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LT1364 Datasheet(PDF) 10 Page - Linear Technology |
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LT1364 Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page 10 LT1364/LT1365 a comparator, peak detector or other open-loop applica- tion with large, sustained differential inputs. Under normal, closed-loop operation, an increase of power dis- sipation is only noticeable in applications with large slewing outputs and is proportional to the magnitude of the differential input voltage and the percent of the time that the inputs are apart. Measure the average supply current for the application in order to calculate the power dissipa- tion. Capacitive Loading The LT1364/LT1365 are stable with any capacitive load. This is accomplished by sensing the load induced output pole and adding compensation at the amplifier gain node. As the capacitive load increases, both the bandwidth and phase margin decrease so there will be peaking in the frequency domain and in the transient response as shown in the typical performance curves. The photo of the small signal response with 200pF load shows 62% peaking. The large signal response shows the output slew rate being limited to 10V/ µs by the short-circuit current. Coaxial cable can be driven directly, but for best pulse fidelity a resistor of value equal to the characteristic impedance of the cable (i.e., 75 Ω) should be placed in series with the output. The other end of the cable should be terminated with the same value resistor to ground. Circuit Operation The LT1364/LT1365 circuit topology is a true voltage feedback amplifier that has the slewing behavior of a current feedback amplifier. The operation of the circuit can be understood by referring to the simplified schematic. The inputs are buffered by complementary NPN and PNP emitter followers which drive a 500 Ω resistor. The input voltage appears across the resistor generating currents which are mirrored into the high impedance node. Comple- mentary followers form an output stage which buffers the gain node from the load. The bandwidth is set by the input resistor and the capacitance on the high impedance node. The slew rate is determined by the current available to charge the gain node capacitance. This current is the differential input voltage divided by R1, so the slew rate is proportional to the input. Highest slew rates are therefore seen in the lowest gain configurations. For example, a 10V APPLICATIONS INFORMATION output step in a gain of 10 has only a 1V input step, whereas the same output step in unity gain has a 10 times greater input step. The curve of Slew Rate vs Input Level illustrates this relationship. The LT1364/LT1365 are tested for slew rate in a gain of –2 so higher slew rates can be expected in gains of 1 and –1, and lower slew rates in higher gain configurations. The RC network across the output stage is bootstrapped when the amplifier is driving a light or moderate load and has no effect under normal operation. When driving a capacitive load (or a low value resistive load) the network is incompletely bootstrapped and adds to the compensa- tion at the high impedance node. The added capacitance slows down the amplifier which improves the phase margin by moving the unity-gain frequency away from the pole formed by the output impedance and the capacitive load. The zero created by the RC combination adds phase to ensure that even for very large load capacitances, the total phase lag can never exceed 180 degrees (zero phase margin) and the amplifier remains stable. Power Dissipation The LT1364/LT1365 combine high speed and large output drive in small packages. Because of the wide supply voltage range, it is possible to exceed the maximum junction temperature under certain conditions. Maximum junction temperature (TJ) is calculated from the ambient temperature (TA) and power dissipation (PD) as follows: LT1364CN8: TJ = TA + (PD x 130°C/W) LT1364CS8: TJ = TA + (PD x 190°C/W) LT1365CN: TJ = TA + (PD x 110°C/W) LT1365CS: TJ = TA + (PD x 150°C/W) Worst case power dissipation occurs at the maximum supply current and when the output voltage is at 1/2 of either supply voltage (or the maximum swing if less than 1/2 supply voltage). For each amplifier PDMAX is: PDMAX = (V+ – V–)(ISMAX) + (V+/2)2/RL Example: LT1365 in S16 at 70 °C, VS = ±5V, RL = 150W PDMAX = (10V)(8.4mA) + (2.5V)2/150Ω = 126mW TJMAX = 70°C + (4 x 126mW)(150°C/W) = 145°C |
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