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LT1816 Datasheet(PDF) 11 Page - Linear Technology |
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LT1816 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LT1818/LT1819 18189f APPLICATIO S I FOR ATIO Slew Rate The slew rate of the LT1818/LT1819 is proportional to the differential input voltage. Highest slew rates are therefore seen in the lowest gain configurations. For example, a 6V output step with a gain of 10 has a 0.6V input step, whereas at unity gain there is a 6V input step. The LT1818/LT1819 is tested for slew rate at a gain of –1. Lower slew rates occur in higher gain configurations, whereas the highest slew rate (2500V/ µs) occurs in a noninverting unity-gain configuration. Power Dissipation The LT1818/LT1819 combine high speed and large output drive in small packages. It is possible to exceed the maximum junction temperature specification (150 °C) under certain conditions. Maximum junction temperature (TJ) is calculated from the ambient temperature (TA), power dissipation per amplifier (PD) and number of ampli- fiers (n) as follows: TJ = TA + (n • PD • θJA) Power dissipation is composed of two parts. The first is due to the quiescent supply current and the second is due to on-chip dissipation caused by the load current. The worst-case load-induced power occurs when the output voltage is at 1/2 of either supply voltage (or the maximum swing if less than 1/2 the supply voltage). Therefore PDMAX is: PDMAX =(V+ – V–) • (ISMAX) + (V+/2)2/RL or PDMAX =(V+ – V–) • (ISMAX) + (V+ – VOMAX) • (VOMAX/RL) Example: LT1819IS8 at 85 °C, VS = ±5V, RL = 100Ω PDMAX = (10V) • (14mA) + (2.5V)2/100Ω = 202.5mW TJMAX = 85°C + (2 • 202.5mW) • (150°C/W) = 146°C Circuit Operation The LT1818/LT1819 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. Complementary NPN and PNP emitter followers buffer the inputs and drive an internal resistor. The input voltage appears across the resistor, generating a current that is mirrored into the high impedance node. Complementary followers form an output stage that buffer the gain node from the load. The input resistor, input stage transconductance and the capacitor on the high imped- ance node determine the bandwidth. 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 step. Highest slew rates are therefore seen in the lowest gain configurations. |
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Similar Description - LT1816 |
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