Rainbow-electronics MAX3799 Bedienungsanleitung PDF herunterladen (Seite 27)

Design Procedure

Programming Bias Current

1) IBIASMAX[7:0] = Maximum_Bias_Current_Value

2) SET_IBIAS

[8:1] = Initial_Bias_Current_Value

Note: The total bias current value is calculated using

the SET_IBIAS[8:0] register. SET_IBIAS[8:1] are the bits

that can be manually written. SET_IBIAS[0] can only be

updated using the BIASINC[4:0] register.

When implementing an APC loop, it is recommended to

use the BIASINC[4:0] register, which guarantees the

fastest bias current update.

3) BIASINC

[4:0] = New_Increment_Value

4) If (SET_IBIAS

[8:1] ≤ IBIASMAX[7:0]),

then (SET_IBIAS

[8:0] = SET_IBIAS

i-1

[8:0] + BIASINC

[4:0])

5) Else (SET_IBIAS

[8:0] = SET_IBIAS

i-1

[8:0])

The total bias current can be calculated as follows:

6) I

BIAS

= [SET_IBIAS

[8:0] + 20] x 40μA

Programming Modulation Current

1) IMODMAX[7:0] = Maximum_Modulation_Current_Value

2) SET_IMOD

[8:1] = Initial_Modulation_Current_Value

Note: The total modulation current value is calculated

using the SET_IMOD[8:0] register. SET_IMOD[8:1] are

the bits that can be manually written. SET_IMOD[0] can

only be updated using the MODINC[4:0] register.

When implementing modulation compensation, it is rec-

ommended to use the MODINC[4:0] register, which

guarantees the fastest modulation current update.

3) MODINC

[4:0] = New_Increment_Value

4) If (SET_IMOD

[8:1] ≤ IMODMAX[7:0]),

then (SET_IMOD

[8:0] = SET_IMOD

i-1

[8:0] + MODINC

[4:0])

5) Else (SET_IMOD

[8:0] = SET_IMOD

i-1

[8:0])

The following equation is valid with assumption of 100Ω

on-chip and 100Ω external differential load (Rextd). The

maximum value that can be set for SET_TXDE[3:0] = 11.

6) I

MOD(Rextd=100Ω)

= [(20 + SET_IMOD

[8:0]) x 40μA]

For general Rextd, the modulation current that is

achieved using the same setting of SET_IMOD

[8:0] as

for Rextd = 100Ω is shown below. It can be written as a

function of I

MOD(Rextd=100Ω)

, still assuming a 100Ω on-

chip load.

Programming LOS Threshold

LOS

= (SET_LOS[5:0] x 1.5mV

P-P

)

Programming Transmit Output

Deemphasis

The TXDE_EN bit must be set to 1 to enable the deem-

phasis function. The SET_TXDE register value is used

to set the amount of deemphasis, which is a percent-

age of the modulation current. Deemphasis percentage

is determined as:

where the maximum SET_TXDE[3:0] = 11.

For an I

MOD

value of 10mA, the maximum achievable

deemphasis value is approximately 20%. Maximum

deemphasis achievable for full I

MOD

range of 12mA is

limited to 15%.

With deemphasis enabled, the value of the modulation

current amplitude is reduced by the calculated deem-

phasis percentage. To maintain the modulation current

amplitude constant, the SET_IMOD[8:0] register must

be increased by the deemphasis percentage. If the sys-

tem conditions like temperature, required I

MOD

value,

etc., change during the transmit operation, the deem-

phasis setting might need to be readjusted. For such an

SET TXDE

(%)

_[:]

×+

()

100 2 3 0

MOD xtd MOD xtd(Re ) (Re )

=×

⎡

=Ω

100

⎣⎣

⎢

⎤

⎦

⎥

×−

⎡

⎣

⎢

⎤

⎦

⎥

_[:]

230

SET TXDE

Transmitter Deemphasis Control Register (SET_TXDE)

Bits 3 to 0: SET_TXDE[3:0]. This is a 4-bit register used to control the amount of deemphasis on the transmitter out-

put. When calculating the total modulation current, the amount of deemphasis must be taken into account. The

deemphasis is set as a percentage of modulation current.

MAX3799

1Gbps to 14Gbps, SFP+ Multirate Limiting

Amplifier and VCSEL Driver

______________________________________________________________________________________ 27

Bit # 7 6 5 4 3 2 1 0 ADDRESS

Name X X X X

SET_TXDE

[3] (MSB)

SET_TXDE

[2]

SET_TXDE

[1]

SET_TXDE

[0] (LSB)

Default Value X X X X 0 0 0 0

H0x10

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Amplifier and VCSEL Driver 1

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