A fully floating BJT-based LED current driver

The circuit in Figure 1 combines a V_BE-referenced current source with a current mirror to implement a simple two-terminal, fully floating LED current sink or source. This approach is well-suited for applications in which tight current accuracy is not required, such as driving LED strings where a 5–10% current tolerance is acceptable.

Figure 1.	A simple, fully floating LED current driver based on a VBE-referenced current source and a BJT current mirror. The circuit operates as either a current sink or source and supports output currents up to 100 mA. Note: R2 = R3. All resistors are ¼ W and 5%.

The LED driver can drive an arbitrary number of series-connected LEDs, provided the available supply voltage is at least 2.3 V. The topology supports both high-side and low-side operation, as shown in Figure 2. Output current ranges from a few milliamps up to 100 mA, with no requirement for heat sinks.

Figure 2.	High-side and low-side operating configurations enabled by the fully floating nature of the LED driver.

The current source formed by BJTs Q1 and Q2 is set by resistor R1. A current mirror implemented with BJTs Q3 and Q4, using equal emitter resistors (R2 = R3), forces nearly equal currents in branches I₁ and I₂, as long as the voltage drop across the emitter resistors is at least 0.5 V. This requirement helps compensate for V_BE mismatch between the transistors. The total LED current is therefore doubled, while power dissipation is evenly shared among the devices.

Experimental data (Table 1) confirm the expected behavior: output current scales with R1, and the minimum supply voltage increases from 2.3 V at 9.3 mA to 2.8 V at 97 mA, consistent with the headroom required by the V_BE-referenced source and mirror.

Table 1.	Experimental data showing R1, R2/R3, and corresponding output current and minimum supply voltage
R1 R2 = R3 Output current (IO) Minimum supply voltage 150 Ω 100 Ω 9.3 mA 2.3 V 82 Ω 56 Ω 18.2 mA 2.4 V 33 Ω 22 Ω 44 mA 2.5 V 15 Ω 10 Ω 97 mA 2.8 V

With a minimum operating voltage of approximately 2.8 V, the circuit dissipates about 280 mW at a maximum output current of 100 mA. Higher supply voltages reduce efficiency due to increased power dissipation in the driver.

Because the LED current is V_BE-dependent, it exhibits temperature sensitivity, with a temperature coefficient of approximately –0.3 %/°C. Using a resistor with a negative temperature coefficient for R1 can partially compensate for this effect.

Materials on the topic

1. Datasheet Fairchild BC550C
2. Datasheet Fairchild BC557B