lt3680 Linear Technology Corporation, lt3680 Datasheet - Page 18

• Manufacturer Part Number: lt3680
• Description: 36v, 3.5a, 2.4mhz Step-down Switching Regulator With 75?a Quiescent Current
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3680
voltage at the V
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
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 0.7 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 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.
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 lay-
ers will spread the heat dissipated by the LT3680. Place
additional vias can reduce thermal resistance further. With
these steps, the thermal resistance from die (or junction)
TYPICAL APPLICATIONS
18
IN
pin of the LT3680 can ring to twice the
6.3V TO 36V
10μF
V
IN
D: ON SEMI MBRA340
L: NEC MPLC0730L4R7
680pF
15k
63.4k
ON OFF
5V Step-Down Converter
f = 600kHz
RUN/SS
V
RT
PG
SYNC
C
V
IN
LT3680
GND
to ambient can be reduced to
100 LFPM airfl ow, this resistance can fall by another 25%.
Further increases in airfl ow will lead to lower thermal re-
sistance. 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 effi ciency measure-
ment and subtracting the catch diode loss and inductor
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.
BD
BOOST
SW
FB
100k
0.47μF
D
536k
4.7μH
L
3680 TA02
47μF
V
5V
3.5A
JA
OUT
= 35°C/W or less. With
3680f