBRS360
D4
IN
pin. Figure 8 shows a circuit that will run only
V
RUN/SS
V
IN
C
LT3980
GND FB
BOOST
C
SW
nodes small so that the ground
IN
and SW pins, the catch
3980 F08
V
BACKUP
OUT
tors can cause problems if the LT3980 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
voltage at the V
nominal input voltage, possibly exceeding the LT3980’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3980 into an
energized supply, the input network should be designed
to prevent this overshoot. Figure 10 shows the waveforms
that result when an LT3980 circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The
fi rst plot is the response with a 4.7μF ceramic capacitor
at the input. The input voltage rings as high as 50V and
the input current peaks at 26A. A good solution is shown
in Figure 10b. 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. For high input voltages its impact on
effi ciency is minor, reducing effi ciency by 1.5 percent for
a 5V output at full load operating from 24V.
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
VIAS TO LOCAL GROUND PLANE
VIAS TO V
D1
OUT
L1
IN
C1
VIAS TO SYNC
pin of the LT3980 can ring to twice the
V
OUT
GND
VIAS TO RUN/SS
VIAS TO PG
C2
R
RT
R
PG
LT3980
VIAS TO V
OUTLINE OF LOCAL
GROUND PLANE
C
C
R1
17
R
C
IN
R2
3980 F09
3980f