LT1010_15 Datasheet, PDF (15/20 Pages) Linear Technology – Fast 150mA Power Buffer

LT1010
APPLICATIONS INFORMATION
Typical thermal calculations for the miniDIP package are
detailed in the following paragraphs.
For 4.8mA supply current (typical at 50°C, 30V supply
voltage—see supply current graphs) to the LT1010 at
±15V, P_D = power dissipated in the part is equal to:
(30V)(0.0048A) = 0.144W
The rise in junction is then:
(0.144W)(100°C/W—This is θ_JA for the N package)
= 14.4°C.
This means that the junction temperature in 50°C ambient
air without driving any current into a load is:
14.4°C + 50°C = 64.4°C
Using the LT1010 to drive 8V DC into a 200Ω load using
±15V power supplies dissipates P_D in the LT1010 where:
( )^{( )} _{PD =}
V⁺ – V_OUT
R_L
V_OUT
=
(15V – 8V)(8V
200Ω
)
=
0.280W
This causes the LT1010 junction temperature to rise another
(0.280W)(100°C/W) = 28°C.
This heats the junction to 64.4°C + 28°C = 92.4°C.
An example of 1MHz operation further shows the limita-
tions of the N (or miniDIP) package. For ±15V operation:
PD at I_L = 0 at 1MHz* = (10mA)(30V) = 0.30W
This power dissipation causes the junction to heat from
50°C (ambient in this example) to 50°C + (0.3W)(100°C/W)
= 80°C. Driving 2V_RMS of 1MHz signal into a 200Ω load
causes an additional
P_D
=


2V
200Ω


•
(15
–
2)
=
0.130W
to be dissipated, resulting in another (0.130W)(100°C/W)
= 13°C rise in junction temperature to 80°C + 13°C = 93°C.
Thermal Resistance of DFN Package
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
The following table lists thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with one ounce
copper.
Table 1. DFN Measured Thermal Resistance
COPPER AREA
THERMAL RESISTANCE
TOPSIDE BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
2500 sq mm 2500 sq mm 2500 sq mm
40°C/W
1000 sq mm 2500 sq mm 2500 sq mm
45°C/W
225 sq mm 2500 sq mm 2500 sq mm
50°C/W
100 sq mm 2500 sq mm 2500 sq mm
62°C/W
For the DFN package, the thermal resistance junction-to-
case (θ_JC), measured at the Exposed Pad on the back of
the die, is 7.5°C/W.
Continuous operation at the maximum supply voltage
and maximum load current is not practical due to thermal
limitations. Transient operation at the maximum supply
is possible. The approximate thermal time constant for a
2500sq mm 3/32" FR-4 board with maximum topside and
backside area for one ounce copper is 3 seconds. This
time constant will increase as more thermal mass is added
(i.e., vias, larger board and other components).
For an application with transient high power peaks, average
power dissipation can be used for junction temperature
calculations as long as the pulse period is significantly less
than the thermal time constant of the device and board.
*See Supply Current vs Frequency graph.
1010fe
15