Regulators, or converters, including a switch, sometimes referred to as a power switch, for transferring energy from an input, such as an AC or DC voltage or current source, to a regulated output are well known. In some regulators, sometimes referred to as switching regulators, the switch turns on and off to regulate the output. In other regulators, sometimes referred to as linear regulators, the switch operates in its active, or saturation region.
Common switching regulator configurations include Buck, Boost, Buck-Boost, flyback, SEPIC, Cúk, half bridge, and full bridge to name a few. As is also well known, various control methodologies for controlling conduction of the power switch can be applied to switching regulators, including Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM), and for each of these control methodologies, various feedback and feed forward techniques are possible including voltage mode control and current mode control.
Switching regulators are often used to provide a regulated current to drive an LED load as may include one or multiple LEDs coupled in series and/or parallel. Such switching regulators control the intensity or brightness of the LEDs by selectively dimming the LEDs. In one type of LED dimming, sometimes referred to as analog dimming, the intensity of the LEDs is adjusted by adjusting the regulated LED current. Analog dimming may utilize an error amplifier which is responsive to a reference signal and to a feedback signal proportional to the regulator output to generate an error signal. The error signal is then used to generate a switch control signal to control conduction of the switch and the resulting regulated current. For example, the error signal may be compared to a ramp signal to generate the switch control signal with a duty cycle suitable for regulating the LED current at a desired level.
In another type of LED dimming, sometimes referred to as PWM dimming, the intensity of the LEDs is adjusted by turning them off and on in response to a PWM signal at a variable duty cycle with a fixed DC current and frequency (typically 100 Hz to 1 KHz). The PWM signal may be externally provided or internally generated.
For LEDs, often PWM dimming is preferred over analog dimming because it minimizes the color shift that can occur when using analog dimming. On the other hand, analog dimming can be less complicated to implement than PWM dimming and PWM dimming pulses the LED current, which can cause visible flicker, audible noise, or EMI issues.
FIG. 1 illustrates a Boost regulator driving an LED load with current mode control that permits both PWM dimming and analog dimming. Analog dimming is achieved by adjusting the reference of the error amplifier (I1) according to the voltage on an external pin. As illustrated in FIG. 1, the IREF pin is the external input that can be used to control the error amplifier reference when the voltage is below 1.0V. When the IREF pin voltage is greater than 1.0V, comparator I10 changes the state of analog multiplexer (“mux”) I13, running the error amplifier off of the internal reference, which may be more accurate than the voltage supplied on the IREF pin. This feature is sometimes implemented by adding an additional positive input to the error amplifier (I1) that overrides the internal reference when the external signal is less than the internal reference.
For PWM dimming, the user controls the intensity of the LEDs by supplying a digital PWM signal into the PWM pin in FIG. 1. As noted, some LED regulators may include the generation of the PWM signal on chip, in which case the user supplies an analog signal that is translated into a PWM duty cycle. Referring also the illustrative waveforms of FIG. 2, when the PWM signal is low the SW node is tri-stated with AND gate I5 to cause the LED current to be disabled. In addition, many LED regulators also tri-state the COMP node by introducing switch SW1. This technique allows the control loop to quickly recover when the regulator is re-enabled on the PWM input rising edge. Without switch SW1 the error amplifier (I1) would have to slew the COMP node to resume operation.
To provide an even faster LED turn on and turn off, some regulators introduce a switch in series with the LEDs that is driven by the PWM input as illustrated by M2 in FIG. 1. Without M2, the turn off characteristic of the LED current would be exponential because the output capacitor (Cout) will continue to supply current to the LEDs until the dynamic resistance in the LEDs discharges Cout enough to turn off the diode component of the LEDs. The dynamic resistance in the LEDs causes the output capacitor to more slowly discharge, which results in the soft turn off characteristic and a small voltage ripple across the output capacitor. This soft turn off characteristic makes the relationship between the PWM pin duty cycle and the LED intensity non-linear, especially at low duty cycles. On the other hand, turning the LED current instantaneously on and off can cause EMI issues in some systems, since fast edges produce more harmonic noise further up in the frequency domain which can be difficult to attenuate. This issue is especially aggravated when there are wires between the LED driver and the LEDs, as shown in FIG. 1, because they act as an antenna for EMI emission.