Datasheet

9
LT1497
or power plane layer either inside or on the opposite side
of the board. Copper board stiffeners and plated through-
holes can also be used to spread the heat generated by the
device. Table 1 lists the thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with 2oz copper.
This data can be used as a rough guideline in estimating
thermal resistance. The thermal resistance for each appli-
cation will be affected by thermal interactions with other
components as well as board size and shape.
Table 1. Fused 16-lead and 8-lead SO Packages
TOTAL θ
JA
θ
JA
TOPSIDE BACKSIDE COPPER AREA (16-LEAD) (8-LEAD)
2500mm
2
2500mm
2
5000mm
2
40°C/W 80°C/W
1000mm
2
2500mm
2
3500mm
2
46°C/W 92°C/W
600mm
2
2500mm
2
3100mm
2
48°C/W 96°C/W
180mm
2
2500mm
2
2680mm
2
49°C/W 98°C/W
180mm
2
1000mm
2
1180mm
2
56°C/W 112°C/W
180mm
2
600mm
2
780mm
2
58°C/W 116°C/W
180mm
2
300mm
2
480mm
2
59°C/W 118°C/W
180mm
2
100mm
2
280mm
2
60°C/W 120°C/W
180mm
2
0mm
2
180mm
2
61°C/W 122°C/W
Calculating Junction Temperature
The junction temperature can be calculated from the
equation:
T
J
= (P
D
)(θ
JA
) + T
A
T
J
= Junction Temperature
T
A
= Ambient Temperature
P
D
= Power Dissipation
θ
JA
= Thermal Resistance (Junction-to-Ambient)
As an example, calculate the junction temperature for the
circuit in Figure 1 assuming an 85°C ambient temperature.
The device dissipation can be found by measuring the
supply currents, calculating the total dissipation and then
subtracting the dissipation in the load and feedback net-
work. Both amplifiers are in a gain of –1.
The dissipation for each amplifier is:
P
D
= (1/2)(86.4mA)(30V) – (10V)
2
/(200||560) = 0.62W
The total dissipation is 1.24W. When a 2500mm
2
PC
board with 2oz copper on top and bottom is used, the
thermal resistance is 40°C/W. The junction temperature
T
J
is:
T
J
= (1.24W)(40°C/W) + 85°C = 135°C
The maximum junction temperature for the LT1497 is
150°C, so the heat sinking capability of the board is
adequate for the application.
If the copper area on the PC board is reduced to 180mm
2
the thermal resistance increases to 61°C/W and the junc-
tion temperature becomes:
T
J
= (1.24W)(61°C/W) + 85°C = 161°C
which is above the maximum junction temperature indi-
cating that the heat sinking capability of the board is
inadequate and should be increased.
APPLICATIONS INFORMATION
WUU
U
COPPER AREA (2oz)
560Ω
–15V
560Ω
–
+
560Ω
560Ω
86.4mA
15V
200Ω
200Ω
1497 F01
–
+
A
–10V
10V
f = 2MHz
Figure 1. Thermal Calculation Example
Slew Rate
Unlike a traditional op amp, the slew rate of a current
feedback amplifier is not independent of the amplifier gain
configuration. There are slew rate limitations in both the
input stage and the output stage. In the inverting mode and
for higher gains in the noninverting mode, the signal
amplitude on the input pins is small and the overall slew
rate is that of the output stage. The input stage slew rate
is related to the quiescent current in the input devices.
Referring to the Simplified Schematic, for noninverting
applications the two current sources in the input stage
slew the parasitic internal capacitances at the bases of Q3
and Q4. Consider a positive going input at the base of Q1
and Q2. If the input slew rate exceeds the internal slew rate,