Datasheet

LT3055
21
3055f
For more information www.linear.com/LT3055
Calculating Junction Temperature
Example: Given an output voltage of 5V, an input voltage
range of 12V ±5%, a maximum output current range of
75mA and a maximum ambient temperature of 85°C, what
is the maximum junction temperature?
The power dissipated by the device equals:
I
OUT(MAX)
• (V
IN(MAX)
– V
OUT
) + I
GND
• V
IN(MAX)
where:
I
OUT(MAX)
= 75mA
V
IN(MAX)
= 12.6V
I
GND
at (I
OUT
= 75mA, V
IN
= 12V) = 3.5mA
So:
P = 75mA • (12.6V – 5V) + 3.5mA • 12.6V = 0.614W
Using a DFN package, the thermal resistance ranges from
36°C/W to 40°C/W depending on the copper area. So the
junction temperature rise above ambient approximately
equals:
0.614W • 40°C/W = 24.6°C
The maximum junction temperature equals the maximum
ambient temperature plus the maximum junction tempera-
ture rise above ambient or:
T
JMAX
= 85°C + 24.6°C = 110°C
Protection Features
The LT3055 incorporates several protection features that
make it ideal for use in battery-powered circuits. In ad-
dition to the normal protection features associated with
monolithic regulators, such as current limiting and thermal
limiting, the device also protects against reverse input
voltages, reverse output voltages and reverse output-to-
input voltages.
Current limit protection and thermal overload protection
protect the device against current overload conditions at
the output of the device. For normal operation, do not
exceed a junction temperature of 125°C (E-, I-grades) or
150°C (MP-, H-grades).
The LT3055 IN pin withstands reverse voltages of 50V. The
device limits current flow to less than 1μA (typically less
than 25nA) and no negative voltage appears at OUT. The
device protects both itself and the load against batteries
that are plugged in backwards.
The LT3055 incurs no damage if its output is pulled below
ground. If the input is left open circuit or grounded, the
output can be pulled below ground by 50V. No current
flows through the pass transistor from the output. However,
current flows in (but is limited by) the feedback resistor
divider that sets the output voltage. Current flows from
the bottom resistor in the divider and from the ADJ pin’s
internal clamp through the top resistor in the divider to
the external circuitry pulling OUT below ground. If the
input is powered by a voltage source, the output sources
current equal to its current limit capability and the LT3055
protects itself by thermal limiting. In this case, grounding
the SHDN pin turns off the device and stops the output
from sourcing current.
APPLICATIONS INFORMATION
Table 4. DFN Measured Thermal Resistance
COPPER AREA
TOPSIDE BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
2500 sq mm 2500 sq mm 36°C/W
1000 sq mm 2500 sq mm 37°C/W
225 sq mm 2500 sq mm 38°C/W
100 sq mm 2500 sq mm 40°C/W
Figure 11. Reverse Output Current
V
OUT
(V)
0
0
OUTPUT CURRENT (µA)
0.1
0.3
0.4
0.5
1.0
0.7
10
20
25 40
3055 F11
0.2
0.8
0.9
0.6
5 15
30
35
V
IN
= 0