LT1969CMS#PBF Datasheet LT1969CMS#PBF, LT1969CMS#TRPBF | P16-P18

LT1969

Figure 10. Receiver Configuration

–V

1969 F10

1:n

–

BIAS

–

LT1813

–

LT1813

= REFLECTED IMPEDANCE

= ATTENUATION OF V

+ R

=SET

+ R

is also 10, but the power consumption has been reduced

to 350mW, a savings of 36% over the previous design.

Note that the reduction of the back-termination resistor

has allowed use of a 1:1 transformer ratio.

Table 2 compares the two approaches. It may seem that

the low power design is a clear choice, but there are further

system issues to consider. In addition to driving the line,

the amplifiers provide back-termination for signals that

are received simultaneously from the line. In order to

reject the drive signal, a receiver circuit is used such as

shown in Figure 10. Taking advantage of the differential

nature of the signals, the receiver can subtract out the

drive signal and amplify the received signal. This method

works well for standard back-termination. If the back-

termination resistors are reduced by positive feedback, a

portion of the received signal also appears at the amplifier

outputs. The result is that the received signal is attenuated

by the same amount as the reduction in the back-termina-

tion resistor. Taking into account the different transformer

turns ratios, the received signal of the low power design

will be one third of the standard design received signal.

The reduced signal has system implications for the sensi-

tivity of the receiver. The power reduction may, or may not,

be an acceptable system tradeoff for a given design.

Controlling the Quiescent Current

The quiescent current of the LT1969 is controlled via two

control pins, CTRL1 and CTRL2. The pins can be used to

either turn off the amplifiers, reducing the quiescent cur-

rent on ±2.5V supplies to less than 500µA per amplifier, or

to control the quiescent current in normal operation.

Figure 11 shows how the control pins are used in conjunc-

tion with external resistors to program the supply current.

In normal operation, each control pin is biased to approxi-

mately 1V above V

–

and by varying the resistor values, the

current from each control pin can be adjusted. It is this

current that sets the supply current of both amplifiers. If

one of the resistors is open, i.e. R2, the supply current of

the amplifiers will be set by CTRL1 and R1. Figure 12

shows supply current vs resistor value.

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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

–

. This will set the amplifiers in nominal mode with a

gain bandwidth of 700MHz and ±200mA minimum I

OUT

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

–

CTRL2

1969 F11

–

Figure 11

RESISTANCE (kΩ)

, BOTH AMPLIFIERS (mA)

20 40 60 80

1969 F12

10010030507090

= ±6V

= 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 500µA 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 I

OUT

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

ONON

OFF

1969 F13

OFF

3.3V/5V FROM V

–

Figure 13

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LT1969

system performance. If left unchecked, the high DC cur-

rent can heat the LT1969 to destruction.

Using DC blocking capacitors to AC couple the signal to

the transformer eliminates the possibility for DC current to

flow under any conditions. These capacitors should be

sized large enough to not impair the frequency response

characteristics required for the data transmission.

Another important fault related concern has to do with

very fast high voltage transients appearing on the tele-

phone line (lightning strikes for example). TransZorbs

varistors and other transient protection devices are often

used to absorb the transient energy, but in doing so also

TransZorb is a registered trademark of General Instruments, GSI

create fast voltage transitions themselves that can be

coupled through the transformer to the outputs of the line

driver. Several hundred volt transient signals can appear

at the primary windings of the transformer with current

into the driver outputs limited only by the back termination

resistors. While the LT1969 has clamps to the supply rails

at the output pins, they may not be large enough to handle

the significant transient energy. External clamping diodes,

such as BAV99s, at each end of the transformer primary

help to shunt this destructive transient energy away from

the amplifier outputs.

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P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

LT1969CMS#PBF

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Manufacturer:

Analog Devices Inc.

Description:

High Speed Operational Amplifiers 700MHz Dual 200mA OA with programmable I supply in MS10

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LT1969CMS#PBF

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