High-side current-sensing switched-mode regulator provides constant-current LED drive

Many suitable circuits exist for driving LEDs in constant-current mode and from low-voltage sources. For example, references 1, 2, and 3 show circuits that use switched-mode-regulator ICs and low-voltage sources to supply LED current. To produce a constant-current output using the circuit in Reference 2, you configure the regulator IC as a boost-mode switcher and use a resistor to sense the load current flowing in the LED string's low side, or negative-return leg. The sense resistor produces a proportional voltage that's applied to the LT1300's Sense input through a 2.5V reference diode. A voltage of 3.3V appearing at the LT1300's feedback-input terminal, Pin 4, indicates that the circuit's output is within regulation.

In applications that require a series string of LEDs to operate with its low side connected to ground, current sensing must take place in the string's high side. You can use either a rail-to-rail op amp and a handful of passive components or a dedicated current-sensing IC, such as Maxim's MAX4073T, to accomplish high-side sensing. However, adding a current-sensing IC increases circuit cost. To complicate matters, in this application, only three conductors are available to connect a remotely mounted LED string, D₃ through D₁₀, and on/off switch S₁ to the regulator circuitry.

In this Design Idea, an LT1300, IC₁, boosts 9V to drive the LED string, which presents a total forward-voltage drop of approximately 12V (Figure 1). Resistor R₄ serves as a current-sense resistor. At a current of approximately 40 mA, transistor Q₁ conducts and forces current through R₃, developing sufficient voltage drop to produce the requisite 3.3V at Sense Pin 4 of IC₁, bringing its output current into regulation. Zener diode D₂ limits the regulator's output voltage in case the LED string or connector opens. Switch S₁ turns on the circuit by grounding IC₁'s Pin 3.