PDM modulation of LED current
A modulated current of an LED device provides a capability to dim of the light produced by a string of LED device. The current modulation takes the form of pulse width modulation (PWM) or pulse density modulation (PDM). The modulation is produced on the primary side of a transformer and coupled to the string of LED diodes that are coupled to the secondary side of the transformer. The modulation is varied to change the current of the LED devices and therefore the light intensity of the LED devices.
1. An LED light dimming circuit, comprising:
a) a string of at least one LED device;
b) a control circuit providing primary side regulation configured to produce drive current and modulation; and
c) said control circuit configured to hold drive current constant at a predetermined level and varying a duty cycle of a pulse density modulator (PDM) to change LED device illumination, wherein said control circuit is configured to produce a hysteresis in the drive current between a fixed PDM duty cycle portion and a variable PDM duty cycle portion of the LED current to enable a smooth transition in emitted LED light.
2. The dimming circuit of claim 1 , wherein said string of at least one LED device is coupled to a secondary side of a transformer which is configured to produce secondary side drive current and modulation.
3. The dimming circuit of claim 1 , wherein said PDM is configured to create a majority of modulated energy at higher frequencies.
4. The dimming circuit of claim 3 , wherein said PDM is a first-order modulator, which uses a carry-out of an accumulator and automatically wraps around on a 2 N count boundary.
5. The dimming circuit of claim 1 , wherein said modulation is pulse width modulation (PWM).
6. The dimming circuit of claim 5 , wherein said PWM configured to create a majority of modulated energy at lower frequencies.
7. A method to create an LED light dimming circuit, comprising:
a) coupling a control circuit to a primary side of a transformer, wherein the primary side is configured to receive power by an alternating voltage applied to a diode bridge circuit;
b) coupling a string of at least one LED device to a secondary side of the transformer; and
c) controlling an illumination of the string of at least one LED device by said control circuit configured to change drive current from a maximum value to a predetermined level and then changing a duty cycle of a pulse density modulator (PDM) to further reduce current wherein said control circuit is configured to create a hysteresis at a transition between a fixed PDM duty cycle and a variable PDM duty cycle in the LED drive current to prevent a noticeable jump in brightness of light emanating from the at least one LED device.
8. The method of claim 7 , wherein said PDM is configured to create a majority of modulated energy at high frequencies.
9. The method of claim 7 , wherein said PDM is formed from a first-order modulator configured to a carry-out of an accumulator and to automatically wrap around on a 2 N count boundary.
10. The method of claim 7 , wherein said modulator is a pulse width modulator (PWM).
11. The method of claim 10 , wherein said PWM modulator is configured to create a majority of modulated energy at lower frequencies.