Digitally synchronized integrator for noise rejection in system using PWM dimming signals to control brightness of light source
An apparatus and method for controlling the operation of a utility device, such as a cold cathode fluorescent lamp that is powered in accordance with a pulse width modulation (PWM) signal, includes an analog sensor which monitors the utility device to derive an output signal representative of the PWM signal. An integrating analog-to-digital converter (ADC), which is coupled to the sensor and has its operation synchronized with an integral multiple of the period of the PWM signal, produces an output representative of an average of the output of the utility device.
1. A method comprising the steps of:
(a) applying a periodically varying analog input signal to an analog-to-digital converter (ADC), said analog-to-digital converter being operative to generate an output value representative of the average amplitude value of said periodically varying analog signal; and
(b) digitally synchronizing the integration period of said analog-to-digital converter with an integral multiple of the period of said analog input signal.
2. The method according to claim 1 , wherein said ADC comprises an integrating ADC, contents of which are controllably incremented during prescribed pulse width portions of said analog input signal, and further including a counter the contents of which are controllably incremented during a prescribed integration interval, and a divider which is coupled to receive count values produced by said ADC and said counter and to generate a value representative of a ratio of the contents of said ADC and said counter as incremented over said integration period, to provide a normalized output value.
3. The method according to claim 1 , wherein said analog input signal is derived from a sensor which is operative to sense light produced by a light source such as a cold cathode fluorescent lamp (CCFL) and to provide said analog input signal representative of a pulse width modulation signal used to energize said light source (CCFL).
4. The method according to claim 3 , wherein said ADC comprises an integrating analog-to-digital converter.
5. A method of controlling the operation of a utility device comprising the steps of:
(a) applying a pulse width modulation (PWM) signal to said utility device;
(b) monitoring an output of said utility device to derive an output signal representative of said PWM signal applied thereto;
(c) coupling said output signal to an analog-to-digital converter (ADC) to produce an output representative of an average of said output of said utility device; and
(d) synchronizing the operation of said ADC with an integral multiple of the period of said PWM signal.
6. The method according to claim 5 , wherein said ADC comprises an integrating ADC having count contents thereof controllably incremented in step (c) during prescribed pulse width portions of said output signal.
7. The method according to claim 6 , further comprising an auxiliary counter contents of which are controllably incremented during a prescribed integration interval, and further including the step (e) of generating a value representative of a ratio of the contents of said ADC and said auxiliary counter as incremented over said prescribed integration interval, to provide a normalized value of the output of said utility device.
8. The method according to claim 5 , wherein said utility device comprises a cold cathode fluorescent lamp (CCFL) or external electrode fluorescent lamp (EEFL) or light emitting diode (LED) or other light source.
9. An apparatus for controlling the operation of a utility device that is driven by a pulse width modulation (PWM) signal, the duty cycle of said PWM signal being controllably adjustable to control an output produced by said utility device, said apparatus comprising:
a sensor which is operative to monitor said output of said utility device to derive an output signal representative of said PWM signal applied thereto; and
an analog-to-digital converter (ADC) unit coupled to said sensor and being operative to produce an output representative of an average of said output of said utility device, and wherein the operation of said ADC is synchronized with an integral multiple of the period of said PWM signal.
10. The apparatus according to claim 9 , wherein said ADC unit comprises an integrating ADC, count contents of which are controllably incremented during prescribed pulse width portions of said output signal.
11. The apparatus according to claim 10 , wherein said ADC unit further comprises an auxiliary counter contents of which are controllably incremented during a prescribed integration interval, and further including a divider which is coupled to receive count values produced by said ADC and said auxiliary counter and to generate a value representative of a ratio of the contents of said ADC and said auxiliary counter as incremented over said prescribed integration interval, to provide a normalized value of the output of said utility device.
12. The apparatus according to claim 9 , wherein said utility device comprises a cold cathode fluorescent lamp (CCFL) or external electrode fluorescent lamp (EEFL) or light emitting diode (LED) or other light source.