In designs using multiple LED strings such as brake lights, daytime running lights, rear lights and other automotive lighting applications, current-balancing circuits supply equal current to all LED strings in order to prevent different brightness levels.
The discrete transistor-based current-balancing circuit is used widely for its simple circuit and use of few components, though there are some performance disadvantages. Mismatches in the transistor base-emitter voltage (VBE), LED forward-voltage drops and variations over temperature can yield mismatched LED brightness. This article compares the technical performance of an operational amplifier (op amp)-based LED current-balancing circuit to the more common discrete transistor-based solution.
Figure 1 shows a simplified schematic of a discrete transistor-based LED current-balancing circuit.
Figure 1 This is how a discrete transistor-based current-balancing circuit looks like. Source: Texas Instruments
The circuit in Figure 1 uses a fixed voltage (VS1) with a resistor (R1) and bipolar NPN transistor (T1) configured as a diode to set the reference current (IREF). IREF is mirrored through transistors T2 and T3 as I1LED and I2LED, respectively. Equation 1 displays the current through the transistors. Ideally, currents IREF, I1LED and I2LED are equal.
IREF = VREF/R1 = VS1-VT1BE/R1 (1)
VT1BE is the base-to-emitter voltage of transistor T1. This voltage can vary greatly over process and temperature for the same given current. In this circuit, there are three legs of current, and only two of them go through LED strings. The power supply must provide the current drawn by T1 to set IREF. This current, drawn by T1, could amount to multiple milliamperes and thus contribute to power loss.
Figure 2 shows a simplified schematic of an op-amp-based LED current-balancing circuit.
Figure 2 The Op-amp-based current-balancing circuit uses a high-side voltage to a current converter to supply a constant current through the LEDs. Source: Texas Instruments
The reference voltage (VREF) is created with a voltage divider using R1, R1B and VS1. The current is regulated by feeding the voltage across sense resistors R2 and R3 back to the inverting inputs of the LM2904B-Q1 dual automotive op amp. Equation 2 calculates the current through transistors T2 and T3:
I1LED = I2LED = VREF/R2 = VS1/R2 × R1/(R1+R1B) (2)
Where R2 is equal to R3.
I1LED1 and I2LED depend on the resistor tolerance and temperature drift coefficient of R1, R1B, R2 and R3. Using precision resistors (for example, 0.1%) will minimize the effect of resistor tolerance and temperature drift; it will also ensure better current matching over process and temperature variations than a discrete transistor-based solution. Because VREF sets the current—unlike in the discrete transistor-based circuit—it is possible to reduce the current through R1 and R1B by increasing their values to minimize power loss.
Effect of NPN VBE mismatch in LED current
Now let’s compare the effects in mismatched component characteristics and effects over temperature and power supply between the two circuits.
A mismatch of the NPN transistor’s VBE for the same current can occur if:
- The transistors differ between manufacturing lots, emitter area or temperature.
- Stress exists on the PCB where the transistors are placed.
- The base resistance and base current of the NPN transistor differ.
Mismatches in VBE affect the current flowing through each LED current string.
A bipolar transistor’s VBE voltage can drift by –2 mV/°C. A temperature difference of 10°C between transistors can create a 20-mV difference in VBE in addition to the initial VBE offset voltage.
Figure 3 displays a discrete transistor-based current-balancing circuit with two strings containing five LEDs. The supply voltage (VS1) is set to 12 V. Resistor R1 and NPN transistor T1 configured as a diode set IREF through T1 to 20 mA at room temperature. Adding voltage sources VF and VBE_OFFSET to the circuit represent a difference in the LED forward-voltage drop and the VBE voltage of the transistors, respectively. Preferably, these voltage sources would not be present in the circuit.
Figure 3 The schematic highlights the effects of VBE and VF of a discrete transistor-based circuit. Source: Texas Instruments
Figure 4 illustrates the effects of variations in VBE voltage between T2 and T3 for the circuit in Figure 3. Sweeping VBE_OFFSET from –25 mV to 25 mV results in a variation in I2LED current from 31.5 mA to 11.5 mA, respectively. There is as much as a 57.5% mismatch between I1LED and I2LED from the set current of 20 mA at a VBE_OFFSET of 0 V. As shown in Equation 1, the current through the LED depends on VT1BE. Therefore, any mismatch in VBE results in an LED string current mismatch, including initial offset errors and variations over temperature.
Figure 4 Effects of VBE voltage differences are shown for a discrete transistor-based circuit. Source: Texas Instruments
Figure 5 shows the recommended current-balancing circuit with op amps and two strings of five LEDs. The component values shown in the schematic generate 20 mA of current in each leg. To make a fair comparison, the variables and sweep conditions are the same as the schematic in Figure 3.
Figure 5 Effects of VBE are shown for an op-amp-based circuit. Source: Texas Instruments
Figure 6 displays the effects of variations in VBE voltage between T2 and T3 for the circuit in Figure 5. Sweeping VBE_OFFSET (which is the difference in VBE voltage of the T2 and T3 transistors) from –25 mV to 25 mV results in a variation in I2LED current from 19.92101 mA to 19.921024 mA, respectively. The 0.00004% mismatch between I1LED and I2LED from a set current of 19.921017 mA at a VBE_OFFSET of 0 V is a significant improvement compared to the discrete transistor-based circuit. This decrease in error is because the design uses the feedback loop of the op amp to ensure a constant voltage across the sense resistor to generate the 20 mA of current.
Figure 6 Effects of VBE voltage differences are shown for the op-amp-based circuit. Source: Texas Instruments
Effect of self-heating on LED currents
The temperature difference across the PCB and localized self-heating has a major effect on LED current-balancing circuits. The effect of self-heating is not modeled in discrete transistors and LED circuit models used for simulation; thus, its effect is not visible in the simulation.
A temperature difference between the T2 and T3 transistors caused by varying distances from the heat source can lead to a significant difference in VBE. This difference will cause more current in the hotter LED string, resulting in an additional temperature increase (higher power dissipation) and a positive feedback cycle. If this thermal-positive feedback remains uncontrolled, a single LED string can consume all of the current from the supply and damage the transistor and LEDs. Designers must consider self-heating if a single source powers all of the LED strings.
The op-amp-based circuit is immune to any self-heating caused by current imbalances because the voltage across R2 and R3 is regulated and constant. If transistor self-heating causes a drift in VBE, the closed-loop configuration changes the base voltage (op-amp output) to drive the transistor and keep the current constant. There will be minimal drift in I2 and I3. However, if offset voltage drift is a concern, consider using a lower-drift device such as the OPA4991-Q1 quad automotive op amp.
Effect of power-supply variation on LED currents
Figure 7 shows the effects of the LED power-supply voltage changing from 12 V to 14 V for the discrete transistor and op-amp-based circuits, respectively. The IREF for the discrete transistor circuit and the VREF for the op-amp circuit both depend on the power-supply voltage. The change in power supply affects the current proportionally. Both circuits have variations in LED current over the change in LED power supply, but the slope of the change in the op-amp-based solution is lower. If the magnitude of the supply change is large, the discrete transistor-based solution will have the most deviation from the initial current.
Figure 7 A comparison shows effects of power-supply variation in the two circuits. Source: Texas Instruments
Using an external voltage reference to set IREF in the circuit shown in Figure 2 may require a Vref with a higher output current, but could minimize the effect of power-supply variations. The circuit in Figure 4 has a low-current load to set Vref.
Effect of LED forward-voltage drop on LED currents
Individual LEDs can have different forward-voltage drops caused by process variations or differences in area or temperature. These differences will cause a cumulative forward-voltage drop difference between LED strings.
Figure 8 shows the simulation result comparing the discrete transistor-based circuit and op-amp-based circuit with the variable, VF, swept from –1 V to 1 V. This emulates the difference in a cumulative forward voltage between LED string 1 and LED string 2. The results show a 330-µA LED string current variation in the discrete transistor-based circuit, while the LED string current stays relatively constant for the op-amp-based circuit.
Figure 8 The simulation result compares the effects of LED forward-voltage drop in the two circuits. Source: Texas Instruments
Comparing two LED current balancing circuits—a discrete transistor-based circuit and an op-amp-based circuit—shows the effects of component mismatch and effects over temperature and supply voltage. As shown in this article, the op-amp-based circuit adjusts to VBE variations caused by process variations and temperature, and is more immune to power supply and LED forward-voltage drop variations.
Sanjeev Manandhar is systems engineer at Texas Instruments (TI).
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