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LTC5100

sn5100 5100fs

LASER BIAS AND MODULATION

Modulator Architecture

The LTC5100 drives common cathode lasers using a

method called “shunt switching”. As shown in Figure 12,

shunt switching involves sourcing DC current into the

laser diode and shunting part of that current with a high

speed current switch to produce the required modulation.

The SRC pin provides the DC current and the MODA,

MODB pins (which are connected on chip) provide the

high speed modulation current. This technique results in

a very fast, single-ended driver that confines the high

speed modulation current to the laser and ground system.

The LTC5100 actually uses a modified shunt switching

scheme in which the source current is delivered through

a “termination” resistor, RT, that is bypassed to ground

with a large capacitor. The resistor brings three advan-

tages to the modulation stage. First, it gives the modulator

a precise resistive output impedance to damp ringing and

absorb reflections from the laser. Second, the resistor

isolates the capacitance of the SRC pin from the high

speed signal path, further improving modulation speed.

Third, the resistor and capacitor heavily filter the high

speed output signal so that it does not modulate the power

supply and cause radiation or interference. On-chip

decoupling of the high speed amplifiers further reduces

power supply noise generation.

OPERATIO

Terminology and Basic Calculations

Figure 12 through Figure 16 define terminology that is

used throughout this data sheet. The current delivered by

the SRC pin is called IS. The average modulation current

delivered by the chip at the MODA, MODB pins is called IM.

The laser bias current, IB, is defined as the average current

in the laser. IB is the difference between the source current

and average modulation current.

The

peak-to-peak modulation current delivered by the

chip is called IMPP. IMPP is twice the value of IM because

the high speed data is assumed to have a 50% duty cycle.

The peak-to-peak modulation current is divided between

the termination resistor and the laser. The peak-to-peak

modulation amplitude

in the laser is called IMOD. The

relationship between IMPP and IMOD depends on the

relative values of the termination resistor and the laser

dynamic resistance.

Figure 12. Simplified Laser Bias and Modulation Circuit

VSS

IS = IB + IM

IMOD

VDD

MODA, MODB

11, 10

SRC

LTC5100

Ω

TYP

10nF

IMPP = 2 • IM

3.2Gbps

MODULATOR

Figure 13. Components of the LTC5100 Source and

Modulation Currents (The Laser Bias Current is Also Shown)

IMPP

5100 F13

The relationships between the source, bias, and modula-

tion currents are as follows.

IB = IS – IM

(1)

IMPP = 2 • IM

(2)

MOD

TLD

MPP

()

•

(3)

where

RT is the termination resistor value.

RLD is the dynamic resistance of the laser, defined in

Figure 15.

The expression for IB in Equation 1 shows that the maxi-

mum achievable laser bias current is a function of the

maximum source current, IS, and the average modulation

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