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OPA620KP Datasheet(PDF) 11 Page - Texas Instruments |
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OPA620KP Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 16 page  11 ® OPA620 Many demanding high-speed applications such as ADC/ DAC buffers require op amps with low wideband output impedance. For example, low output impedance is essential when driving the signal-dependent capacitances at the inputs of flash A/D converters. As shown in Figure 3, the OPA620 maintains very low closed-loop output impedance over frequency. Closed-loop output impedance increases with frequency since loop gain is decreasing with frequency. 100 1k 10k 100k 1M 10M 100M Frequency (Hz) 10 1 0.1 0.01 G = +10V/V G = +1V/V G = +2V/V FIGURE 3. Small-Signal Output Impedance vs Frequency. THERMAL CONSIDERATIONS The OPA620 does not require a heat sink for operation in most environments. The use of a heat sink, however, will reduce the internal thermal rise and will result in cooler, more reliable operation. At extreme temperatures and under full load conditions a heat sink is necessary. See “Maximum Power Dissipation” curve, Figure 4. FIGURE 4. Maximum Power Dissipation. The internal power dissipation is given by the equation P D = P DQ + PDL, where PDQ is the quiescent power dissipation and P DL is the power dissipation in the output stage due to the load. (For ±V CC = ±5V, PDQ = 10V x 23mA = 230mW, max). For the case where the amplifier is driving a grounded load (R L) with a DC voltage (±VOUT) the maximum value of PDL occurs at ±V OUT = ±V CC/2, and is equal to PDL, max = ( ±V CC) 2/4R L. Note that it is the voltage across the output transistor, and not the load, that determines the power dissipated in the output stage. When the output is shorted to ground, P DL = 5V x 150mA = 750mW. Thus, P D = 230mW + 750mW ≈ 1W. Note that the short-circuit condition represents the maximum amount of internal power dissipation that can be generated. Thus, the “Maximum Power Dissipation” curve starts at 1W and is derated based on a 175 °C maximum junction temperature and the junction-to-ambient thermal resistance, θ JA, of each package. The variation of short-circuit current with tempera- ture is shown in Figure 5. FIGURE 6. Driving Capacitive Loads. OPA620 C L R L R S (R typically 5 to 25 ) S ΩΩ CAPACITIVE LOADS The OPA620’s output stage has been optimized to drive resistive loads as low as 50 Ω. Capacitive loads, however, will decrease the amplifier’s phase margin which may cause high frequency peaking or oscillations. Capacitive loads greater than 20pF should be buffered by connecting a small resistance, usually 5 Ω to 25Ω, in series with the output as shown in Figure 6. This is particularly important when driving high capacitance loads such as flash A/D converters. In general, capacitive loads should be minimized for optimum high frequency performance. Coax lines can be driven if the cable is properly terminated. The capacitance of coax cable (29pF/foot for RG-58) will not load the amplifier when the coaxial cable or transmission line is terminated in its characteristic impedance. FIGURE 5. Short-Circuit Current vs Temperature. 250 200 150 100 50 –75 –50 –25 0 +25 +50 +75 +100 +125 Ambient Temperature (°C) +I SC – I SC Cerdip Package 1.2 1.0 0.8 0.6 0.4 0.2 0 0 +25 +50 +75 +100 +125 +150 Ambient Temperature (°C) Plastic DIP, SO-8 Packages |
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