Maxim Design Support Technical Documents Reference Designs

Maxim Design Support Technical Documents Reference Designs
Maxim > Design Support > Technical Documents > Reference Designs > Amplifier and Comparator Circuits > APP 4122
Maxim > Design Support > Technical Documents > Reference Designs > Optoelectronics > APP 4122
Keywords: MAX4007, Current Monitor, Current monitoring, current regulator, current mirror, photodiode
current
REFERENCE DESIGN 4122 INCLUDES: Tested Circuit Schematic Description Test Data Simple Regulator Provides Constant Voltage for
High-Side Current Monitors
By: Arpit Mehta, Strategic Applications Engineer, Amplifiers & Sensors
Dec 21, 2007
Abstract: In a typical photodiode current-monitoring application, the voltage drop between the current
monitor and the avalanche photodiode (APD) varies with the temperature and current flowing through it,
thereby changing the overall gain. This application note describes a regulator circuit that solves this
problem by maintaining a constant voltage drop across the current monitor and APD.
Overview
The voltage drop between a current monitor and avalanche photodiode (APD) varies with the
temperature and current flowing through the photodiode current-monitoring application. Therefore, in
applications such as fiber and instrumentation transmission systems, the ability to detect the average
photodiode current is essential for efficient system management.
Typical Application Circuit
The MAX4007/MAX4008 are precision, high-side, high-voltage current monitors specifically designed for
monitoring photodiode currents. These devices provide a connection point (REF) for the reference
current and a monitor output that provides a signal proportional to the reference current. A suitable APD
(avalanche photodiode) or PIN (positive-intrinsic-negative) photodiode is connected at the REF pin of the
IC as shown in Figure 1. The REF pin provides the source current to the cathode of the photodiode.
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Figure 1. Typical application circuit using the MAX4007 high-side current monitor.
Due to the steep slope of the photodiode's V-I characteristics, a slight change in the voltage across the
photodiode causes a significant change in its current, thereby changing the overall gain of the fiber
application circuit. The photodiode's voltage drop varies with the temperature and current flowing through
it. The typical voltage drop (VBIAS - VREF) across the MAX4007 series is 0.8V, with a guaranteed
maximum of 1.1V.
Figure 2 shows the variation of the reference voltage with the change in REF current IREF. The plot
shows that in the conventional unregulated circuit VREF varies by as much as 0.4V with the 1µA to 4mA
photodiode-current range. This variation in VREF voltage might be unacceptable in some applications. A
major challenge in regulating the REF voltage is that VBIAS can be as high as 76V.
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Figure 2. Variation of the reference voltage with IREF.
Regulator Circuit
The regulator circuit shown in Figure 3 solves both of the above-mentioned problems by maintaining a
constant voltage drop from VSUPPLY to VREF. It consists of a 2.048V stable reference (the MAX6007)
and a low-bias-current op amp (the MAX4037) with a 1.2V internal buffered reference. Resistor R1 sets
the bias current.
The op amp supply will always see a difference across its supply of 3.248V (2.048V + 1.2V) over the full
5V to 76V supply-voltage range. The output of the MAX4037 op amp is fed to the BIAS pin of the
MAX4007, and its REF pin connects to the inverting input of the op amp. The op amp absorbs any
change in the REF voltage by keeping this voltage fixed to the voltage at its noninverting input (VSUPPLY
- 2.048V). The MAX4037 is able to source currents over the entire 1µA to 4mA photodiode-current
range.
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Figure 3. Regulator circuit for the MAX4007/MAX4008 current monitor.
Based on the plot of the regulated output in Figure 2, one can conclude that for the 1µA to 4mA
photodiode-current variation the REF voltage (VSUPPLY - VREF) is constant at 2.047V. Figure 4
illustrates that this voltage remains constant for supply voltages from 5V to 76V for different reference
currents of 1mA, 3mA, and 4mA.
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Figure 4. VSUPPLY - VREF vs. VSUPPLY for different values of bias currents.
A slight voltage error is introduced by a couple of error sources. One is the offset voltage of the
MAX4037 op amp, but this is very low (±2.0mV) and drifts only by 100µV/°C over temperature. The
second one is the breakdown-voltage variation of the MAX6007, which varies by ±1.3mV over the
current range and 75ppm/°C over temperature. Despite these error sources, the proposed regulator
circuit represents a significant improvement over the unregulated circuit.
Conclusion
The circuit proposed in this application note nullifies any variation in the voltage drop across the
MAX4007/MAX4008 current monitor. Consequently, it provides a stable, known voltage for the
photodiode in fiber applications over the full range of load currents and supply voltages.
Related Parts
MAX4007
High-Accuracy, 76V, High-Side Current Monitors in
SOT23
Free Samples MAX4008
High-Accuracy, 76V, High-Side Current Monitors in
SOT23
Free Samples MAX4037
Low IBIAS , +1.4V/800nA, Rail-to-Rail Op Amps with
+1.2V Buffered Reference
Free Samples More Information
For Technical Support: http://www.maximintegrated.com/support
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For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 4122: http://www.maximintegrated.com/an4122
REFERENCE DESIGN 4122, AN4122, AN 4122, APP4122, Appnote4122, Appnote 4122
Copyright © by Maxim Integrated Products
Additional Legal Notices: http://www.maximintegrated.com/legal
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