Do you want to power a group of lighting LEDs? You’re probably thinking: "That’s easy. Wire LEDs plus series resistors... job done!" While this offers a basic solution, what we have here is a neat, deluxe version to provide extremely accurate and stable current source for up to 100 white LEDs.

Light is the common theme of many seasonal festivals held around the world. I built a power supply for LEDs, with which the operating currents can be adjusted very precisely and stably. A “programmable current regulator” such as the Analog Devices LT3092 IC proved to be ideal for this application. This is an integrated, serial current source to provide an adjustable current between 0.5 and about 300 mA. Powered from a 24 V supply it can drive up to six white LEDs connected in series.

A Current Source IC

The LT3092 datasheet indicates that the IC can dissipate a maximum of about 1 W of power. It also shows a basic circuit (Figure 1) to configure the chip as a constant current source and gives the formula for selecting the resistors to set the current value and also specifies the minimum and maximum voltage dropped across the IC.

Current source IC fig1
Figure 1: The basic circuit of the LT3092 IC and the formula to set the current.

The forward voltage drop of a “standard” white LED is typically around 3.3 V at a nominal current of 20 mA. If you are using the constant current chip to drive just one LED, you could get away with using a supply voltage of 5 V because 3.3 V will be dropped across the LED and 1.2 V across the chip, which comes to a total of 4.5 V. Using a 12 V supply allows three LEDs connected in series and at 24 V six LEDs in series are possible. If you want to take it to the max, one chip powered from a 36 V supply can drive up to 10 white LEDs in series. Using LEDs of another color that have a lower forward voltage drop will allow even more LEDs to be used in series. White LEDs with a higher power rating will typically have a higher forward voltage drop of around 3.6 V, which means fewer of them can be connected in series in this application. You will need to take this into consideration in your own particular setup.

According to the formula in Figure 1, the current output level results from the voltage drop at RSET through ROUT. Since 10 µA flows through RSET, a voltage drop of 1 V will be produced when RSET is 100 kΩ. This voltage drop is added to the minimum voltage across the IC. Using a value of 100 kΩ for RSET gives a total voltage drop across the current source circuit of at least 2.2 V, meaning that a single white LED would not start conducting if a 5 V supply were used. A value of 30 kΩ is more practicable. Using a standard leaded 3- or 5-mm LED at 20 mA with an ROUT of 15 Ω results in a total voltage drop across the circuit of a more acceptable 1.5 V.

As already mentioned, the maximum power dissipation in the IC is 1 W. The LT3092IST#PBF used here is in a three-pin SO223 package. The resulting heat dissipated in the chip is transferred to the outside world via an “exposed pad,” which should be connected to the output as the fourth pin. In the Board Area and Thermal Resistance table, you can see how large the required board area should be in order to keep the chip temperature within its operating limits (Table 1).

Table 1: Board area and thermal resistance.
Table 1: Board area and thermal resistance.

The Circuit

The circuit shown in Figure 2 is just a simple tenfold copy/paste of the basic circuit. The value of resistors R11 to R20 is set to 30 kΩ (available in the E24 range of values), while R1 to R10 are 15 Ω to provide a suggested current of 20 mA for the LEDs. Choosing 1% tolerance resistors ensures the current through all the LEDs will be closely matched thereby minimizing differences in brightness.

Current Source LEDs project
Figure 2: The final tenfold current source
circuit is based on Figure 1. Click to
download the PDF and to zoom in.

Increasing the values of R1 to R10 will reduce current and LED brightness. 30 Ω resistors in these positions will result in 10 mA through the LEDs, while 62 Ω will give almost 5 mA. The PCB for this project has been designed to supply a current maximum of 25 mA; it cannot dissipate too much heat due to the limited board area available. The layout is, however, very neat and compact. The layout files in Target3001 format can be downloaded free of charge here. The circuit board gets its power via a two-pole barrel connector; up to 10 series-connected LEDs (when operated at 36 V) can be connected (observing correct polarity) to each of the J1 to J10 connectors. The author’s prototype can be seen in Figure 3.

The IC requires a minimum of 1.2 V to operate, and the voltage drop across ROUT is 0.3 V. The minimum supply voltage should therefore be at least n x forward voltage + 1.5 V, where n is the number of series-connected LEDs. The maximum supply voltage is given by the power dissipated in the circuit: If the power dissipation per IC is set to a reasonable value of 100 mW, it can operate with a voltage drop of 5 V. In practice, the operating voltage range of the circuit can be calculated using n x 3.3 V + 1.5 V to 5.3 V for white LEDs.

The author’s prototype.
Figure 3: The author’s prototype.

To Sum Up

The LT3092 IC currently retails at around €4. You can cut costs if you are driving fewer than 100 LEDs by not populating the whole PCB. For example, to drive 30 LEDs you only need three ICs running from 36 V or 5 ICs at 24 V. If more LEDs are required later, you can always retrofit them to give maximum flexibility. The voltage rating of C1 should be chosen to be above the supply voltage used. For example, if the circuit is powered by a 12 V supply, the capacitor voltage rating should be 16 V or above. Even though SMD components are used, they are reasonably large versions and can be easily soldered by hand.

If cost is an important consideration, the LT3082 type IC can be substituted, which would save almost €1 per IC at current pricing. A small mains adapter type power supply with an appropriate output voltage can be used to power the circuit. Compared to linear stabilized models, small switched-mode power supplies offer better efficiency. Unstabilized power supplies should not be used, they may result in excessive power dissipation in the ICs. Since all current flows directly through the LEDs, the required current capacity can also be easily calculated. Using all 10 LED channels, each driving 20 mA, requires a power supply rating of at least 200 mA, which is well within the range of almost every available mains adapter.

Editor's Note: This article appeared in the Elektor Circuit Special 2022.

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