Datasheet

Detailed Description
The DS3923 contains circuits to create an APD bias
voltage and to monitor the APD bias current. The device
includes three current mirrors, which allow for a wide
dynamic monitoring range. The mirror response time is
fast enough to comply with GPON Rx burst-mode moni-
toring requirements. The device has a built-in current-lim-
iting feature to protect APDs. This current limit is adjust-
able using an external resistor. The APD current can also
be shut down by ISRC/SHDN or thermal shutdown. The
internal HV FET and resistor-divider may be optionally
used in conjunction with a DC-DC boost controller to pre-
cisely create the APD bias voltage. A dual range sample/
hold circuit with a range selection multiplexeor is included
to hold the analog voltage while an external ADC per-
forms the conversion.
Current Mirrors
The DS3923 includes three current mirrors as shown in
Figure 2. One is a 10:1 (10%) mirror connected at IOUT.
The other two are a 1.25:1 (80%) and the 5:1 (20%) mirror
connected to VIP1 and VIP2 as well as the internal sample/
hold circuit. On pin VIP1 and VIP2, resistors to ground
should be selected such that the maximum voltage is
0.625V (2.5V after the 4x gain of the built-in amplifier).
For example, if the maximum monitored current through
the APD is 1mA, the mirror is 1.25:1 ratio, and then the
correct resistor is approximately 780Ω.
Current Mirror Filter
The DS3923 includes a filter to stabilize the MIROUT
voltage. An external capacitor must be connected to
pin MIRCAP. See specification t
MIRCAP
in the Electrical
Characteristics table for filter and capacitor settings.
Current Source for Mirror Bias
The current mirror response time is improved by providing
a continuous current source. This source is adjustable by
changing the resistor connected to the ISRC/SHDN pin.
However, only one value is allowed to guarantee perfor-
mance (see Electrical Characteristics table).
ISRC ISRC
I (6 / R ) 6%= ±
Typical R
ISRC
value should be 30kΩ.
Figure 2. DS3923 Current Mirrors
0.1I
1
0.2I
1
0.8I
1
1.0I1
CURRENT
LIMIT
TEMPERATURE
LIMIT
MIRCAP
RLIM
MIROUT VIP1 VIP2 IOUT
MIRIN
HV BIAS
DS3923
DS3923 High-Speed Current Mirror
with Sample/Hold Output
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