LTC5100EUF datasheet(19/52 Pages) LINER

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LTC5100

sn5100 5100fs

monitor photodiode current. In each case the fundamental

calculation is the same. The LTC5100’s digital controller

multiplies the nominal value of the quantity (IB, IM or IMD)

by a quadratic function of temperature. Temperature mea-

surements are supplied either by an on-chip temperature

sensor or by an external microprocessor, according to the

setting of Ext_temp_en. The general temperature com-

pensation formula is:

I = I_nom • (TC2 • 2–18 •

∆T2 + TC1 • 2–13 • ∆T + 1) (12)

where I is the digital representation of the laser bias

current, modulation current or monitor diode current (IB,

IM or IMD).

When using the internal temperature sensor (Ext_temp_en

= 0), the temperature measurements are taken by the on-

chip ADC, and

∆T is the change in the LTC5100 die tem-

perature relative to a user defined nominal temperature:

∆T = T_int_adc – T_nom

(13)

When using an external temperature source (Ext_temp_en

= 1), the temperature measurements are provided in

digital form by a microprocessor or host computer and

∆T

is the change in temperature relative to a user defined

nominal temperature:

∆T = T_ext –T_nom

(14)

T_int_adc, T_ext, and T_nom are 10-bit, unsigned numbers

scaled at 0.5K/LSB. The maximum temperature that can be

represented is therefore 210 • 0.5

°K = 512°K or 239°C.

TC1 and TC2 are the first and second order temperature

coefficients. They correspond to the registers Im_tc1 and

Im_tc2 for modulation current, Ib_tc1 and Ib_tc2 for bias

current and Imd_tc1 and Imd_tc2 for monitor diode

current. In each case TC1 and TC2 are 8-bit signed

numbers in two’s complement format. The range of the

temperature coefficients is therefore –128 to +127. When

TC1 is multiplied by its weighting coefficient of 2–13 in

Equation 12, the effective value of the first order tempera-

ture coefficient is 122ppm/

°C per LSB. The full-scale

range is approximately

±15500 ppm/°C. When TC2 is

multiplied by its weighting coefficient of 2–18 in Equation

12, the effective value of the second order temperature

coefficient is 3.81ppm/

°C2 per LSB. The full-scale range is

approximately

±484 ppm/°C2.

Note that Equation 12 is applied to the digital representa-

tion of the currents, not the physical current themselves.

This is a particularly important point where monitor diode

current is concerned, because the digital representation of

the monitor diode current is the logarithm of the current.

Thus the temperature compensation is of the logarithm of

the monitor diode current and not the current itself.

Notation Used for Registers and Bit Fields

The LTC5100 has a large set of registers, many of which

are subdivided into fields of bits. Register names are given

in all capitals (SYS_CONFIG) and bit fields are given in

mixed case (Apc_en). For example, the bit that enables

Automatic Power Control mode is contained in the System

Configuration register. This bit is denoted by:

SYS_CONFIG.Apc_en

In many cases this bit field will simply be referred to as

“Apc_en.”

The functional diagrams of Figure 3 and Figure 4 show

registers and bit fields within registers between horizontal

bars. For example, the “Data” field in the ADC register is

shown as:

USER_ADC.Data

A write operation to this register is shown as:

USER_ADC.Data

A register read operation is shown as:

Peaking

Range Selection for the Source

and Modulation Currents

The source and modulation currents each have four ranges

of operation to optimize ADC and DAC resolution as well

as high frequency performance. The source current range

is controlled by two bits called Is_rng. Similarly, the modu-

lation current range is controlled by two bits called Im_rng.

The maximum current that can be delivered is proportional

to the range, so the current output is 1, 2, 3 or 4 times the

typical base value of 9mA for the source current and

4.5mA for the average modulation current or 9mA peak-

to-peak.

OPERATIO

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