The circuit shown provides a continuous, regulated, and adjustable dc current as required for certain electronic functions, such as the LED backlighting of LCD display panels (see the figure). A digital interface permits an external microcontroller to make adjustments to compensate for variations in temperature, aging, and other effects. Switchmode regulation delivers power levels well beyond 1 W at load voltages as low as 4 V.
Maxim’s MAX749 (IC2), a digitally adjustable LCD-bias generator, features a 6-bit digital-to-analog converter that allows 64 uniformly spaced levels of regulated output current. It usually operates as an inverting regulator, with its feedback terminal at 0 V during equilibrium.
To obtain some “headroom” on the low side of IC2, R3 biases the IC above ground. Meanwhile, the Zener diode (D1) establishes a proper supply voltage across it (6 V is the maximum specified). IC2, L1, and D2 (referred to ground) now form an ordinary buck regulator.
The OP-07 (IC3), Q1, and R1 mirror and scale the RSENSE current, making it compatible with the range of feedback current expected at IC2’s FB terminal (6 to 20 mA). To extend the current-regulation range down to zero, resistors R7 and R8 are used to remove the offset (30% of full scale) by simulating a load current.
The logic inputs CTRL and ADJ provide control of the internal counter and digital-to-analog converter according to instructions in the data sheet. For proper operation, these inputs must be level-shifted with respect to IC2’s ground terminal. For V+ below 18 V, the low-cost dual SPST analog switch shown (IC1) is a good choice for this task.