How To Add Analog Dimming To Virtually Any LED Driver

Jan 5, 2012 3:16 PM
Hector F. Arroyo

Analog dimming of LEDs has key advantages, including standalone operation, without the need for additional circuitry to generate a PWM signal (such as a microcontroller) and a patent-free environment.

Dimming LEDs is an important feature in many of today’s solid state lighting applications. While many implementations for dimming LEDs in the market today rely on PWM modulation of the LED driving current (to allow for digital control, achieve specific color temperature characteristics, etc.), direct modulation of the LED current (aka “analog” dimming) has recently grown considerably in popularity.

One issue designers face when looking to implement analog dimming control is that many of the existing LED drivers currently in the marketplace, while including an option for PWM dimming, they do not offer a native option for analog dimming. We will look at alternatives to implement analog dimming on conventional LED driver circuits, and a complete solution based on a simple, minimal-external-components architecture will be presented and analyzed.

The architecture of a common constant-current, closed-loop-control LED driver circuit is shown in Fig. 1. In this and most other LED driver architectures, either switching or linear, the LED current is sensed through a resistor in series with the LED. The voltage across the resistor, proportional to the LED current, is then used by the feedback (FB) input pin of the regulator (also called CS or Current Sense by some manufacturers) to adjust the control mechanisms and modify the voltage applied to the LED in order to maintain the current through it constant.

From that simple closed-loop operation, it can be clearly seen that by artificially manipulating the signal going into the regulator’s FB pin, it may be possible to alter the LED driver’s constant-current value, and thus implement an adjustable constant-current version of it, or in other words, analog dimming.

One goal of analog dimming is to be able to adjust the LED current (and hence the LED brightness) through a DC control voltage. In some instances, the goal is to use a simple potentiometer to adjust LED brightness.

The circuits that may be used to achieve such functionality may be diverse in their actual implementation, but their functionality will basically always need to be the same:

The circuit will need to still monitor the LED current and use that information to keep the LED current regulated; e.g. “constant” through closed-loop control.

The circuit will need to take an external control signal (DC voltage or potentiometer signal) to modify the value of the current through the LED and thus control its brightness.

Op-Amp Architecture

One architecture that easily comes to mind that can satisfy both of the conditions above is one using an operational amplifier. Through such a circuit, an external control voltage (analog dimming signal) and the actual LED current info may be combined and used together by the FB pin of the driver IC. While such a circuit should work very well to satisfy the analog dimming needs, one of the advantages of the analog dimming technique, as mentioned before, is simplicity. Not needing an additional IC to generate a PWM signal is certainly an advantage, however, such an advantage goes away if a proposed analog dimming circuit does need an additional IC to run, like in the case of an op-amp. To overcome such disadvantage, a method in which no additional IC is needed to combine an external control signal and the original feedback voltage is needed.

Current Injection Architecture

In regular voltage-output regulator circuits, a common technique used to adjust output voltage through an external control signal, is current injection [1]. Fig. 2 shows a simplified version of such architecture. Basically, in a current-injection approach, a DC control voltage is applied to FB node through a resistor (R3). Since there is a voltage differential between the FB node and the applied control voltage (V_CTRL), a small current is developed in resistor R3. An IC FB pin is a high impedance input, and thus the current the FB pin may sink or source is negligible. Assuming the FB pin current is basically zero, the only path the current through R3 may use is through the bottom feedback resistor, Rfb2. As this current flows through Rfb2, the voltage across this resistor changes, effectively modifying the voltage applied to the FB pin input and eventually manipulating the output voltage of the regulator through the applied control voltage.

This current injection concept, in principle, could be used in LED driver circuits, however, as can be quickly realized, the bottom resistor sensing current through the LED is usually a very small value resistor (sub-ohm) and in order to develop a significant voltage across it, a larger current will need to be injected, one basically in the same order as the LED current. With today’s high brightness LEDs (HBLEDs) being biased in the order of hundreds to thousands of milliamperes, the current injection approach, as traditionally implemented, is impractical.

Floating Voltage Divider Architecture

Fig. 3 shows an alternate architecture in which voltages from an external source and from the LED current sense resistor are being combined. In such an architecture, an arbitrary control voltage is able to control the LED current in its complete range. In its general case, this voltage can come from an external source, a potentiometer wiper, a DAC, etc.

Through the proper selection of the R1 to R2 ratio as well as the R_SNS value, basically any LED current can be programmed and controlled in its full range by an arbitrary DC voltage. The minimum range of V_CTRL goes from 0V to 2x V_REF (e.g. 0V to 400mV for a driver IC with a V_REF voltage of 200mV), but its upper limit may extend as high as needed, being able to use common control voltage ranges such as 0V to 5V, 0V to 10V, etc.

Circuit Analysis

In a “floating” voltage divider similar to the one formed by R1 and R2 in Fig. 3, the first parameter to analyze would usually be to find the value of the midpoint between the resistors (V_FB) given the values of the voltages at each end (V_CTRL and V_LED) and the resistor values (R1 and R2). Equation 1 shows this relationship. Its important to note, however, that in the particular case of this application, by the action of closed-loop regulation on the LED driver, the feedback voltage (V_FB) will track V_REF, and will be, for practical purposes, a fixed value. Given R1, R2, R_SNS and V_REF are all constants, it makes then sense to solve for V_LED instead, in order to ultimately calculate the LED’s forward current (I_F) in our circuit as a function of the other variables. Equation 2 shows Equation 1 solved for V_LED.

Equation 3 shows the relationship between V_LED and I_F. Using that relationship in Equation 2, gives in Equation 4 the general expression to calculate I_F as a function of V_CTRL.

Fig. 4 shows Equation 4 in action, and plots the control transfer function realized by the circuit in Fig. 3 with R1, R2 and R_SNS selected, in this example, to provide a 0 to 1A LED current range controlled by a DC voltage in the 0V to 5V window.

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