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LT1815/LT1816/LT1817

181567fb

APPLICATIONS INFORMATION

Power Dissipation

The LT1815/LT1816/LT1817 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)iscalculatedfromtheambienttemperature(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 one-half of either supply voltage (or the

maximum swing if less than one-half the supply voltage).

Therefore, PDMAX is:

PDMAX = (V+ – V–) • (ISMAX) + (V+/2)2/RL or

PDMAX = (V+ – V–) • (ISMAX) + (V+ – VOMAX) • (VOMAX/RL)

Example: LT1816IS8 at 85°C, VS = ±5V, RL=100Ω

PDMAX = (10V) • (11.5mA) + (2.5V)2/100Ω = 178mW

TJMAX = 85°C + (2 • 178mW) • (150°C/W) = 138°C

Circuit Operation

The LT1815/LT1816/LT1817 circuit topology is a true volt-

age 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 volt-

age appears across the resistor, generating current that

is mirrored into the high impedance node.

Complementary followers form an output stage that buf-

fers 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.

Figure 1. Programming Resistor Between ISET and V–

Figure 2. Gain Bandwidth Product vs RSET Programming Resistor

Figure 3. Supply Current vs RSET Programming Resistor

ISET

V–

181567 F01

V+

RSET

–5V

–

LT1815S6

RSET PROGRAMING RESISTOR (Ω)

100

150

200

250

10k

40k

181567 F02

100

VS = 5V

TA = 25°C

RL = 500Ω

RL = 100Ω

RSET PROGRAMMING RESISTOR (Ω)

10k

40k

181567 F03

100

VS = ±5V

TA = 25°C

PER AMPLIFIER

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