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LT5554

5554f

APPLICATIONS INFORMATION

The differential outputs can also be converted to single-

ended 50Ω load using a center-tap transformer interface

shown in Figure 6 and Figure 16.

The internal 400Ω differential resistor (RO) sets the output

impedance and the maximum voltage gain (GMAX) to 36dB

when outputs OUT+, OUT– are open.

Figure 7 shows the Voltage and Power Gains as a func-

tion of ROUT, which is the total output loading at the open

collector ampliﬁer output including the internal resistor

RO = 400Ω.

Voltage clipping will occur with ROUT >140Ω, in which case

the instantaneous voltage at each OUT+ and OUT– outputs

is either <2V or >8V.

The output OP1dB = 20dBm can be achieved when ROUT =

130Ω. In this case, the LT5554 outputs reach both current

and voltage limiting for maximum output power.

Gain Control Interface

The MODE pin selects the interface to the LT5554 gain

control pins.

The PGx and STROBE control inputs can be conﬁgured

to be either DC-coupled (for TTL interface) or AC-coupled

(for ECL or low-voltage CMOS interfaces).

In addition, the STROBE input can be driven such that the

LT5554 gain state is updated asynchronously (PGx latch

control in transparent-mode) or controlled by positive

STROBE transition (PGx latch control in strobed-mode).

There are several options available for coupling type and

latch control which are given in the following tables:

Table1. MODE Input Options

MODE

(State)

COUPLING TYPE

PGx (Latch Control)

STROBE

PGx

LOW

AC Positive

Transition

Strobe

OPEN

DC >2.2V

Transparent

OPEN

0.6 to 2.2V

Strobe

HIGH

DC >2.2V

Transparent

HIGH

0.6 to 2.2V

Strobe

Table2. MODE Input Levels

MODE

(State)

MODE

(Min Level)

MODE

(Max Level)

LOW

0.6V

OPEN

1.5V

2.5V

HIGH

VCC – 0.4V

VCC

Alternatively, the MODE pin can be left open (2V

internal).

Figure 7. Maximum Voltage and Power Gain vs ROUT

The gain vs ROUT relationship is given by the following

equations:

GV = 20log(GM • ROUT) in dB

GP = 10log(RIN • GM2 • ROUT) in dB

Where RIN = 50Ω and GM = 0.15 siemens at GMAX

For wide band applications, the ampliﬁer bandwidth can

be extended by inductive peaking technique. The inductor

in series with the LT5554 outputs (OUT+ OUT–) can have

a value up to some tens of nH depending on ROUT value

and board capacitance.

The current limiting will occur with ROUT <140Ω, in which

case the instantaneous signal current at the output exceeds

IODC = 45mA.

ROUT (Ω)

100

400

5554 F07

1000

VOLTAGE GAIN

POWER GAIN

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