Simple voltage regulator circuit provides constant voltage for high-side current monitor

Overview

The voltage drop between the current monitor and the avalanche photodiode (APD) varies with temperature and the current flowing through the photodiode. Therefore, in fiber optic transmission and instrumentation systems, sensing the average photodiode current is particularly important for effective system management.

Typical application circuit

The MAX4007/MAX4008 are high-precision, high-side, high-voltage current monitors designed to monitor the current of a photodiode. These devices provide a connection point for a reference current (REF) and a sense output proportional to the reference current. Connect an appropriate APD (avalanche photodiode) or PIN (anode-intrinsic-cathode) photodiode to the REF pin of the IC, as shown in Figure 1. The REF pin sources current to the cathode of the photodiode.

Figure 1. Typical application circuit for the MAX4007 high-side current monitor.

Due to the steep slope in the photodiode's volt-ampere characteristic, a small change in the photodiode's voltage results in a large change in the current, which changes the overall gain of the fiber-optic application circuit. The voltage drop across the photodiode varies with temperature and current. The voltage drop (VBIAS - VREF) for the MAX4007 device is typically 0.8V and has a maximum of 1.1V. Figure 2 shows the variation of the reference voltage with the REF current, I REF. This graph shows the results of a traditional, unregulated circuit test, where VREF changes by as much as 0.4V when the photodiode current varies from 1μA to 4mA. In some applications, such a large VREF change may not be acceptable. The biggest problem with regulating the REF voltage is that VBIAS can be as high as 76V.

Figure 2. Reference voltage changes with IREF.

Voltage stabilization circuit

The voltage regulation circuit shown in Figure 3 solves both of these problems by maintaining a fixed voltage drop from VSUPPLY to VREF. The circuit consists of a 2.048V regulated reference (MAX6007) and a low bias current op amp (MAX4037) with an internal buffered 1.2V reference. Resistor R1 sets the bias current. The supply voltage on the op amp is maintained at 3.248V (2.048V + 1.2V) over the entire 5V to 76V supply voltage range. The output of the MAX4037 op amp is fed into the BIAS pin of the MAX4007, and its REF pin is connected to the inverting input of the op amp. The op amp absorbs any changes in the REF voltage and keeps that voltage fixed to the noninverting input (VSUPPLY - 2.048V). The MAX4037 is capable of sourcing photodiode currents over the entire range of 1μA to 4mA.

[page] Figure 3. Voltage-regulation circuit for the MAX4007/MAX4008 current monitor Based on the test results of the voltage-regulation circuit shown in Figure 2, it can be concluded that the reference supply (VSUPPLY - VREF) remains constant at 2.047V when the photodiode current varies from 1μA to 4mA. As can be seen in Figure 4, the reference voltage remains constant for different reference currents of 1mA, 3mA, and 4mA when the supply voltage ranges from 5V to 76V.

Figure 4. VSUPPLY - VREF vs. VSUPPLY for different bias currents

There are many sources of error that can cause voltage errors. One of them is the offset voltage of the MAX4037 op amp, but in reality, the offset voltage is very low (±2.0mV) and has a very small temperature drift of only 100μV/°C. The second error source is the breakdown voltage of the MAX6007, which varies by ±1.3mV over the entire current range and has a temperature coefficient of 75ppm/°C over the entire temperature range. Even considering these error sources, the voltage regulation circuit presented in this article still improves system performance significantly compared to the unregulated circuit.

in conclusion

This application note presents a circuit that cancels out the voltage-drop variation of the MAX4007/MAX4008 current monitors, thereby maintaining a stable, known voltage across the entire load current range and supply voltage range for photodiodes in fiber-optic applications.