LT1818 データシートの表示（PDF） - Linear Technology
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LT1818 Datasheet PDF : 18 Pages
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LT1818/LT1819
APPLICATIONS INFORMATION
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 maxi-
mum 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 amplifiers
(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 cur-
rent 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 buf-
fer the gain node from the load. The input resistor, input
stage transconductance and the capacitor on the high
impedance 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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