ADuCM310 Hardware Reference Manual
UG-549
Rev. C | Page 35 of 192
IDAC3 Shutdown
IDAC3 with IDAC6 are designed to control a semiconductor optical amplifier (SOA) stage of a tunable laser. IDAC3 is the main current
source DAC. IDAC6 is a selectable current sink that is connected to the same external pins as IDAC3; the pin name in the
data sheet is IDAC3.
The SOA IDAC (IDAC3) is different from the other five IDACs because it can also be configured as a current sink.
The SOA IDAC (IDAC3) has two separate data registers: IDAC3DAT and IDAC6DAT.
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The pull-down stage shown in Figure 9 is controlled by an 11-bit IDAC; the data register for this IDAC is IDAC6DAT, which allows
the sink current to be set from 0 mA to 80 mA. When the current sink is enabled, the SOA pin voltage is pulled negative by the high
voltage analog die to drain the current out of the SOA diode. To protect the SOA diode, an internal clamp circuit is in place to ensure
that the voltage seen by the diode never drops below −3.5 V. The absolute minimum pull-down voltage is −3.7 V.
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IDAC3DAT is the normal current source data register, which sets the IDAC output when used as a current source in the range of
0 mA to 250 mA.
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HVCON is the user control register for the high voltage die. HVCON[1] must be cleared to 0 to enable the SOA sink current. Setting
HVCON[1] to 1 disables the SOA sink current on the high voltage die.
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HVCON[11] controls the SOA shutdown clamping voltage. The SOA clamping voltage is controlled in two ways:
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When HVCON[11] = 0, the default negative shutdown voltage is clamped at approximately −1.75 V.
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When HVCON[11] = 1, the full trimmed clamping circuit is enabled, meaning the shutdown voltage is clamped in the range of
−3.0 V to −3.5 V.
To avoid an undershoot when shutting down the SOA IDAC and enabling the current sink, observe the following procedure to ensure fast
and safe shutdown of the SOA IDAC:
1.
Enable SOA current sink via IDAC6CON.
2.
Configure negative shutdown current via IDAC6DAT.
3.
Disable SOA current source, IDAC3CON = 0x3.
These three steps result in a negative voltage on the SOA pin with a voltage setting of greater than −1.75 V.
Next, turn on the full clamping circuit by setting HVCON[11] = 1, which results in the shutdown voltage dropping to −3.0 V to −3.5 V.
For normal SOA operation, when IDAC3 is used as a current source and when there is no requirement for sinking current from the
SOA diode, ensure that IDAC6 is configured as follows:
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IDAC6CON = 0x80
•
IDAC6DAT = 0x00
Not configuring the IDAC6 registers as shown previously results in an offset error on IDAC0, IDAC1, IDAC4, and IDAC5.
If any of the IDACs are unused, the recommended setting is to power it on and set the data code to 0:
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IDACxCON = 0x80
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IDACxDAT = 0x00
IDAC0, IDAC1, IDAC2, IDAC4, and IDAC5 also have a small current sink capability to minimize the positive offset current when the
data register is set to 0. The IDACxCON[1] bit can enable a pull-down current source to PGND. This pull-down current is typically 100 µA.
