Single Resistor Provides Extra Current from a Linear Regulator
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 (IMIN) 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 (IOUT) drops below IMIN, the output voltage rises above the regulated level, as high as VIN, 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 IOUT drops below IMIN.
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).