Occasionally, the need arises for a simple current source that can be easily adjusted over a relatively wide range of current levels. The circuit shown in the figure uses a 200-kHz pulse-width modulation (PWM) step-down switching-regulator IC to generate a programmable constant current of approximately 50 mA to 1.5 A. The eight-pin IC requires few external components and features an internal switch, current-sense resistor, thermal shutdown, and undervoltage lockout. The input voltage can range from 8 (undervoltage lockout) to 30 V. The PROG pin is used to set the output current.
The output current is programmed by sinking current from the PROG pin. Output current is approximately 2000 times greater than the current out of the PROG pin. Several different current-programming methods may be used:
- Because the voltage on the PROG pin is a stable 2.465 V, placing a resistor from the PROG pin to ground generates a constant current out of the PROG pin. This produces a current 2000 times greater out of the IC's BAT pin (see the figure, a, for the program resistor equation).
- The output current can be programmed via a dc voltage from either a voltage source or a digital-to-analog converter (DAC). The voltage source must be able to sink approximately 500 µA of current (see the figure, b).
- A variable-duty-cycle PWM signal can also be used to program the output current (see the figure, c). For example, a 10% duty-cycle signal will produce 10% of full current; a 50% duty cycle will produce 50% of full current; and so on. The frequency of the PWM signal isn't critical and can range from approximately 100 Hz to 10 kHz or more. But lower frequencies cause higher ripple current.
The IC package includes a 1.5-A power switch. To adequately dissipate the heat generated by the IC, the pc board must have generous amounts of copper, especially around the leads. Most of the heat generated by the IC is conducted through the copper leads to the pc-board copper, which is the heatsink. Using a double-sided pc board with feedthrough vias to the backside copper increases the ability of the board to spread the heat to the surrounding air.
For currents up to approximately 500 mA and output voltages to 10 V, the minimum parts configuration of the circuit is adequate (see the figure, a, again). But the modified circuit is recommended for higher currents and/or higher output voltages (see the figure, d). The IC features a boost pin that, when used, reduces the saturation voltage of the switch. This increases efficiency, lowers the board temperature, and allows more output current. These few additional components and a 3- to 6-V source (at approximately 25 mA) can drop the circuit temperature by up to 20°C.