Duty cycle controller

A variety of electronic circuits (i.e., drivers) are used to drive loads. One way to control power to these loads is to control a duty cycle of a driver. Loads, more particularly, may include strings of series-connected light-emitting diodes (LEDs), which, in some examples, form an LED display, or, more particularly, a backlight for a display, for example, a liquid crystal display (LCD). Thus, changing the duty cycle can change the brightness of the LEDs.

In one aspect, a duty cycle controller includes a first port configured to receive a voltage bias signal, a second port configured to receive an input voltage signal, a third port configured to provide an output signal of the duty cycle controller having a duty cycle and a n-bit digital-to-analog converter (DAC) configured to receive the voltage bias signal and to provide a DAC output signal to a comparator. The DAC output signal has a peak value. The duty cycle controller also includes the comparator configured to compare the DAC output signal from the n-bit DAC with the input voltage signal to provide a comparator output signal. The comparator output signal is used to provide the output signal of the duty cycle controller and the duty cycle changes with changes to the input voltage signal.

In another aspect, an integrated circuit (IC) includes a first pin configured to receive a voltage bias signal, a second pin configured to receive an input voltage signal, a third pin configured to provide a duty cycle signal having a duty cycle and a n-bit digital-to-analog converter (DAC) configured to receive the voltage bias signal and to provide a DAC output signal to a comparator. The DAC output signal has a peak value. The IC also includes the comparator configured to compare the DAC output signal from the n-bit DAC with the input voltage signal to provide a comparator output signal. The comparator output signal is used to provide the output signal of the duty cycle controller and the duty cycle changes with changes to the input voltage signal.

In a further aspect, a method includes providing an integrated circuit (IC) comprising a first pin to provide an output signal, setting a duty cycle of the output signal in response to: a first resistor, R 1 , coupled to a second pin of the IC and a voltage bias, a second resistor, R 2 , coupled to the first resistor and a third pin of the IC, and a clock signal provided to a fourth pin of the IC.

FIG. 1 is a block diagram of a system that includes a duty cycle controller;

FIG. 2 is a circuit diagram of an example of the duty cycle controller;

FIG. 3A are waveform timing diagrams of signals used with the duty cycle control circuit;

FIG. 3B is a waveform diagram of an example of a DAC output signal and an example of an input signal to the duty cycle controller.

Described herein are techniques to control a duty cycle. For example, a duty cycle control circuit is described that provides a signal that controls a duty cycle of a driver in response to changes to an input voltage signal to the duty cycle control circuit. The techniques herein do not require a capacitor or trim, provide more accurate duty cycle control and have less variation from temperature changes. As will be shown further herein, the duty cycle is independent of the frequency of a clock signal. Further, the duty cycle controller described herein may still function even when using different voltage bias values.

The various connections described herein may be referred to herein interchangeably with the signal carried by the respective connection. For example, reference character 122 c may be used interchangeably to refer to the connection between the n-bit counter 118 and the latch circuit 126 and a signal associated with such a connection.

Referring to FIG. 1 , a system 10 includes a duty cycle control circuit 20 , a driver 30 and a load 40 . The duty cycle control circuit 20 receives an oscillator signal 22 a (e.g., a clock signal), a voltage bias signal 22 b (i.e., a fixed voltage) and an input voltage signal 22 c and generates an output signal 22 d . By changing the input voltage signal 22 c , the duty cycle control circuit 20 changes the duty cycle of the output signal 22 d . In one example, the driver 30 is a field effect transistor. In one example, the load 40 is one or more light emitting diodes (LEDs) that may, in one particular example, form a liquid crystal display (LCD) display.

Referring to FIG. 2 , an example of a duty cycle control circuit 20 is a duty cycle control circuit 20 ′. The duty cycle control circuit 20 ′ includes a digital-to-analog converter (DAC) 116 (i.e., an n-bit DAC), a counter 118 (i.e., an n-bit counter), a comparator 120 and a latch circuit 126 .

The DAC 116 receives the voltage bias signal 22 b from a voltage bias 104 , which is coupled to ground through a resistor R 1 and a resistor R 2 ; and provides a DAC output signal 122 a to the comparator 120 . In one example, the DAC 116 is a 10-bit DAC. In general, the more bits used by the DAC 116 the better resolution the DAC output provides, which in turn provides a more accurate duty cycle control.

In one example, the duty cycle control circuit 20 ′ is formed on an integrated circuit (IC). One or more of the oscillator 102 , the voltage bias 104 , the input voltage signal 22 c and the resistors R 1 , R 2 and R 3 are provided off-chip (i.e., external to the IC). For example, the oscillator signal 22 a is provided to a first pin on the IC, the voltage bias signal 22 b is provided to a second pin on the IC and the input voltage signal 22 c is provided to a third pin on the IC. In one particular example, the voltage bias 104 is provided by another pin on the IC.

The counter 118 receives the oscillator signal 22 a from an oscillator 102 which is coupled to ground through a resistor R 3 and provides a maximum count value signal 122 c to an S port of the latch circuit 126 . In one example, the maximum count value signal 122 c is high when the counter 118 reaches the maximum count value. In some examples, the maximum count value is the maximum count possible. For example, for a 10-bit counter the maximum count possible is 1111111111. In other examples, the maximum count value is less than the maximum count possible. For example, for a 10-bit counter, a value of 1010111110 may be selected as the maximum count value.

The counter 118 is also coupled to the DAC 116 by n connections 128 and is configured to provide to the DAC 116 a signal for each of the n bits. The counter 118 provides values from zero to the maximum count value to the n-bit DAC 116 . For example, if the DAC 116 is a 10-bit DAC there would be ten connections 128 coming from the counter 118 , which would be a 10-bit counter. In one example, each connection of the n connections 128 is connected to a respective flip-flop (not shown) in the counter 118 and the flips flops are tied in series.

The comparator 120 receives, at a negative terminal, the input voltage signal 22 c from a node 132 between the resistor R 1 and the resistor R 2 . The input voltage signal 22 c is equal to (R 2 /(R 1 +R 2 )) times the Voltage Bias. The comparator 120 receives, at a positive terminal, the DAC output signal 122 a and provides a comparator output signal 122 b to an R port of the latch circuit 126 . The comparator output signal 122 b is high when the DAC output signal 122 a is above the input voltage 22 c.

The comparator 120 also receives the oscillator signal 22 a . The comparator 120 uses the oscillator signal 22 a to remove or blank out artifacts from the DAC 116 . For example, the DAC 116 may produce artifacts after each clock signal trigger. In other examples, the DAC 116 does not produce any artifacts so that the comparator 120 does not need to be configured to remove artifacts.

The latch circuit 126 provides the output signal 22 d of the duty cycle control circuit 20 ′ from a Q port. The output signal 22 d is high when the DAC output signal 122 a is below the input voltage 22 c . The maximum count value signal 122 c received at the S port of the latch circuit 126 resets the latch circuit.

In operation, when the oscillator signal goes from low to high, for example, the counter 118 increments in a binary fashion. In particular, the counter 118 increases from zero to the maximum count value and then resets to zero. For example, with 10-bits the n-bit connections provides a 0000000000 value initially and after the oscillator signal 22 a goes from low to high the counter 118 provides a 0000000001 value. In one example, if the maximum count value is 1111111111, the values increase until the 1111111111 value is achieve and then after the next clock signal trigger, the counter 118 resets to a 0000000000 value. In another example, if the maximum count value is 1010111110, the counter 118 is reset after the 1010111110 value is reached where the 1010111110 value is 702 in decimal numbers.

Referring to FIG. 3A , the DAC 116 provides the DAC output signal 122 a as a sawtooth waveform. A peak voltage value of the sawtooth corresponds to the maximum count value. In one example, the DAC output signal 122 a is the DAC output signal 122 a ′. In one example, the DAC output signal 122 a ′ increases from zero volts to a peak voltage value equal to the voltage bias 104 when the maximum count value is the maximum count value possible. In one particular example, for a 10-bit DAC, the maximum count value is 1111111111 and the output voltage increases in 1024 steps where zero equals 0 volts and 1024 equals the voltage bias 104 .

In another example, the DAC output signal 122 a ′ increases from zero volts to a peak voltage value that is less than the voltage bias 104 . In one particular example, for a 10-bit DAC, the maximum count value is 1010111110 and the output voltage increases in 702 steps where zero equals 0 volts and 702 equals 0.685547 times the voltage bias 104 (e.g., if the voltage bias is 5.25 volts the DAC output signal 122 a ′ increases from zero volts to 3.6 Volts) where 702/1024 equals 0.685547.

An example of the output signal 22 d is an output signal 22 d ′ and an example of the input voltage 22 c is the input voltage 22 c ′. The output signal 22 d ′ is high when the DAC output signal 122 a ′ is below the input voltage 22 c ′. Thus, as the input voltage signal 22 c ′ increases the duty cycle of the output signal 22 d ′ increases. The duty cycle is equal to:

[((Voltage Bias)/(Peak Voltage Value)* R 2)/( R 1 +R 2)]*100%, or

[(Input Voltage)/(Peak Voltage Value)]*100%,

where the Peak Voltage Value is the peak voltage value of the DAC output signal 122 a . If the peak value voltage is equal to the voltage bias, then the duty cycle is equal to:

[( R 2)/( R 1 +R 2)]*100%, or

[(Input Voltage)/(Voltage Bias)]*100%,

An example of the comparator output signal 122 b is a comparator output signal 122 b ′. The comparator output signal 122 b ′ is high when the DAC output signal 122 a ′ is above the input voltage 22 c′.

An example of the maximum count value signal 122 c is a maximum count value signal 122 c ′. The maximum count value signal 122 c ′ is high when the DAC output signal 122 a ′ reaches its peak value which corresponds to the counter 118 reaching the maximum count value.

Referring to FIG. 3B , another example of a DAC output signal 122 a is the DAC output signal 122 a ″ and another example of the input voltage 22 c is the input voltage 22 c ″. The input voltage 22 c ″ is the same as the input voltage 22 c ′. The DAC output signal 122 a ″ has the same peak value as the DAC output signal 122 a ′; however, the DAC output signal 122 a ″ has twice the frequency of the DAC output signal 122 a ′, which means the oscillator signal 22 a has twice the frequency. Even though the frequency of the oscillator signal 22 a has doubled, the duty cycle remains the same. Thus the duty cycle of the output signal 22 d is independent of the input signal 22 c.

The voltage bias signal 104 may be as low as 1.5 volts, for example, for the duty cycle control circuit 20 ′ to function.

Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Other embodiments not specifically described herein are also within the scope of the following claims.