Datasheet

LT3008 Series
15
3008fc
APPLICATIONS INFORMATION
So,
P = 20mA(12.6V – 3.3V) + 0.3mA(12.6V) = 189.8mW
The thermal resistance ranges from 65°C/W to 85°C/W
depending on the copper area. So, the junction temperature
rise above ambient approximately equals:
0.1898W(75°C/W) = 14.2°C
The maximum junction temperature equals the maximum
junction temperature rise above ambient plus the maximum
ambient temperature or:
T
J(MAX)
= 85°C + 14.2°C = 99.2°C
Protection Features
The LT3008 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 protec-
tion protect the device against current overload condi-
tions at the output of the device. For normal operation,
do not exceed a junction temperature of 125°C. The
typical thermal shutdown circuitry temperature threshold
is 160°C.
The IN pin withstands reverse voltages of 50V. The device
limits current fl ow to less than 30µA (typically less than
1µA) and no negative voltage appears at OUT. The device
protects both itself and the load against batteries that are
plugged in backwards.
For example, a 4-layer, 1 ounce copper PCB board with
3 thermal vias from the DFN exposed backside or the
3 fused TSOT-23 GND pins to inner layer GND planes
achieves 45°C/W thermal resistance. Demo circuit DC
1388A’s board layout achieves this 45°C/W performance.
This is approximately a 30% improvement over the lowest
numbers shown in Tables 2 and 3.
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input voltage
range of 12V ±5%, an output current range of 0mA to 20mA
and a maximum ambient temperature of 85°C, what will
the maximum junction temperature be for an application
using the DC package?
The power dissipated by the device is equal to:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
) + I
GND
(V
IN(MAX)
)
where,
I
OUT(MAX)
= 20mA
V
IN(MAX)
= 12.6V
I
GND
at (I
OUT
= 20mA, V
IN
= 12.6V) = 0.3mA
Table 3: Measured Thermal Resistance for TSOT-23 Package
COPPER AREA
BOARD
AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)TOPSIDE* BACKSIDE
2500mm
2
2500mm
2
2500mm
2
65°C/W
1000mm
2
2500mm
2
2500mm
2
67°C/W
225mm
2
2500mm
2
2500mm
2
70°C/W
100mm
2
2500mm
2
2500mm
2
75°C/W
50mm
2
2500mm
2
2500mm
2
85°C/W
*Device is mounted on the topside.