LTC6090 datasheet(11/18 Pages) AD

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LT1210X

1210Xfa

For more information www.linear.com/LT1210X

APPLICATIONS INFORMATION

When the LT1210X is used to drive capacitive loads, the

available output current can limit the overall slew rate.

In the fastest configuration, the LT1210X is capable of a

slew rate of over 1V/ns. The current required to slew a

capacitor 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 150V/µs, determined by the current

limit of 1.5A.

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.

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 LT1210X will operate from single or split supplies

from ±5V (10V total) to ±15V (30V total). It is not neces-

sary to use equal value split supplies, however the offset

voltage and inverting input bias current will change. The

offset voltage changes about 500µV per volt of supply

mismatch. The inverting bias current can change as much

as 5µA per volt of supply mismatch, though typically the

change is less than 0.5µA per volt.

Power Supply Bypassing

To obtain the maximum output and the minimum distor-

tion from the LT1210X, the power supply rails should be

well bypassed. For example, with the output stage pour-

ing 1A current peaks into the load, a 1 power supply

impedance will cause a droop of 1V, reducing the available

output swing by that amount. Surface mount tantalum

and ceramic capacitors make excellent low ESR bypass

elements when placed close to the chip. For frequencies

above 100kHz, use 1µF and 100nF ceramic capacitors.

If significant power must be delivered below 100kHz,

capacitive reactance becomes the limiting factor. Larger

ceramic or tantalum capacitors, such as 4.7µF, are recom-

mended in place of the 1µF unit mentioned above.

Inadequate bypassing is evidenced by reduced output

swing and “distorted” clipping effects when the output

is driven to the rails. If this is observed, check the supply

pins of the device for ripple directly related to the output

waveform. Significant supply modulation indicates poor

bypassing.

Thermal Considerations

For the TSSOP16-E package, 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 electri-

cally 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

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

RF = RG = 3k

RL = ∞

VS = ±15V

1210X F06

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