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TZA3047B

30 Mbits/s up to 1.25 Gbits/s laser drivers

Philips Electronics 

Philips Semiconductors

30 Mbits/s up to 1.25 Gbits/s laser drivers

Product specification

TZA3047A; TZA3047B

12 APPLICATION INFORMATION

12.1 Design equations

12.1.1 BIAS AND MODULATION CURRENTS

The bias and modulation currents are determined by the

voltages on pins BIASIN and MODIN. These voltages are

applied by the BIASOUT and MODOUT pins for dual-loop

control. For average loop control the BIASIN voltage is

applied by the BIASOUT pin and the MODIN voltage is

applied by an external voltage source or an external

resistor RMODIN.

For direct setting of bias and the modulation current, the

BIASIN and MODIN voltages have to be applied by

external voltage sources or by RBIASIN and RMODIN

external resistors connected on BIASIN and MODIN pins:

IBIAS = (RBIASIN × 100 µA − 0.5 V) × gm(bias) [mA]

Imod = (RMODIN × 100 µA − 0.5 V) × gm(mod) + 5 [mA]

The bias and modulation current sources operate with an

input voltage range from 0.5 to 1.5 V. The output current is

at its minimum level for an input voltage below 0.4 V;

see Figs 3 and 4.

The bias and modulation current sources are temperature

compensated and the adjusted current level remains

stable over the temperature range.

The bias and modulation currents increase with increasing

resistor values for RBIASIN and RMODIN respectively, this

allows resistor tuning to start at a minimum current level.

handbook, halfpage

110

I BIAS

(mA)

gm(bias) =

110 mA/V

I BIAS (min)

0.2

0

0.5

VBIASIN (V)

1.5

MGT890

Fig.3 Bias current as a function of BIASIN voltage.

handbook, halfpage

105

Imod = Io(LA)

(mA)

gm(mod) =

100 mA/V

I o(LA)(min)

5

0

0.5

VMODIN (V)

1.5

MGT891

LA current when LA output is on.

Vo(LA) = VCCO.

Fig.4 Modulation current as a function of MODIN

voltage.

12.1.2 AVERAGE MONITOR CURRENT AND EXTINCTION

RATIO

The average monitor current Iav(MON) in dual-loop or

average loop operation is determined by the source

current (IAVR) of the AVR pin. The current can be sunk by

an external current source or by an external resistor (RAVR)

connected to ground:

Iav(MON) = 1580 − 5.26 × IAVR =1580 − 5.26 × R-V----AA---VV----RR- [µA]

The extinction ratio in dual-loop operation is determined by

the source current (IER) of the ER pin. The current can be

sunk by an external current source or by an external

resistor (RER) connected to ground:

ER = 20 – 2----I--µE---R-A---- = 20 – 2------1µ----A-- × R-V----EE---RR--

The average monitor current and the extinction ratio as a

function of the IAVR and IER current are illustrated in Fig.5.

The average monitor current increases with a decreasing

IAVR or increasing RAVR, this allows resistor tuning of RAVR

to start at minimum IAVR current level.

The formulas used to program AVR and ER are valid for

typical conditions; tuning is necessary to achieve good

absolute accuracy of AVR and ER values.

2003 Jun 05

15

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