Patent Application
20110063009
1. Field of Disclosure
The present disclosure relates to a circuit. More particularly, the present disclosure relates to a voltage trimming circuit.
2. Description of Related Art
The reference voltage in a circuit, such as a low drop out linear regulator, has to be very precise. However, the reference voltage generating circuit may suffer from many undesirable effects resulting in an inaccurate output voltage. Thus, a trimming mechanism is needed to tune the inaccurate output voltage back to the desired value of the output voltage to provide the accurate reference voltage.
Nevertheless, the conventional design of the voltage trimming circuit provides only up-trimming mechanism to up-trim an initial output voltage from a value lower than the target output voltage. Once the initial output voltage is higher than the target output voltage, the up-trimming mechanism fails to reach the target output voltage. In addition, if the initial voltage with lower value is generated first, it may have to take more bits of trimming voltage steps to reach the target output voltage. Thus, the more bits of trimming voltage steps it takes to trim the output voltage, the more the LSB error will be generated.
Thus, what is needed is a voltage trimming circuit provides both up-trimming and down-trimming mechanism. The present disclosure addresses such a need.
A voltage trimming circuit has an input stage, an up-trimming resistor ladder, a down-trimming resistor ladder and a control means. The input stage has a first input, a second input and an output, wherein the first output is to receive an input voltage, the second input is connected to a connection point and the output is to provide an output voltage based on a difference between the voltage of the first input and the second input. The up-trimming resistor ladder is connected between the output of the input stage and the connection point and the down-trimming resistor ladder connected between a ground potential and the connection point. The control means is to control the resistance of the up-trimming resistor ladder and the down-trimming resistor ladder. Wherein the output voltage is not trimmed when the resistance of the up-trimming resistor ladder and the down-trimming resistor ladder are equal, the output voltage is up-trimmed when the control means makes the resistance of the up-trimming resistor ladder larger than resistance of the down-trimming resistor ladder and the output voltage is down-trimmed when the control means makes the resistance of the down-trimming resistor ladder larger than the resistance of the up-trimming resistor ladder.
It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the disclosure as claimed.
The disclosure can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows:
Reference will now be made in detail to the present embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
Please refer to
The input stage 10 has a first input, a second input and an output. The input stage 10 in the present disclosure is an operational amplifier 10. Therefore, the first input of the input stage 10 is the inverting input (depicted in
Please refer to
In the present embodiment, the first stage comprises eight resistors connected in parallel, the second stage comprises four resistors connected in parallel, the third stage comprises two resistors connected in parallel, the fourth stage comprises one resistor and the fifth stage comprises two resistors connected in serial. All the resistors in each stage are the same. Thus, the resistance of the second stage is double the resistance of the first stage, the resistance of the third stage is four times larger than the resistance of the first stage, the resistance of the fourth stage is eight times larger than the resistance of the first stage and the resistance of the fifth stage is sixteen times larger than the resistance of the first stage.
Each of the five switches 200-204 of the control means is connected in parallel with a trimming resistor stage, as depicted in
The output voltage Vo can be represented as the following equation:
Vo=Vi*(1+Ru/Rd)
wherein the Ru is the resistance of the up-trimming resister ladder 12 and the Rd is the resistance of the down-trimming resister ladder 14. When the switches 200-204 are closed, the input voltage Vi is tuned to the half of a target output voltage. Due to the symmetrical structure of the up-trimming resistor ladder 12 and the down-trimming resistor ladder 14, the resistance of the up-trimming resistor ladder 12 and the down-trimming resistor ladder 14 are the same. Therefore, according to the above equation, the output voltage Vo is about twice that of the input voltage Vi. However, the output voltage Vo may not be exactly twice of the input voltage Vi because the circuit is imperfect. Therefore, a trimming mechanism is needed.
Because there are five switches 200-204, the switches 200-204 are able to perform thirty-two connecting modes. When the switch 200 corresponding to the first stage is open, the total resistance of the up-trimming resistor ladder 12 becomes higher. The total resistances of the up-trimming resistor ladder 12 and the down-trimming resistor ladder 14 becomes asymmetrical. Thus, the output voltage Vo becomes higher due to the up-trimming mechanism provided by the up-trimming resistor ladder 12. If the output voltage Vo is still lower than the target output voltage, an appropriate connecting mode of the switches 200-204 can be chosen to further pull up the output voltage Vo and tune the output voltage Vo to the target output voltage. The switches 200-204 are able to perform thirty-two connecting modes, therefore thirty-two up-trimming steps of voltage are provided to up-trim the output voltage Vo.
It's noticed that in the present embodiment, the resistance of the N-th trimming resistor stage of the up-trimming resistor ladder 12 is 2N-1 times of the resistance of the first trimming resistor stage of the up-trimming resistor ladder 12, as described above. According to the above characteristic, the voltage increment of the N-th trimming step becomes 2N-1 times of voltage increment of the first trimming step. Therefore, the voltage trimming circuit 1 provides a bit-wise up-trimming mechanism. The connecting modes can be represented as a 5-bit binary number.
Please refer to
Ru(01011)=R1+R2+R2/4+R2/8
The total resistance of the up-trimming resistor ladder 12 increases. If all the switches of the down-trimming resistor ladder 14 are still closed, the resistance of the down-trimming resistor ladder 14 is unchanged and is still R1. Therefore, the resistance of the up-trimming resistor ladder 12 becomes larger than the resistance of the down-trimming resistor ladder 14. The output voltage is up-trimmed due to the asymmetric resistance of the current connecting mode.
Likewise, when the switch corresponding to the first stage in the down-trimming resistor ladder turns open, the total resistance of the down-trimming resistor ladder 14 becomes higher. The total resistances of the up-trimming resistor ladder 12 and a down-trimming resistor ladder 14 becomes asymmetrical. Thus, the output voltage Vo becomes lower due to the down-trimming mechanism provided by the down-trimming resistor ladder 14. If the output voltage Vo is still higher than the target output voltage, an appropriate connecting mode of the switches in the down-trimming resistor ladder 14 can be chosen to further pull down the output voltage Vo and tune the output voltage Vo to the target output voltage. The switches are able to perform 32 connecting modes, therefore 32 down-trimming steps of voltage is provided to down-trim the output voltage Vo.
It's noticed that in the present embodiment, the resistance of the N-th trimming resistor stage of the down-trimming resistor ladder 14 is 2N-1 times the resistance of the first trimming resistor stage of the down-trimming resistor ladder 14, as described above. According to the above characteristic, the voltage drop of the N-th trimming step becomes 2N-1 times the voltage drop of the first trimming step. Therefore, the voltage trimming circuit 1 provides a bit-wise down-trimming mechanism. The connecting modes can be represented as a 5-bit binary number.
The number of the trimming resistor stages in the up-trimming resistor ladder 12 and a down-trimming resistor ladder 14 can be different in other embodiments. When the number of the trimming resistor stages of each of the up-trimming and the down-trimming resistor ladder is M, the switches of each of the up-trimming and the down-trimming resistor ladder have 2M connecting modes to perform 2M up-trimming steps and 2M down-trimming steps. Also, the structure of each stage in the up-trimming resistor ladder and the down-trimming resistor ladder can be different in other embodiment.
In another embodiment, the up-trimming resistor ladder and the down-trimming resistor ladder is not necessarily symmetrical, and the input voltage is not necessarily the half of the target output voltage. However, the asymmetrical structure is not easy to design and may have to spend more time to adjust the structure of the resistance in each stage. Thus, the symmetrical structure of the above embodiment is much more desirable.
In the conventional design, the voltage trimming circuit provides only up-trimming mechanism to up-trim an initial output voltage from a value lower than the target output voltage. Once the initial output voltage is higher than the target output voltage, the up-trimming mechanism fails to reach the target output voltage. In addition, if the initial voltage with lower value is generated first, it may have to take more bits of trimming voltage steps to reach the target output voltage. Thus, the more bits of trimming voltage steps it takes to trim the output voltage, the more the LSB error will be generated, which is an undesirable result.
Thus, the voltage trimming circuit of the present disclosure provides both an up-trimming and a down-trimming mechanism. Fewer bits are required to tune the output voltage to the target output voltage. If the same range of the voltage trimming steps is provided in the conventional design and the present disclosure, the LSB error in the conventional design is greater. Take ten voltage trimming steps for example, the voltage trimming circuit of the present disclosure takes 25 which is 32 bits for up-trimming steps, and takes 25 which is 32 bits for down-trimming steps as well. However, the voltage trimming circuit of the conventional design takes 210, which is 1024 bits for up-trimming steps. If each of the resistors of the trimming stages has a deviation from the correct value of the resistance, the stage with higher resistance will have larger scale of error. This is the so-called LSB error. Thus, the voltage trimming circuit of the present disclosure further provides a trimming mechanism with lesser LSB error. Therefore, the accurate output voltage generated by the voltage trimming circuit in the present disclosure can be further used as the reference voltage of other circuit.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims.
