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

TOPSIDE

BACKSIDE

COPPER AREA

(16-LEAD)

(8-LEAD)

40

°C/W

80

°C/W

46

°C/W

92

°C/W

48

°C/W

96

°C/W

49

°C/W

98

°C/W

56

°C/W

112

°C/W

58

°C/W

116

°C/W

59

°C/W

118

°C/W

60

°C/W

120

°C/W

61

°C/W

122

°C/W

Calculating Junction Temperature

The junction temperature can be calculated from the

equation:

As an example, calculate the junction temperature for the

circuit in Figure 1 assuming an 85

°Cambienttemperature.

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:

board with 2oz copper on top and bottom is used, the

thermal resistance is 40

°C/W. The junction temperature

The maximum junction temperature for the LT1497 is

150

°C, so the heat sinking capability of the board is

adequate for the application.

the thermal resistance increases to 61

°C/W and the junc-

tion temperature becomes:

which is above the maximum junction temperature indi-

cating that the heat sinking capability of the board is

inadequate and should be increased.

COPPER AREA (2oz)

560

Ω

– 15V

560

Ω

560

Ω

560

Ω

86.4mA

15V

200

Ω

200

Ω

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,