LTC6090 datasheet(9/18 Pages) AD

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LT1210X

1210Xfa

For more information www.linear.com/LT1210X

APPLICATIONS INFORMATION

Although the optional compensation works well with

capacitive loads, it simply reduces the bandwidth when

it is connected with resistive loads. For instance, with a

10 load, the bandwidth drops from 35MHz to 26MHz

when the compensation is connected. Hence, the com-

pensation was made optional. To disconnect the optional

compensation, leave the COMP pin open.

Figure 1

FREQUENCY (MHz)

–6

–2

100

1210X F01

–4

VS = ±15V

CL = 200pF

RF = 1.5k

COMPENSATION

RF = 3.4k

NO COMPENSATION

RF = 3.4k

COMPENSATION

Shutdown/Current Set

If the shutdown feature is not used, the SHUTDOWN pin

must be connected to ground or V–.

The Shutdown pin can be used to either turn off the bias-

ing for the amplifier, reducing the quiescent current to

less than 200µA, or to control the quiescent current in

normal operation.

The total bias current in the LT1210X is controlled by

the current flowing out of the Shutdown pin. When the

Shutdown pin is open or driven to the positive supply,

the part is shut down. In the shutdown mode, the output

looks like a 70pF capacitor and the supply current is typi-

cally less than 100µA. The Shutdown pin is referenced to

the positive supply through an internal bias circuit (see

the Simplified Schematic). An easy way to force shutdown

is to use open-drain (collector) logic. The circuit shown

in Figure 2 uses a 74C906 buffer to interface between 5V

logic and the LT1210X. The switching time between the

active and shutdown states is about 1µs. A 24kΩ pull-up

resistor speeds up the turn-off time and ensures that the

LT1210X is completely turned off. Because the pin is

referenced to the positive supply, the logic used should

have a breakdown voltage of greater than the positive

supply voltage. No other circuitry is necessary as the

internal circuit limits the Shutdown pin current to about

500µA. Figure 3 shows the resulting waveforms.

For applications where the full bandwidth of the amplifier

is not required, the quiescent current of the device may be

reduced by connecting a resistor from the Shutdown pin

to ground. The quiescent current will be approximately 65

times the current in the Shutdown pin. The voltage across

the resistor in this condition is V+ – 3VBE. For example,

a 82kΩ resistor will set the quiescent supply current to

9mA with VS = ±15V.

The photos in Figures 4a and 4b show the effect of reduc-

ing the quiescent supply current on the large-signal

response. The quiescent current can be reduced to 9mA

in the inverting configuration without much change in

response. In noninverting mode, however, the slew rate

is reduced as the quiescent current is reduced.

Figure 2. Shutdown Interface

–

LT1210X

15V

–15V

VIN

24k

ENABLE

VOUT

1210X F02

15V

74C906

Figure 3. Shutdown Operation

AV = 1

RF = 825Ω

RL = 50Ω

RPULL-UP = 24k

VIN = 1VP-P

VS = ±15V

1210X F03

VOUT

ENABLE

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