July 17, 1997

Switching regulator turns into light source

Donald V Comiskey Jr, Power Trends Inc, Warrenville , IL

The switching regulator, IC₁, in Figure 1 normally functions as a voltage regulator; that is, it provides a constant output voltage to a variable load. However, by adding a small amount of external circuitry, you can transform the IC into a constant-intensity light source. In this application, the regulator provides a variable-output voltage to a variable load to maintain a constant intensity. Using a switching regulator, which can provide typical efficiencies of 85 to 95%, in this application provides much better performance than using the alternative bulky, inefficient linear device.

The constant-intensity application uses the adjust pin of IC₁ with an external feedback mechanism. Internally, the adjust pin connects to the center point of a high-impedance resistive-divider network that samples the regulator's output voltage. For proper operation, the external feedback circuit of this constant-intensity source must appear as a low-impedance voltage source capable of dominant control of the adjust pin. For greatest accuracy, the characteristic impedance of this voltage source should be less than 1 kilohm. Using a current-driven, low-impedance sense resistor is one way to establish this dominant voltage source.

Normally, IC₁'s internal circuitry monitors and regulates the IC's output voltage to maintain a voltage of 1.25V at its adjust pin. The addition of the external feedback circuit instead forces the IC to monitor and regulate the light-source intensity to maintain the same 1.25V at the adjust pin. The external circuit uses a photodetector and a current-sense resistor to provide a voltage translation of the delivered light intensity. The circuit scales this intensity-to-voltage translation so that the desired light intensity equates to a voltage of 1.25V. You accomplish the scaling by selecting the value of the current-sense resistor for a known photodetection current.

The circuit uses a phototransistor as an emitter follower and produces a certain photodetection current at the desired light intensity. The detection current flows through the low-impedance current-sense resistor, which develops a voltage drop of 1.25V. In effect, IC₁ regulates the photodetection current from the phototransistor, which corresponds to the desired light intensity.

As an example, assume that at the desired light intensity, the photodetection current is 5 mA. Then, R_SENSE=1.25V/5 mA=250 ohms. The closed-loop circuit regulates the 1.25V across the 250 ohms resistor to maintain a constant phototransistor current of 5 mA. The maintained 5-mA detection current corresponds to a regulated intensity from the light source.

Depending on the response characteristics of the load and the detector, you may need external compensation, C_F, to ensure stable operation. You may also need an optional zener diode, D₁, to limit the maximum output voltage of IC₁. Such limiting may be necessary when the circuit is unable to achieve the desired light intensity because of degradation of the light source or an obstructed light path. You should select D₁ to clamp the output voltage of IC₁ to V_ZENER+1.25V. Without D₁, the output of IC₁ could approach V_IN in an effort to obtain the desired light intensity. You should design the constant-intensity source so that the output voltage of the regulator IC doesn't exceed 25V.

Although this application configures IC₁ as a constant-intensity source, this IC still functions as a buck-type step-down regulator and requires a certain amount of head room between the input and output voltages to guarantee regulation. The required head room for the Power Trends PT6100 series is typically 2.5V. Thus, the desired light intensity (the voltage developed across the light source) should not exceed V_IN­2.5V. Also, you need to choose the appropriate regulator in the series to accommodate the current demand of the light source.

I tested the circuit concept using an npn silicon phototransistor and a 12V filament-type bulb. The phototransistor you need depends on the desired sensitivity and the spectrum of the light source. (DI #2057)

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