LT3581 LINER [Linear Technology], LT3581 Datasheet - Page 18

• Manufacturer Part Number: LT3581
• Description: 3.3A Boost/Inverting DC/DC Converter with Fault Protection
• Manufacturer: LINER [Linear Technology]

LT3581
applicaTions inForMaTion
where T
perature, P
shown in Table 4, and θ
the silicon junction to the ambient air.
The published (http://www.linear.com/designtools/packag-
ing/Linear_Technology_Thermal_Resistance_Table.pdf)
θ
package and 45°C/W for the 16-lead MSOP package. In
practice, lower θ
performed with appropriate grounding (accounting for heat
sinking properties of the board) and other considerations
listed in the Layout Guidelines section. For instance, a
θ
MSE and DFN packages of the LT3581 (at V
12V, I
optimized as per the suggestions in the Board Layout
Guidelines section.
Junction Temperature Measurement
The duty cycle of the CLKOUT signal is linearly propor-
tional to die junction temperature, T
reading, measure the duty cycle of the CLKOUT signal and
use the following equation to approximate the junction
temperature:
where DC
is the die junction temperature in °C. Although the actual
die temperature can deviate from the above equation by
±15°C, the relationship between change in CLKOUT duty
cycle and change in die temperature is well defined. Basi-
cally a 1% change in CLKOUT duty cycle corresponds to a
3.33°C change in die temperature. Note that the CLKOUT
pin is only meant to drive capacitive loads up to 50pF .

JA
JA
T
value of ~24°C/W was consistently achieved for both
J
value is 43°C/W for the 4mm × 3mm 14-pin DFN
OUT
=
J
DC
= Die Junction Temperature, T
= 0.83A, f
CLKOUT
TOTAL
CLKOUT
0 3
. %
JA
is the final result from the calculations
is the CLKOUT duty cycle in % and T
values are realizable if board layout is
–
OSC
35
%
JA
= 2MHz) when board layout was
is the thermal resistance from
J
. To get a temperature
A
= Ambient Tem-
IN
= 5V, V
OUT
=
J
Thermal Lockout
A fault condition occurs when the die temperature exceeds
165°C (see Operation Section), and the part goes into
thermal lockout. The fault condition ceases when the die
temperature drops by ~5°C (nominal).
SWITCHING FREQUENCY
There are several considerations in selecting the operat-
ing frequency of the converter. The first is staying clear
of sensitive frequency bands, which cannot tolerate any
spectral noise. For example, in products incorporating RF
communications, the 455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz and in
that case a 1.5MHz switching converter frequency may be
employed. The second consideration is the physical size
of the converter. As the operating frequency goes up, the
inductor and filter capacitors go down in value and size.
The tradeoff is efficiency, since the losses due to switch-
ing dynamics (see Thermal Considerations), Schottky
diode charge, and other capacitive loss terms increase
proportionally with frequency.
Oscillator Timing Resistor (R
The operating frequency of the LT3581 can be set by the
internal free-running oscillator. When the SYNC pin is driven
low (< 0.4V), the frequency of operation is set by a resistor
from the R
capacitor resides inside the IC. The oscillator frequency
is calculated using the following formula:
where f
can be calculated from the desired frequency (in MHz)
using:
R
f
OSC
T
=
OSC
=
87 6
f
OSC
R
T
87 6
is in MHz and R
.
T
pin to ground. An internally trimmed timing
.
+
–
1
1
T
is in k. Conversely, R
T
)
T
(in k)
3581f