LT3579 LINER [Linear Technology], LT3579 Datasheet - Page 20

• Manufacturer Part Number: LT3579
• Description: 6A Boost/Inverting DC/DC Converter with Fault Protection
• Manufacturer: LINER [Linear Technology]

LT3579/LT3579-1
APPLICATIONS INFORMATION
The published (http://www.linear.com/designtools/
packaging/Linear_Technology_Thermal_Resistance_
Table.pdf) θ
package and 34°C/W for the 4mm × 5mm QFN 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
value of ~22°C/W was consistently achieved for both TSSOP
and QFN packages of the LT3579 (at V
I
as per the suggestions in the Layout Guidelines section.
Junction Temperature Measurement
The duty cycle of the CLKOUT signal on the LT3579 is linearly
proportional to die junction temperature, T
duty cycle on the LT3579-1 is fi xed at ~50%). To get an
accurate reading, measure the duty cycle of the CLKOUT
signal and use the following equation to approximate the
junction temperature:
where DC
the die junction temperature in °C. Although the absolute
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 defi ned. A
3% increase in CLKOUT duty cycle corresponds to ~10°C
increase in die temperature:
Note that the CLKOUT pin is only meant to drive capacitive
loads up to 50pF.
Thermal Lockout
A fault condition occurs when the die temperature exceeds
~165°C (see Operation – FAULT Section), and the part goes
into thermal lockout. The fault condition ceases when the
die temperature drops by ~5°C (nominal).
20
OUT
T
J
= 1.7A, f
=
DC
CLKOUT
CLKOUT
JA
OSC
value is 38°C/W for the TSSOP Exposed Pad
0 3
. %
= 1MHz) when board layout was optimized
JA
is the CLKOUT duty cycle in % and T
–
values are realizable if board layout is
35
%
IN
= 5V, V
J
(the CLKOUT
OUT
= 12V,
J
JA
is
SWITCHING FREQUENCY
There are several considerations in selecting the operating
frequency of the converter. The fi rst 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 fi lter capacitors go down in value and size.
The tradeoff is effi ciency, since the switching losses due to
NPN base charge (see Thermal Considerations), Schottky
diode charge, and other capacitive loss terms increase
proportionally with frequency.
Oscillator Timing Resistor (R
The operating frequency of the LT3579 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 RT pin to ground. An internally trimmed timing
capacitor resides inside the IC. The oscillator frequency
is calculated using the following formula:
where f
kΩ) can be calculated from the desired frequency (in
MHz) using:
R
f
OSC
T
=
OSC
=
87 6
f
OSC
R
87 6
.
is in MHz and R
T
.
+
–
1
1
T
is in kΩ. Conversely, R
T
)
T
35791f
(in