Datasheet

LT3090
20
3090fa
For more information www.linear.com/LT3090
applicaTions inForMaTion
junction temperature. The dual-in-line pin arrangement
allows metal to extend beyond the ends of the package
on the topside (component side) of the PCB. Connect this
metal to IN on the PCB. The multiple IN and OUT pins of
the LT3090 further assist in spreading heat to the PCB.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PCB and its
copper traces. Copper board stiffeners and plated through-
holes can also be used to spread the heat generated by
power devices.
Table 3. Measured Thermal Resistance for DFN Package
COPPER AREA BOARD AREA THERMAL
RESISTANCE
Top Side* Bottom Side
2500mm
2
2500mm
2
2500mm
2
34°C/W
1000mm
2
2500mm
2
2500mm
2
34°C/W
225mm
2
2500mm
2
2500mm
2
35°C/W
100mm
2
2500mm
2
2500mm
2
36°C/W
*Device is mounted on topside
Table 4. Measured Thermal Resistance for MSOP Package
COPPER AREA BOARD AREA THERMAL
RESISTANCE
Top Side* Bottom Side
2500mm
2
2500mm
2
2500mm
2
33°C/W
1000mm
2
2500mm
2
2500mm
2
33°C/W
225mm
2
2500mm
2
2500mm
2
34°C/W
100mm
2
2500mm
2
2500mm
2
35°C/W
*Device is mounted on topside
Tables 3 and 4 list thermal resistance as a function of
copper area in a fixed board size. All measurements were
taken in still air on a 4 layer FR-4 board with 1oz solid
internal planes and 2oz top/bottom external trace planes
with a total board thickness of 1.6mm. The four layers
were electrically isolated with no thermal vias present. PCB
layers, copper weight, board layout and thermal vias affect
the resultant thermal resistance. For more information
on thermal resistance and high thermal conductivity test
boards, refer to JEDEC standard JESD51, notably JESD51-7
and JESD51-12. Achieving low thermal resistance neces
-
sitates attentions to detail and careful PCB layout.
Calculating
Junction T
emperature
Example: Given an output voltage of –2.5V and input
voltage of –3.3V ± 5%, output current range from 1mA
to 500mA, and a maximum ambient temperature of 85°C,
what is the maximum junction temperature?
The LT3090’s power dissipation is:
I
OUT(MAX)
• (V
IN(MAX)
– V
OUT
) + I
GND
• V
IN(MAX)
where:
I
OUT(MAX)
= –500mA
V
IN(MAX)
= –3.465V
I
GND
(at I
OUT
= –500mA and V
IN
= –3.465V) = –6.5mA
Thus:
P = (–0.5A) • (–3.465V + 2.5V) + (–6.5mA) • (– 3.465V)
= 0.505W
Using a DFN package, the thermal resistance is in the
range of 34°C/W to 36°C/W depending on the copper area.
Therefore, the junction temperature rise above ambient
approximately equals:
0.505W • 35°C/W = 18°C
The maximum junction temperature equals the maxi-
mum ambient temperature plus the maximum junction
temperature rise above ambient:
T
JMAX
= 85°C + 18°C = 103°C
Overload Recovery
Like many monolithic power regulators, the LT3090
incorporates safe-operating-area (SOA) protection. The
SOA protection activates at output-to-input differential
voltage greater than 7V. The SOA protection decreases
current limit as output-to-input differential increases and
keeps the power transistor inside a safe operating region
for all values of output-to-input voltage up to the LT3090’s
Absolute Maximum Ratings. The LT3090 provides some
level of output current for all values of output-to-input
differential. Refer to the Current Limit curve in the Typical
Performance Characteristics section. When power is first
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