Datasheet LT1886 (Analog Devices) - 9

LT1886
U U W U APPLICATIO S I FOR ATIO Input Considerations
the efficiency of the PC board as a heat sink. The PCB material can be very effective at transmitting heat between The inputs of the LT1886 are an NPN differential pair the pad area attached to the V– pin and a ground or power protected by back-to-back diodes (see the Simplified plane layer. Copper board stiffeners and plated through- Schematic). There are no series protection resistors holes can also be used to spread the heat generated by the onboard which would degrade the input voltage noise. If device. Table 1 lists the thermal resistance for several the inputs can have a voltage difference of more than 0.7V, different board sizes and copper areas. All measurements the input current should be limited to less than 10mA with were taken in still air on 3/32" FR-4 board with 2oz copper. external resistance (usually the feedback resistor or source This data can be used as a rough guideline in estimating resistor). Each input also has two ESD clamp diodes—one thermal resistance. The thermal resistance for each appli- to each supply. If an input drive exceeds the supply, limit cation will be affected by thermal interactions with other the current with an external resistor to less than 10mA. components as well as board size and shape. The LT1886 design is a true operational amplifier with high
Table 1. Fused 8-Lead SO Package
impedance inputs and low input bias currents. The input
COPPER AREA (2oz) TOTAL
offset current is a factor of ten lower than the input bias
TOPSIDE BACKSIDE COPPER AREA
θ
JA
current. To minimize offsets due to input bias currents, 2500 sq. mm 2500 sq. mm 5000 sq. mm 80°C/W match the equivalent DC resistance seen by both inputs. 1000 sq. mm 2500 sq. mm 3500 sq. mm 92°C/W The low input noise current can significantly reduce total 600 sq. mm 2500 sq. mm 3100 sq. mm 96°C/W noise compared to a current feedback amplifier, especially 180 sq. mm 2500 sq. mm 2680 sq. mm 98°C/W for higher source resistances. 180 sq. mm 1000 sq. mm 1180 sq. mm 112°C/W
Layout and Passive Components
180 sq. mm 600 sq. mm 780 sq. mm 116°C/W 180 sq. mm 300 sq. mm 480 sq. mm 118°C/W With a gain bandwidth product of 700MHz the LT1886 180 sq. mm 100 sq. mm 280 sq. mm 120°C/W requires attention to detail in order to extract maximum 180 sq. mm 0 sq. mm 180 sq. mm 122°C/W performance. Use a ground plane, short lead lengths and a combination of RF-quality supply bypass capacitors (i.e., 470pF and 0.1µF). As the primary applications have
Calculating Junction Temperature
high drive current, use low ESR supply bypass capacitors The junction temperature can be calculated from the (1µF to 10µF). For best distortion performance with high equation: drive current a capacitor with the shortest possible trace lengths should be placed between Pins 4 and 8. The TJ = (PD)(θJA) + TA optimum location for this capacitor is on the back side of TJ = Junction Temperature the PC board. The DSL driver demo board (DC304) for this T part uses a Taiyo Yuden 10µF ceramic (TMK432BJ106MM). A = Ambient Temperature P The parallel combination of the feedback resistor and gain D = Device Dissipation setting resistor on the inverting input can combine with θJA = Thermal Resistance (Junction-to-Ambient) the input capacitance to form a pole which can cause As an example, calculate the junction temperature for the frequency peaking. In general, use feedback resistors of circuit in Figure 1 assuming an 85°C ambient temperature. 1kΩ or less. The device dissipation can be found by measuring the
Thermal Issues
supply currents, calculating the total dissipation and then subtracting the dissipation in the load. The LT1886 enhanced θJA SO-8 package has the V– pin fused to the lead frame. This thermal connection increases 9