Single Resistor Provides Extra Current from a Linear Regulator

Adding a 33Ω resistor between the input and output of a linear regulator, as shown in Figure 1, boosts the regulator's output current to 200mA. Note: this technique requires that the application draws a known minimum output current.

IC1 by itself provides a maximum output current of 150mA, but for applications that require a slightly higher maximum while maintaining a finite minimum, the small resistor offers a simple and stable solution. For the Figure 1 circuit, adding the 33Ω resistor boosts the maximum by 50mA while imposing a minimum output current (I_MIN) of 50mA:




Figure 1. Adding a 33Ω resistor boosts the output current of this linear regulator from 150mA to 200mA (the application must draw a known minimum output current).

Like most linear regulators, IC1 is unable to maintain regulation by sinking current. If the output current (I_OUT) drops below I_MIN, the output voltage rises above the regulated level, as high as V_IN, according to Kirchoff's Law:



Figure 2 compares load regulation for the Figure 1 circuit with and without the extra resistor. The dotted line represents output voltage (with the resistor in place) when I_OUT drops below I_MIN.


Figure 2. Output-load regulation for the Figure 1 circuit shows that the application must draw a minimum output current (50mA, in this case).

This design idea appeared in the October 2005 issue of Electronic Techniques (China).