Many suitable circuits exist for driving LEDs in constant-current mode and from low-voltage sources. For example, references 1, and 2 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 1, 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 2.5 V reference diode. A voltage of 3.3 V 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.
|Figure 1.||A single switched-mode-regulator IC drives a series-connected string of
LEDs in constant-current mode.
In this Design Idea, an LT1300, IC1, boosts 9 V to drive the LED string, which presents a total forward-voltage drop of approximately 12 V (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 3.3 V 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.