Patent Publication Number: US-7215186-B2

Title: Method for operational amplifier sharing between channels

Description:
BACKGROUND OF THE INVENTION 
   Current sources and current sinks are commonly used to provide regulated currents in circuits of all types. As shown in  FIG. 1A , a current sink can be constructed as a combination of a sense resistor, a MOSFET and an operational amplifier. The operational amplifier adjusts the voltage at the gate of the MOSFET to minimize the voltage difference between the inputs of the op amp. In a perfect system, the voltage at the source of the MOSFET, V s , equals the voltage on the positive terminal of the amplifier, V set , and the current is given by I=V set /R.  FIG. 1B  shows a current source constructed using a similar combination of components. 
   For some applications, it is desirable to use a series of current sinks or sources driven using the same set voltage, V set . In an arrangement of this type, each current sink or current source defines a separate channel for current flowing to ground. 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=V set /R+V OS /R. Consider for example, a hypothetical low power implementation where R is 2 Ohms. If V os  is in the range of −10 mV to 10 mV, then V OS /R can be as large as 5 mA. This would be significant for the case where V set /R is 20 mA (which would not be unusual for low power devices). 
   For this reason, U.S. patent application Ser. No. 10/970,061 (incorporated in this document by reference) describes a method for sharing a single operational amplifier between a series of channels. As shown in  FIG. 2 , this method uses two multiplexers. The first allows the output of an operational amplifier to be switched between channels. The second multiplexer allows the feedback voltage to the operational amplifier to be switch in the same fashion. The overall result is that the operational amplifier is shared, with each channel being selected in a (typically) rotating sequence. A problem encountered with this method arises because the operational amplifier takes time to adapt as it is switched between channels. If two channels are operating at significantly different values, regulation of the channel selected second will be bobbled as the operational amplifiers adapts to conditions of the second channel. The second channel starts with the conditions from the previous channel and then the current has to be changed to the final desired value. 
   SUMMARY OF THE INVENTION 
   The present invention includes a pre-charge method for amplifier sharing 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. 
   For a typical implementation of the pre-charge method, each channel is selected in sequence (e.g., Channel A followed by Channel B, followed by Channel C, followed by Channel A, etc.). As each channel is selected, a two-phase refresh cycle is initiated. During the first phase of the refresh cycle, the amplifier is set into a state that is close to the actual operating condition of the selected channel, before it is used to drive that channel. This is accomplished by first setting the amplifier into a unity gain configuration, with its positive input being driven by the gate of the selected channel MOSFET and its holding capacitor. During the second phase of the refresh cycle, the amplifier is used to adjust the current flowing through the selected channel to a desired level. 
   Two multiplexers are used to perform channel selection (M 1  and M 2 ). As each channel is selected, these multiplexers are configured to:
     (1) connect the selected channel&#39;s current sense node to a node S (by operation of M 1 ); and   (2) connect the selected channel&#39;s MOSFET gate to a node G (by operation of M 2 ).   

   An additional two multiplexers (M 3  and M 4 ) and a switch (SW 1 ) are used to implement the two-phase refresh cycle. For the first phase of the refresh cycle, the switch and the multiplexers M 3  and M 4  are configured to:
         (1) disconnect the output of the operational amplifier from the node G (by operation of SW 1 );   (2) connect the negative input of the amplifier to its output (by operation of M 3 ); and   (3) connect the positive input of the amplifier to the node G (by operation of M 4 ).       

   For the second phase of the refresh cycle, the switch and the multiplexers M 3  and M 4  are configured to:
         (1) connect the output of the operational amplifier to the node G (by operation of SW 1 );   (2) connect the negative input of the amplifier to the node S (by operation of M 3 ); and   (3) connect the positive input of the amplifier to the set voltage V set  (by operation of M 4 ).       

   In practice, the use of the two-phase refresh cycle minimizes current variations as the operational amplifier is switched between channels. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a block diagram of a prior art current sink. 
       FIG. 1B  is a block diagram of a prior art current source. 
       FIG. 2  is a block diagram of a multi-channel current sink. 
       FIG. 3  is a block diagram of a multi-channel current sink as provided by an embodiment of the present invention. 
       FIG. 4A  is a block diagram showing a circuit established during the first part of the two-phase refresh cycle provided by the present invention. 
       FIG. 4B  is a block diagram showing a circuit established during the second part of the two-phase refresh cycle provided by the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention includes a pre-charge method for amplifier sharing in multi-channel current sink and current sources.  FIG. 3  shows a representative embodiment of a multi-channel current sink  300  that implements the pre-charge method. As shown in  FIG. 3 , multi-channel current sink  300  includes a series of three separate channels, labeled  302   a  through  302   c . The number of channels  302  is entirely implementation dependent and can be more or less than the three shown. Each channel includes a sense resistor and a MOSFET. Each channel  302  also includes an optional capacitor which helps maintain its MOSFET gate voltage between refresh cycles. Channels  302  regulate current for associated sub-circuits which may be, for example white LEDs. The sub-circuits may also be the respective elements of an RGB LED or any other type of circuit that requires current regulation. 
   Channels  302  are selected in a (typically) rotating sequence. For the three channel implementation shown, channel  302   a  would typically be selected, followed by channel  302   b , channel  302   c  and back to channel  302   a . It should be appreciated that other selection strategies and algorithms may also be used. Multiplexers M 1  and M 2  are used to perform channel selection. To select a channel  302 , multiplexer M 1  is used to connect the channel&#39;s current sense node to a node S. Multiplexer M 2  is used to connect the channel&#39;s MOSFET gate to a node G. A variable shift register (not shown) is typically used to control the channel selection by multiplexers M 1  and M 2 . The shift register 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. 
   Multi-channel current sink  300  also includes an operational amplifier  304 . As each channel  302  is selected, a two-phase refresh cycle is initiated. During the first phase of the refresh cycle, amplifier  304  is set into a state that is close to the actual operating condition of the selected channel  302 , before it is used to drive that channel  302 . This is accomplished by first setting amplifier  304  into a unity gain configuration, with its positive input being driven by the gate of the selected channel  302  and its holding capacitor. During the second phase of the refresh cycle, amplifier  304  is used to adjust the current flowing through the selected channel  302  to a desired level. 
   Multiplexers (M 3  and M 4 ) and a switch (SW 1 ) are used to implement the two-phase refresh cycle. For the first phase of the refresh cycle, switch SW 1  is opened and multiplexers M 3  and M 4  are configured to select their “A” inputs. The result is the circuit shown in  FIG. 4A . In that circuit:
         (1) the output of amplifier  304  is connected to the negative input of amplifier  304 ; and   (2) the positive input of amplifier  304  is connected to the node G (i.e., the gate of the MOSFET of the selected channel  302 ).       

   This circuit is maintained for a period of time (approximately 4 uS for current implementations), allowing the output of amplifier  304  output to charge to the gate voltage of the selected channel  302  (also referred to as pre-charging of operational amplifier  304 ). For the second phase of the refresh cycle, switch SW 1  is closed and the M 3  and M 4  are configured to select their “B” inputs. The result is the circuit shown in  FIG. 4B . In that circuit:
         (1) the output of amplifier  304  is connected to node G (i.e., the gate of the MOSFET of the selected channel  302 );   (2) the negative input of amplifier  304  is connected to the node S (i.e., the current sense node of the selected channel  302 ); and   (3) the positive input of amplifier  304  is connected to the set voltage V set .       

   To avoid charge injection and allow the circuit to operate as intended the switch SW 1  and Multiplexers M 1 –M 4  are sequence in a specific order:
         (1) SW 1  is opened,   (2) M 3  and M 4  are changed to the “A” setting,   (3) M 1  and M 2  are shifted to the next channel to be refreshed,   (4) Pre-charging of operational amplifier  304  occurs,   (5) M 3  and M 4  are changed to the “B” setting,   (6) SW 1  is closed, and   (7) The operational amplifier adjusts the current in the selected channel based on the set voltage V set .       

   A small break before make time is set between settings on M 3  and M 4 . 
   The circuit shown in  FIG. 4B  is maintained until the current in the selected channel matches the target set by the set voltage V set . The duration of time in which the circuit of  FIG. 4B  is maintained may also be varied to change the duty cycle for the selected channel  302 . This can be used, for example where the sub-circuits are elements of an RGB LED and the duty cycle of each element is determined by a color to be displayed (i.e., field sequential display). For applications of this type, where only a single channel is active at a given time, it is possible to use a single sense resistor that is shared between channels. 
   The implementations described above are based, in part on the current sink topology of  FIG. 1A . It should be noted, however that the same techniques may be used with current sources. The implementations are also based on the use of MOSFET technology. It should be noted, however that other transistor types may be used including bipolar transistors (typically with different holding capacitors).