LT1207CS datasheet(11/16 Pages) LINER

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LT1207

Figure 5c. Large-Signal Response, AV = 2

Capacitance on the Inverting Input

Current feedback amplifiers require resistive feedback

from the output to the inverting input for stable operation.

Take care to minimize the stray capacitance between the

output and the inverting input. Capacitance on the invert-

ing input to ground will cause peaking in the frequency

response (and overshoot in the transient response), but it

does not degrade the stability of the amplifier.

Power Supplies

The LT1207 will operate from single or split supplies from

±5V (10V total) to ±15V (30V total). It is not necessary to

use equal value split supplies, however the offset voltage

and inverting input bias current will change. The offset

voltage changes about 500

µV per volt of supply mis-

match. The inverting bias current can change as much as

µA per volt of supply mismatch, though typically the

change is less than 0.5

µA per volt.

Thermal Considerations

Each amplifier in the LT1207 includes a separate thermal

shutdown circuit which protects against excessive inter-

nal (junction) temperature. If the junction temperature

exceeds the protection threshold, the amplifier will begin

cycling between normal operation and an off state. The

cycling is not harmful to the part. The thermal cycling

occurs at a slow rate, typically 10ms to several seconds,

which depends on the power dissipation and the thermal

time constants of the package and heat sinking. Raising

the ambient temperature until the device begins thermal

shutdown gives a good indication of how much margin

there is in the thermal design.

Heat flows away from the amplifier through the package’s

copper lead frame. Heat sinking is accomplished by using

the heat spreading capabilities of the PC board and its

copper traces. Experiments have shown that the heat

spreading copper layer does not need to be electrically

connected to the tab of the device. The PCB material can

be very effective at transmitting heat between the pad area

attached to the tab of the device and a ground or power

plane layer either inside or on the opposite side of the

board. Although the actual thermal resistance of the PCB

material is high, the length/area ratio of the thermal

APPLICATI

I FOR ATIO

LT1207 • F05c

RF = 750Ω

RL = 50Ω

Differential Input Signal Swing

The differential input swing is limited to about

±6V by an

ESD protection device connected between the inputs. In

normal operation, the differential voltage between the

input pins is small, so this clamp has no effect; however,

in the shutdown mode the differential swing can be the

same as the input swing. The clamp voltage will then set

the maximum allowable input voltage. To allow for some

margin, it is recommended that the input signal be less

than

±5V when the device is shut down.

LT1207 • F06

Figure 6. Large-Signal Response, CL = 10,000pF

VS = ±15V

RF = RG = 3k

RL =

∞

at this rate is 1mA per picofarad of capacitance, so

10,000pF would require 10A! The photo (Figure 6) shows

the large-signal behavior with CL = 10,000pF. The slew

rate is about 60V/

µs, determined by the current limit of

600mA.

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