This application claims the priority benefit of European Patent Application Number 13306303.2, filed on Sep. 24, 2013, entitled “FEEDBACK NETWORK FOR LOW-DROP-OUT GENERATOR” which is hereby incorporated by reference to the maximum extent allowable by law.
This disclosure relates to a feedback network for a low-drop-out generator.
Low-drop-out (LDO) regulators are widely used, and may have programmable output voltages. The output voltage selection for an LDO regulator may be implemented within its feedback network.
New feedback network designs for use with LDO regulators are desirable.
A feedback network for a LDO regulator may have the following additional features. The first resistance set of the chain, adjacent its first end terminal, may include two series-coupled resistance subsets with an intermediate node between these two resistance subsets. In addition, the feedback network may also include a first programmable current generator with a current output terminal coupled to the intermediate node between the resistance subsets of the first resistance set of the chain. The first programmable current generator may be suitable for producing a controlled value of a current flowing at its current output terminal.
Thus, the first programmable current generator in combination with the resistance subset closest to the first end terminal of the chain may produce a variable voltage which adds to that produced by the chain of resistance sets itself. Because the first programmable current generator may be independent from the set of switches and the controller dedicated to these switches, the variable voltage generated by the first current generator may add to the output voltage contribution produced by the chain in accordance with the selected one of the chain nodes. This may lead to a total number of available values for the LDO output voltage which is higher, although the number of the resistance sets in the chain may not be increased. This is obtained via the first programmable current generator which is effective with any of the nodes in the chain.
In particular, the number of resistance sets contained in the chain may be reduced when implementing the first programmable current generator, while maintaining the total number of voltage values available for the LDO regulator output as constant.
The resistance sets of the chain may have respective values which are selected so that the voltage of the first end terminal varies with a constant increment upon variation along the chain of the selected node which is electrically coupled to the feedback terminal. The tuning which is provided by the chain of resistance sets for the output voltage of the LDO regulator may thus be simpler than that of conventional approaches.
Advantageously, the first programmable current generator may be adapted so that the current which flows at the current output terminal of this first programmable current generator is digitally controlled. The additional tuning which is provided for the output voltage of the LDO regulator, further to the selection of the coupled node within the chain, may thus be easier. The total number of available values for the LDO regulator output voltage may be equal to the product of the value available for the current output by the first current generator, with the number of node selections provided by the controller.
The last resistance set of the chain adjacent the second chain end terminal may be two other series-coupled resistance subsets with another intermediate node which is arranged between these two other resistance subsets. Then, the feedback network may also include a second programmable current generator with a current output terminal which is coupled to the other intermediate node between the two other resistance subsets of the last resistance set of the chain. This second programmable current generator may be suitable for producing a controlled value of a current flowing at the current output terminal of this second programmable current generator. Thus, third tuning circuitry may be available for adjustment the value of the LDO output voltage.
For easier use of this third tuning, the second programmable current generator may also be adapted so that the current which flows at the current output terminal of this second programmable current generator is digitally controlled.
Advantageously, when both first and second programmable current generators are implemented, these may be oriented so that the current flowing at the current output terminal of one of them is originating from the corresponding intermediate node, and the current flowing at the current output terminal of the other is flowing toward the respectively corresponding intermediate node. Thus, tuning may be available by using both programmable current generators which operate by increasing and decreasing the LDO output voltage with respect to the value as resulting from the chain node selection.
A low-drop-out (LDO) generator is also proposed which includes a reference voltage supply, and a differential amplifier having inverting and non-inverting input terminals and an output terminal, with the non-inverting input terminal being coupled to the reference voltage supply. The LDO generator may also include a feedback network as described before, with the first and second end terminals of the chain being coupled respectively to the output terminal and the power reference terminal of the differential amplifier, and the feedback terminal of the feedback network being to the inverting input terminal of the differential amplifier.
In an application of the LDO regulator, the first programmable current generator may be designed so that a maximum current value which is output by this first programmable current generator, multiplied by the value of the resistance subset between the first chain end terminal and the intermediate node within the first resistance set, is less than a minimum voltage increment obtained for the output terminal of the differential amplifier when varying the selected node which is coupled to the feedback terminal. Thus, the chain of resistance sets together with the switch arrangement and the feedback network controller may provide a coarse tuning of the output voltage of the LDO regulator, and the first programmable current generator may provide a fine tuning of this LDO regulator output voltage.
When the voltage increment due to the node selection within the chain is constant, the first current generator may be designed so that varying a control of this first current generator causes the voltage of the output terminal of the differential amplifier to further vary with a constant secondary increment, with this secondary increment being equal in absolute value to the increment related to the chain node selection, divided by a number of output current values which are available for the first programmable current generator. Thus, the feedback network may provide complete tuning of the LDO output voltage according to the secondary increment.
Finally, an integrated circuit chip, which includes several LDO regulators as described is also proposed. The programmable current generators of the feedback networks of these LDO regulators may contain respective digital-to-analog converters, which are arranged adjacent one another in a chip portion apart from a remaining portion chip which contains remaining circuit parts. In particular, these remaining circuit parts may include the chains of resistance sets of the feedback networks, and also the differential amplifiers of the low-drop-out regulators. Such a chip arrangement may be advantageous since it may not involve re-designing the layout of the remaining chip portion for adding or removing some of the digital-to-analog converters for implementation.
Each one of the first and second programmable current generators may comprise a fixed-current generator for supplying the digital-to-analog converter of this programmable current generator. Then, the fixed-current generator may be shared by several ones of the LDO regulators.
With reference to
R1, R2, . . . , Rm-1, Rm denote the respective resistance values of the resistance sets 1, 2, . . . , m−1, m of the chain. Each resistance set may include several resistance units arranged to produce the resistance value desired for this resistance set. S1, S2, . . . , Sm-1 denote the control signals which are supplied by the controller 204 respectively to the switches 10, 20, . . . . , Y If the controller 204 is digital, it may be fed at input with a control word <S1:Sm-1>, with a word bit-length which is suitable such that the control signals S1, S2, . . . Sm-1 can be deduced from the value of the control word <S1:Sm-1>. In
The switch arrangement represented is for illustrative purpose in this disclosure, and other arrangements may be used equivalently for coupling a single one of the nodes 12, 23, . . . , X to the feedback terminal 203. The switch arrangement represented here may be useful because each switch conducts very little current when in the coupling state due to the high input impedance of the amplifier terminal, so that the switches may be small and designed for occupying a reduced silicon substrate area.
For example, the chain may contain m=257 resistance sets, with 256 switches, leading to 256 voltage steps which may have a constant increment value for the output voltage VOUT of the LDO regulator 1000 if the resistance values R1, R2, . . . , Rm are selected appropriately. Then, the controller 204 is to be fed with a 8-bit word for being capable of selecting one of the switches to drive it into the coupling state while maintaining the other switches in the isolating state.
But the feedback network 200 has the certain drawbacks. For example, the feedback network 200 includes a number of resistance sets and switches which is equal to the number of available output voltage values which are desired for the LDO regulator 1000, the number of which may be of interest in new circuit designs. This can result in the silicon substrate area which is occupied by the feedback network being large. In addition, the bit-length of the control word which is supplied to the controller 204 may increase with the desired number of available output voltage values, thereby using a more complex controller design.
Reference is made to
Resistance sets are involved because the corresponding resistance values may be produced by parallel and/or serially coupling several resistance units. The resistances may be of the same type, for example doped semiconducting material or diffusion-modified material.
The resistance set 1 has been replaced with two series-connected resistance subsets 1D and 1′, with respective resistance values R1D and R1′. An intermediate node 11 is thus added in the chain, between the resistance subsets 1D and 1′. In addition, a programmable current generator 210 is coupled to the intermediate node 11, so as to extract the current I11 from the chain of resistance sets, between the resistance subsets 1D and 1′. The programmable current generator 210 may be arranged so that the current I11 is positive when the power supply voltage VDD is also positive with respect to the power reference terminal 104. Then, the output voltage of the LDO regulator is:
VOUT=G·VREF+R1D·I11 (2)
The programmable current generator 210 may be itself comprised of a digital-to-analog converter and a fixed-current generator 2101, which will now be described with reference to
For the particular embodiment described here in detail, the chain of the resistance sets corresponds to m=17. Then, the controller may be addressed with another four-bit word for producing the control signals S1 to S16 intended for the switches. The following table gathers the numeral values which have been used:
In this table, the unit of resistance is the kΩ (kilo-ohm) and the unit for VREF is the V (volt). Generally, it may not be helpful to split the last resistance set Rm into two resistance subsets Rm′ and RmD coupled in series. Thus, both resistance subsets R17′ and R17D may be replaced by a single resistance set of 173.9 kΩ in the particular embodiment reported. With these values and α and R of the fixed-current source 2101 suitably selected, varying the control four-bit word which is transmitted to the controller leads the LDO output voltage VOUT to vary from 0.9 V to 2.4 V with a constant increment of 0.1 V. This corresponds to the first term in the second member of equation (2) above. The second term R1D·I11 equals j·6.25 mV (millivolt) where j is the integer corresponding to the binary value of the four-bit word <F1:F4>. Given that 16×6.25 mV equals 0.1 V, the voltage increment involved by the controller appears as a coarse increment, whereas the voltage increment involved by the programmable current generator appears as a fine increment, which is a divider of the coarse increment.
Thus, the implementation of the programmable current generator allows a tuning of the LDO output voltage VOUT with 16×16=256 available values. The same VOUT value number obtained by using an appropriate chain of resistance sets 1, 2, . . . , m−1, m may have instead involved m−1=256 switches referenced as 10, 20, . . . , Y and also the controller 204 being an eight-bit controller. So, the teachings of this disclosure allow a reduction of the number of chain switches from 256 to 16, plus the four switches internal to the programmable current generator 210. The eight-bit controller is also replaced by two four-bit controllers, which is simpler. This leads to a reduction in the area occupied by the feedback network 200 from 70% to 20% of the control loop area.
VOUT=G·VREF+R1D·I11−RmD·(G−1)·Im1 (3)
where Im1 is the programmable current which is injected by the current generator 220 into the chain of resistance sets at the node m1, between the resistance subsets Rm′ and RmD. Formula 3 may be re-written in the following way:
VOUT−G·(VREF−RmD·ImD)+(R1D·I11+RmD·Im1) (3′)
Thus, the programmable current generator 220 produces a trimming function by varying the voltage value which is effective for the application of the division factor G of the feedback network 200. The current generator 220 may be designed so that the current Im1 flows towards the intermediate node m1 when the power supply voltage VDD is positive. Thus, the trimming of the VOUT-value which is enabled by the current generator 220 is opposite in sign to the VOUT-tuning provided by the current generator 210. Easier overall adjustment of the LDO output voltage is thus obtained.
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Number | Date | Country | |
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20150084609 A1 | Mar 2015 | US |