Datasheet LT1497 (Analog Devices) - 9

LT1497
U U W U APPLICATIONS INFORMATION
or power plane layer either inside or on the opposite side thermal resistance is 40°C/W. The junction temperature of the board. Copper board stiffeners and plated through- TJ is: holes can also be used to spread the heat generated by the T device. Table 1 lists the thermal resistance for several J = (1.24W)(40°C/W) + 85°C = 135°C different board sizes and copper areas. All measurements The maximum junction temperature for the LT1497 is were taken in still air on 3/32" FR-4 board with 2oz copper. 150°C, so the heat sinking capability of the board is This data can be used as a rough guideline in estimating adequate for the application. thermal resistance. The thermal resistance for each appli- If the copper area on the PC board is reduced to 180mm2 cation will be affected by thermal interactions with other the thermal resistance increases to 61°C/W and the junc- components as well as board size and shape. tion temperature becomes:
Table 1. Fused 16-lead and 8-lead SO Packages
TJ = (1.24W)(61°C/W) + 85°C = 161°C
COPPER AREA (2oz) TOTAL
θ
JA
θ
JA TOPSIDE BACKSIDE COPPER AREA (16-LEAD) (8-LEAD)
which is above the maximum junction temperature indi- 2500mm2 2500mm2 5000mm2 40°C/W 80°C/W cating that the heat sinking capability of the board is 1000mm2 2500mm2 3500mm2 46°C/W 92°C/W inadequate and should be increased. 600mm2 2500mm2 3100mm2 48°C/W 96°C/W 560Ω 180mm2 2500mm2 2680mm2 49°C/W 98°C/W 180mm2 1000mm2 1180mm2 56°C/W 112°C/W 560Ω A 15V – 180mm2 600mm2 780mm2 58°C/W 116°C/W 86.4mA 180mm2 300mm2 480mm2 59°C/W 118°C/W + 200Ω 180mm2 100mm2 280mm2 60°C/W 120°C/W 10V 560Ω 180mm2 0mm2 180mm2 61°C/W 122°C/W –10V 560Ω – f = 2MHz
Calculating Junction Temperature
+ The junction temperature can be calculated from the 200Ω equation: – 15V 1497 F01 TJ = (PD)(θJA) + TA
Figure 1. Thermal Calculation Example
TJ = Junction Temperature TA = Ambient Temperature
Slew Rate
PD = Power Dissipation θ Unlike a traditional op amp, the slew rate of a current JA = Thermal Resistance (Junction-to-Ambient) feedback amplifier is not independent of the amplifier gain As an example, calculate the junction temperature for the configuration. There are slew rate limitations in both the circuit in Figure 1 assuming an 85°C ambient temperature. input stage and the output stage. In the inverting mode and The device dissipation can be found by measuring the for higher gains in the noninverting mode, the signal supply currents, calculating the total dissipation and then amplitude on the input pins is small and the overall slew subtracting the dissipation in the load and feedback net- rate is that of the output stage. The input stage slew rate work. Both amplifiers are in a gain of –1. is related to the quiescent current in the input devices. The dissipation for each amplifier is: Referring to the Simplified Schematic, for noninverting applications the two current sources in the input stage PD = (1/2)(86.4mA)(30V) – (10V)2/(200||560) = 0.62W slew the parasitic internal capacitances at the bases of Q3 The total dissipation is 1.24W. When a 2500mm2 PC and Q4. Consider a positive going input at the base of Q1 board with 2oz copper on top and bottom is used, the and Q2. If the input slew rate exceeds the internal slew rate, 9