Current sources and current sinks are commonly used to provide regulated currents in circuits of all types. As shown in
Duplication of the current sink or source structure can produce additional channels with nearly matched currents when referred to the same set voltage, Vset. For the currents in each channel to be equal, all duplicated elements must exactly match in value and characteristics. Unfortunately, mismatches inevitably result because manufacturing variations are unavoidable. Though mismatch between sense-resistors can be minimized with careful layout, random offset within each amplifier is more difficult to correct and can contribute directly to mismatch between channel currents. In fact, random offset is often the main contributor to mismatch—particularly where R is small since I=Vset/R+Vos/R. Consider for example, a hypothetical low power implementation where R is 2 Ohms. If Vos is in the range of −10 mV to 10 mV, then Vos/R can be as large as 5mA. This would be significant for the case where Vset/R is 20 mA (which would not be unusual for low power devices).
The present invention includes a topology for multi-channel current sink and current sources. For a representative embodiment, a series of current sinks are controlled using a single operational amplifier. Each current sink includes a MOSFET connected through a sense resistor to ground. A feedback sense node is defined for each current sink as the voltage over the sense resistor. The voltage at the feedback sense node is proportional to the current flowing through the MOSFET. That current is used to drive a load, such as an LED.
Two multiplexors are used to select one current sink at a time. When selected, one multiplexor connects the feedback sense node of the selected current sink to one of the inputs of the operational amplifier. The second multiplexor connects the output of the operational amplifier to the MOSFET gate of the selected current sink. The operational amplifier compares the feedback sense node voltage to a set voltage Vset and causes the selected current sink to draw a regulated current proportional to Vset. Each current source is selected in sequence. When non-selected, gate capacitance causes the disconnected MOSFETS to maintain their regulated currents. By sequencing through the different current sinks at a predetermined rate, regulation of each current sink is maintained. Additional capacitance can be added at the gate of each MOSFET to decrease the refresh frequency of the current sinks.
The use of a single amplifier multiplexed between current sinks eliminates the contribution of amplifier offset to current mismatch. This topology also reduces power consumption by minimizing the number of active devices.
The topology just described provides an effective driver for multiple white LEDs. To drive RGB LEDs, the individual sense resistors are replaced with a common sense resistor. A PWM signal is then used to drive the separate red, blue and green elements of the RGB LED. The single sense resistor works because only one LED color element is active at any time. The duty cycle of each color element is varied to control the color and intensity of the LED output.
It should also be noted that a similar topology may be used to drive multiple current sources with a single multiplexed amplifier.
The present invention includes a multi-channel current sink. As shown in
Multiplexor 208 and multiplexor 212 are controlled so that each channel is selected in sequence. When selected, a particular channel 202 is connected to the amplifier 210 and behaves exactly as the circuit in
The use of a single amplifier 210 multiplexed between each channel 202 eliminates the contribution of amplifier offset to current mismatch. This topology also reduces power consumption by minimizing the number of active devices.
Building on the topology just described,
A variable shift register 318 is used to control the channel selection of the multiplexors 308 and 312. The shift register 318 is preferably configured to skip over any channel that has been disabled and refresh only those channels that are intended to conduct current. Typically, this is accomplished using a second register that includes one enable/disable bit per channel. To prevent current flow, it is preferable to ground the gates of all disabled channels.
The red, blue and green LEDS 402 are driven using a pulse width modulation (PWM) scheme. For this scheme, each LED 402 is selected in sequence. The amplifier 408 is then connected to drive the MOSFET 404 associated with the selected LED 402. The amplifier 408 regulates the current through the LED 402 as illustrated for the sink structure of
The implementations described above are based, in part on the current sink topology of