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LT1969

Using CTRL1 and CTRL2 to set the supply current effec-

tively places R1 and R2 in parallel obtaining a net resis-

tance, and Figure 12 can still be utilized in determining

supply current.

The use of two pins to control the supply current allows

for applications where external logic can be used to place

the amplifiers in different supply current modes. Figure

13 illustrates a partial shutdown with direct logic on each

control pin. If both logic inputs are low, the control pins

will effectively see a resistance of 13k//49.9k = 10k to

V –. This will set the amplifiers in nominal mode with a

gain bandwidth of 700MHz and

±200mA minimum IOUT.

The electrical characteristics are specified in nominal

mode. Forcing R1’s input logic high will partially shut

down the part, putting it in a low power mode. By keeping

the output stage slightly biased, the output impedance

–

CTRL1

V–

V+

CTRL2

1969 F11

V–

Figure 11

RESISTANCE (k

Ω)

1969 F12

100

030

VS = ±6V

TA = 25°C

Figure 12. Supply Current vs Control Resistance (R1//R2)

remains low, preserving the line termination. The Typical

Performance Characteristics curve Output Impedance vs

Supply Current shows the details. Both logic inputs high

further reduces the supply current and places the part in

a “standby” mode with less than 500

µA per amplifier

quiescent current.

Output Loading in Low Current Modes

The LT1969 output stage has a very wide bandwidth and

is able to source and sink large amounts of current. The

internal circuitry of the output stage incorporates a posi-

tive feedback boost loop giving it high drive capability. As

the supply current is reduced, the sourcing drive capability

also reduces. Maximum sink current is independent of

supply current and is limited by the short-circuit protec-

tion at 500mA. If the amplifier is in a low power or

“standby” mode, the output stage is slightly biased and is

not capable of sourcing high output currents. The Typical

Performance Characteristics curve Maximum IOUT Sourc-

ing vs Quiescent Current shows the maximum output

current for a given quiescent current.

Considerations for Fault Protection

The basic line driver design presents a direct DC path

between the outputs of the two amplifiers. An imbalance

in the DC biasing potentials at the noninverting inputs

through either a fault condition or during turn-on of the

system can create a DC voltage differential between the

two amplifier outputs. This condition can force a consid-

erable amount of current, 500mA or more, to flow as it is

limited only by the small valued back-termination resis-

tors and the DC resistance of the transformer primary.

This high current can possibly cause the power supply

voltage source to drop significantly impacting overall

CTRL1

CTRL2

OFF

1969 F13

OFF

3.3V/5V FROM V–

Figure 13

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