Selection circuit

A selection circuit having a comparator with comparator inputs connected to first and second voltage inputs), respectively, and a comparator output connected to a control input of a first controllable switch and an inverter. The selection circuit also has a second controllable switch having a second control input connected to the inverter. The first voltage input is connectable to a selection circuit output by the first controllable switch and the second voltage input is connectable to the selection circuit output by the second controllable switch. The inverter has a power supply connector connected to the first voltage input and the comparator has a power supply connector connected to the second voltage input.

FIELD OF THE INVENTION

The invention relates to a selection circuit and method for operating the selection circuit. For example, the invention can be used for choosing the maximum supply voltage out of a set of supply voltages.

BACKGROUND OF THE INVENTION

The power-up of chips that have several power supply sources is a difficult problem. Indeed, to work correctly any chip needs a minimum supply voltage. When there are several power supply sources with different voltage on each, it is necessary to determine which of these sources can provide a high enough voltage to the chip. Once this decision is taken, a mix of these valid power supply sources can be done to supply the chip. The decision or selection circuit is difficult to design because the decision circuit itself is also not supplied by one constant power supply source but by several and it has to work whatever the values of the voltages on these sources are.

As known from the prior artFIG. 1shows a solution for a decision circuit which is used to generate a power supply from two power supply sources VDD1and VDD2. It is based on a feedback loop.

The circuit comprises a comparator CMP with a comparator input1connected to a first voltage input IN1and a comparator input2connected to a second voltage input IN2. The first comparator input1is applied with the first supply voltage VDD1while the second comparator input2is applied with the second supply voltage VDD2. A first diode D1, which is connected in parallel to a first controllable switch SW1, is connected on the one side to the first voltage input IN1and on the other side to a circuit output OVDD. A second diode D2, which is connected in parallel to a second controllable switch SW2, is connected on the one side to the second voltage input IN2and on the other side to the circuit output OVDD. The control input of the first controllable switch is controlled by the output3of the comparator CMP. The output3of the comparator CMP is also connected to an inverter INV, whereby the output6of the inverter INV controls the control input of the second controllable switch SW2. Both power supply connectors4and7of the comparator CMP and the inverter INV are connected to the output voltage VDD.

The first switch SW1is conducting, if the first control voltage SWVDD1is lower than the difference between VDD1−Vt or if the first control voltage SWVDD1is lower than the difference between VDD−Vt. In this case the first voltage VDD is VDD1. If the first control voltage SWVDD1is higher than the difference between VDD1−Vt and higher than the difference between VDD−Vt, the first switch SW1is not conducting and the voltages VDD and VDD1are independent. The voltage Vt is a constant voltage. In principle this is also valid for the second switch SW2. The second switch SW2is conducting, if the second control voltage SWVDD2is lower than the difference between VDD2−Vt or if the second control voltage SWVDD2is lower than the difference between VDD−Vt. In this case is the output voltage VDD is VDD2. If the second control voltage SWVDD2is higher than the difference between VDD2−Vt and higher than the difference between VDD−Vt, the second switch SW2is not conducting and the voltages VDD and VDD2are independent.

At power-up, the diodes D1and D2pull up the node OVDD, which is the output of the circuit, to the voltage VDD equal to the maximum of the three values VDD1−Vdiode, VDD2−Vdiode or O. The voltage Vdiode is the diode voltage and is equal for both diodes D1and D2.

If this voltage VDD is lower than the constant voltage Vt, the comparator output3and the inverter output6will be floating and therefore be undefined. In some cases, this can lead to a large short circuit current. Let's assume for example that the first supply voltage VDD1=Vt+Vdiode, the second supply voltage VDD2=0, the first control voltage, also called comparator output voltage, SWVDD1=0 and also the second control voltage, also called inverter output voltage, SWVDD2=0. The first switch SW1is conducting and will try to force the voltage VDD=VDD1=Vt+Vdiode on the node OVDD. But because the control voltage SWDVDD2=0 and this is lower than VDD−Vt=VDD1−Vt=Vt30Vdiode−Vt, the second switch SW2is also conducting and will try to force an output voltage VDD=0 on the node OVDD.

In the following, the voltage difference VDD1−Vdiode or the voltage difference VDD2−Vdiode is supposed to be higher than the constant voltage Vt. The stable state of the circuit will be determined. The comparator output3will be 0 if the first voltage VDD1is higher than the second voltage VDD2and the comparator output3will be the maximum voltage if the second voltage VDD2is higher than the first voltage VDD1. The maximum voltage is the higher value of the two values VDD1−Vdiode or VDD2−Vdiode. The inverter INV, also supplied with the output voltage VDD, will produce a second control voltage SWVDD2, which is the maximum of VDD1−Vdiode and VDD2−Vdiode, if the first control voltage SWVDD1=0. The inverter INV will produce a second control voltage SWVDD2=0 if the first control voltage SWVDD1is the maximum of VDD1−Vdiode and VDD2−Vdiode. The first control voltage SWVDD1switches on the first switch SW1, this means the first switch SW1is conducting, and the second control voltage SWVDD2switches off the second switch SW2, this means the second switch SW2is not conducting, when the first voltage VDD1is higher than the second voltage VDD2. The control voltages SWVDD1and SWVDD2switch off the first switch SW1and switch on the second switch SW2when the second voltage VDD2is higher than the first voltage VDD1. The output voltage VDD is then going from max (VDD1, VDD2)−Vdiode to max (VDD1, VDD2). Similarly, the control voltages SWVDD1and SWVDD2will either stay on 0 or go up to max (VDD1, VDD2).

This circuit has several drawbacks. It will not function, if the first voltage VDD1and second voltage VDD2are lower than Vt+Vdiode. If the first voltage VDD1and the second voltage VDD2are lower than Vt+Vdiode, there can be a high short circuit parasitic current.

SUMMARY OF THE INVENTION

An object of the invention is to provide a selection circuit and a method for operating the selection circuit which chooses the maximum supply voltage from two different supply voltages independent of the amount of these supply voltages.

An advantage of the selection circuit and the method for operating the selection circuit is that no short circuit currents can occur.

The selection circuit according to the invention comprises a comparator with comparator inputs connected to a first and a second voltage input and a comparator output connected to a control input of a first controllable switch and an inverter. The selection circuit further comprises a second controllable switch with a second control input connected to the inverter. The first voltage input is connectable to a selection circuit output by means of the first controllable switch and the second voltage input is connectable to the selection circuit output by means of the second controllable switch. The inverter with its power supply connector is connected to the first voltage input and at least the comparator with its power supply connector is connected to the second voltage input.

The method for operating a selection circuit according to the invention comprises the following steps:

A first voltage is applied at the first voltage input and/or a second voltage is applied at the second voltage input.

If the first voltage is higher than the second voltage, a pull down element pulls the output of the comparator down and the first voltage input is connected to the selection circuit output by means of the first controllable switch.

If the first voltage is lower than the second voltage, a further pull down element pulls the output of the inverter down and the second voltage input is connected to the selection circuit output by means of the second controllable switch.

In an embodiment according to the invention the selection circuit further comprises a controllable pull down element connected between the comparator output and a reference potential.

In another embodiment of the invention the selection circuit further comprises a further controllable pull down element connected between the inverter output and the reference potential.

Advantageously in the selection circuit according to the invention the pull down element and the further pull down element are NMOS transistors.

According to a preferred embodiment of the selection circuit the controllable pull down element comprises a control input connected to the first voltage input and can be controlled by a first supply voltage.

According to a another preferred embodiment of the selection circuit the further controllable pull down element comprises a control input connected to the second voltage input and can be controlled by a second supply voltage.

At least in the selection circuit according to the invention the comparator can be an operational amplifier.

Contrary to the solution presented above inFIG. 1, the solution according to the invention as shown inFIG. 2is not based on a feedback loop.

The selection circuit comprises a comparator CMP with a comparator input1connected to a first voltage input IN1and a comparator input2connected to a second voltage input IN2. The first comparator input1is applied with a first supply voltage VDD1while the second comparator input2is applied with the second supply voltage VDD2via the voltage inputs IN1and IN2. A first controllable switch SW1is connected on the one side to the first voltage input IN1and on the other side to a circuit output O. A second controllable switch SW2is connected on the one side to the second voltage input IN2and on the other side to the circuit output O. The control input CI1of the first controllable switch SW1is controlled by the output3of the comparator CMP. The output3of the comparator CMP is also connected to an inverter INV whereby the output6of the inverter INV controls the control input CI2of the second controllable switch SW2. The power supply connector4of the comparator CMP is supplied with the second voltage VDD2while the power supply connector7of the inverter INV is supplied with the first voltage VDD1. The output3of the comparator CMP is further connected to a first pull down element PD1, which can be nonlinear. If the voltage at the control input CI3of the first pull down element PD1is higher than the voltage Vt, the output3of the comparator is pulled down to ground GND by means of the first pull down element PD1. The control input CI3of the first pull down element PD1is supplied with the first voltage VDD1. The output6of the inverter INV is further connected with a second pull down element PD2, which also can be nonlinear. If the voltage at the control input CI4of the second pull down element PD2is higher than the voltage Vt, the output6of the inverter INV is pulled down to ground GND by means of the second pull down element PD2. The control input CI4of the second pull down element PD2is supplied with the second voltage VDD2.

The pull down elements PD1and PD2are preferably transistors and work in the same way. If the voltages at the control inputs CI3or CI4of the pull down elements PD1or PD2are lower than the voltage Vt, the voltages SWVDD1or SWVDD2will be undefined. If the voltages at the pull down elements PD1or PD2are floating, the pull down elements can be ignored.

The pull down elements PD1and PD2are used to extend the range where the circuit is working. Without them the circuit would work only when the two supply voltages VDD1and VDD2are both higher than the voltage Vt. Normally, the first pull down element PD1is a weak NMOS transistor and the second pull down element PD2is also a weak NMOS transistor.

When the first supply voltage VDD1is higher than the voltage Vt and the second supply voltage VDD2is also higher than the voltage Vt, the pull down elements PD2and PD1enter in conflict with the output3of the comparator CMP or the output6of the inverter INV because both pull down elements PD1and PD2try to pull down the control voltages SWVDD1and SWVDD2to GND while either the comparator CMP or the inverter INV tries to pull these nodes SWVDD1and SWVDD2up to the second supply voltage VDD2or the first supply voltage VDD1. But as the pull down elements PD2and PD1strengths are much lower than the comparator CMP and the inverter INV, it are these lattest which fix the voltages SWVDD1and SWVDD2.

When the first supply voltage VDD1is lower than the voltage Vt and the voltage Vt is lower than the second supply voltage VDD2, all the elements using the first power supply VDD1have their outputs floating. The first pull down element PD1can be ignored and the inverter output6is floating. But thanks to the second pull down element PD2, the node SWVDD2is pulled down to GND. Therefore the output O is connected to the second supply voltage VDD2through the switch SW2.

When the second supply voltage VDD2is lower than the voltage Vt and the voltage Vt is lower than the first supply voltage VDD1, all the elements using the second power supply VDD2have their outputs floating. The pull down element PD2can be ignored and the comparator output3is floating. But thanks to the first pull down element PD1, the node SWVDD1is pulled down to GND. Therefore the output O is connected to the first supply voltage VDD1through the switch SW1.

The switches SW1and SW2are preferably transistors and work in the same way. As example, in the following the way of working of the first switch SW1is described.

The first switch SW1is conducting, if the first control voltage SWVDD1is lower than the difference between VDD1−Vt or the first control voltage SWVDD1is lower than the difference between VDD−Vt. In this case the output voltage VDD is VDD1. If the first control voltage SWVDD1is higher than difference between VDD1−Vt and higher than difference between VDD−Vt, the first switch SW1is not conducting and the voltages VDD and VDD1are independent.

The comparator CMP works as follows. If the voltage VDD2at the comparator input2is higher than the voltage VDD1at the comparator input1, the comparator output3delivers a first control voltage SWVDD1which is equal to the second voltage VDD2. If the voltage VDD2at the comparator input2is lower than the voltage VDD1at the comparator input1, the comparator output3delivers a first control voltage SWVDD1which is equal to 0. If the second voltage VDD2is lower than the voltage Vt, the comparator output3is floating.

The inverter INV generates a second control voltage SWVDD2=0, if the voltage at its input5is VDD2. If the voltage at the input5of the inverter INV is 0, the inverter INV generates a second control voltage SWVDD2=VDD1. If the first voltage VDD1, which is the operation voltage of the inverter INV, is lower than the voltage Vt, the inverter output6is floating.

There are basically five cases of functioning:

1. The second voltage VDD2is lower than the constant voltage Vt:

Then the first control voltage SWVDD1at the comparator output3is floating. If the first voltage VDD1is higher than the constant voltage Vt, then the first pull down element PD1will pull the first control voltage SWVDD1to 0 and the output O is connected to the first voltage input IN1.

2. The first voltage VDD1is lower than the constant voltage Vt:

Then the inverter output6is floating. If the second voltage VDD2is higher than the constant voltage Vt, then the first control voltage SWVDD1at the comparator output3is equal to the second voltage VDD2. The second pull down element PD2will pull the second control voltage SWVDD2at the inverter output6to0.

3. The first voltage VDD1is higher than the second voltage VDD2and the second voltage VDD2is higher than the voltage Vt:

The first control voltage SWVDD1at the comparator output3goes to0and the second control voltage SWVDD2at the inverter output6adopts the value of the first voltage VDD1.

4. The second voltage VDD2is higher than the first voltage VDD1and the first voltage VDD1is higher than the voltage Vt:

The first control voltage SWVDD1at the comparator output3becomes the second voltage VDD2and the second control voltage SWVDD2at the inverter output6becomes0.

5. The first voltage VDD1is lower than the voltage Vt and the second voltage VDD2is lower than the voltage Vt:

Then the comparator output3and the inverter output6will be floating and the output voltage VDD at the circuit output O will be floating.

In all cases, the value of the output voltage VDD is always the maximum of the two voltages VDD1and VDD2.

By using several selection circuits as shown inFIG. 2, it is possible to choose the maximum supply voltage from a set of different supply voltages V1−Vn, whatever their number n is, thanks to the formula:
max(V1,V2,V3,V4, . . . )=max( . . . max(max(max(V1,V2),V3),V4) . . . )

The advantages of the selection circuit according to the invention are among other things the following.

The selection circuit will work also with low voltages. If the first voltage VDD1or the second voltage VDD2is above the voltage Vt, the selection circuit delivers a non floating output voltage VDD at its output O.

Another advantage is the low power consumption of the circuit. In general, the comparator CMP is consuming much more current than the inverter INV. So it is possible to optimize the power consumption on one power supply by connecting it to VDD1.

In all cases, this circuit is faster than the prior art solution as shown inFIG. 1because it does not rely on a loop and it reaches its final state faster.

Having illustrated and described preferred embodiments for a novel selection circuit, it is noted that variations and modifications in the circuit can be made without departing from the spirit of the invention or the scope of the appended claims.