LT3681
APPLICATIONS INFORMATION
Hot Plugging Safely
125°C. Whenoperatingathighambienttemperatures, the
maximum load current should be derated as the ambient
temperature approaches 125°C.
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3681 circuits. However, these
capacitors can cause problems if the LT3681 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,
Power dissipation within the LT3681 can be estimated
by calculating the total power loss from an efficiency
measurement. The die temperature is calculated by
multiplying the LT3681 power dissipation by the thermal
resistance from junction to ambient.
Alsokeepinmindthattheleakagecurrentoftheintegrated
power Schottky diode, like all Schottky diodes, goes up
with junction temperature. The curves in Figure 11 show
how the leakage current in the power Schottky diode
varies with temperature and reverse voltage. When the
power switch is closed, the power Schottky diode is in
parallel with the power converter’s output filter stage. As
a result, an increase in a diode’s leakage current results
in an effective increase in the load, and a corresponding
increase in input power.
and the voltage at the V pin of the LT3681 can ring
IN
to twice the nominal input voltage, possibly exceeding
the LT3681’s rating and damaging the part. If the input
supply is poorly controlled or the user will be plugging
the LT3681 into an energized supply, the input network
should be designed to prevent this overshoot. Figure 10
shows the waveforms that result when an LT3681 circuit
is connected to a 24V supply through six feet of 24-gauge
twisted pair. The first plot (10a) 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 inrush 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. Another effective method
of reducing the overshoot is to add a 22µF aluminum
electrolytic capacitor, as shown in Figure 10c.
10000
V
V
V
= 10V
= 25V
= 40V
R
R
R
1000
100
10
1
–50
0
50
100
150
TEMPERATURE (°C)
High Temperature Considerations
3681 F12
The PCB must provide heat sinking to keep the LT3681
cool. The Exposed Pads on the bottom of the package
mustbesolderedtocopperpours,whichinturnshouldbe
tied to large copper layers below with thermal vias; these
layerswillspreadtheheatdissipatedbytheLT3681. Place
additional vias to reduce thermal resistance further. With
these steps, the thermal resistance from die (or junction)
Figure 11. Like all Schottky Diodes, the LT3681 Integrated Power
Diode Leakage Current Varies with Temperature and Applied
Reverse Voltage V .
R
Other Linear Technology Publications
Application Notes 19, 35, 44 and 76 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.
to ambient can be reduced to θ = 35°C/W or less. With
JA
100LFPMairflow,thisresistancecanfallbyanother25%.
Furtherincreasesinairflowwillleadtolowerthermalresis-
tance. Becauseofthelargeoutputcurrentcapabilityofthe
LT3681, it is possible to dissipate enough power to raise
thejunctiontemperaturebeyondtheabsolutemaximumof
3681f
18
Linear [ Linear ]
