Datasheet

LT3680
18
Rev C
For more information www.analog.com
TYPICAL APPLICATIONS
and the voltage at the V
IN
pin of the LT3680 can ring
to twice the nominal input voltage, possibly exceeding
the LT3680’s rating and damaging the part. If the input
supply is poorly controlled or the user will be plugging
the LT3680 into an energized supply, the input network
should be designed to prevent this overshoot. Figure 10
shows the waveforms that result when an LT3680 circuit
is connected to a 24V supply through six feet of 24-gauge
twisted pair. The first 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 0.7
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. For high input voltages
its impact on efficiency is minor, reducing efficiency by 1.5
percent for a 5V output at full load operating from 24V.
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3680
cool. The Exposed Pad on the bottom of the package
must be soldered to a ground plane. This ground should
be tied to large copper layers below with thermal vias;
these layers will spread the heat dissipated by the LT3680.
Place additional vias can reduce thermal resistance fur-
ther. With these steps, the thermal resistance from die
(or junction) to ambient can be reduced to
JA
= 35°C/W
or less. With 100 LFPM airflow, this resistance can fall
by another 25%. Further increases in airflow will lead
to lower thermal resistance. Because of the large output
current capability of the LT3680, it is possible to dissipate
enough heat to raise the junction temperature beyond
the absolute maximum of 125°C. When operating at high
ambient temperatures, the maximum load current should
be derated as the ambient temperature approaches 125°C.
Power dissipation within the LT3680 can be estimated by
calculating the total power loss from an efficiency mea
-
surement and subtracting the catch diode loss and induc-
tor loss. The die temperature is calculated by multiplying
the LT3680 power dissipation by the thermal resistance
from junction to ambient.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 100
shows how to generate a bipolar output supply using a
buck regulator.
APPLICATIONS INFORMATION
5V Step-Down Converter
SW
FB
V
C
PG
RT
V
IN
BD
V
IN
6.3V TO 36V
V
OUT
5V
3.5A
10µF
0.47µF
47µF
100k
f = 600kHz
D: ON SEMI MBRA340
L: NEC MPLC0730L4R7
D
15k
63.4k
L
4.7µH
536k
GND
680pF
ON OFF
LT3680
3680 TA02
RUN/SS BOOST
SYNC
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.