ltc5100 Linear Technology Corporation, ltc5100 Datasheet - Page 19

• Manufacturer Part Number: ltc5100
• Description: 3.3v, 3.2gbps Vcsel Driver
• Manufacturer: Linear Technology Corporation

OPERATIO
monitor photodiode current. In each case the fundamental
calculation is the same. The LTC5100’s digital controller
multiplies the nominal value of the quantity (I
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:
where I is the digital representation of the laser bias
current, modulation current or monitor diode current (I
I
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:
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_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 2
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
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
12, the effective value of the second order temperature
coefficient is 3.81ppm/ C
approximately 484 ppm/ C
M
I = I_nom • (TC2 • 2
or I
T = T_int_adc – T_nom
T = T_ext –T_nom
MD
).
U
–18
2
• T
10
per LSB. The full-scale range is
• 0.5 K = 512 K or 239 C.
2
.
2
+ TC1 • 2
–13
–18
• T + 1)
B
in Equation
, I
M
or I
–13
(13)
(14)
(12)
MD
B
in
)
,
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:
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:
A write operation to this register is shown as:
A register read operation is shown as:
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.
SYS_CONFIG.Apc_en
USER_ADC.Data
Peaking
USER_ADC.Data
LTC5100
sn5100 5100fs
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