Controller and voltage detection enabling circuit thereof

A voltage detection enabling circuit is disclosed. The voltage detection enabling circuit includes a reference voltage generating unit, an enabling protection unit, and an enabling judgment unit. The reference voltage generating unit is coupled to a driving voltage, and generates a reference voltage signal. The enabling protection unit receives the reference voltage signal and outputs an enabling judgment signal when the reference voltage signal is higher than a voltage parameter. Particularly, the voltage parameter is a component parameter of an electronic component. Then the enabling judgment unit determines whether an enabling signal is generated or not according to the enabling judgment signal and the driving voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a controller and a voltage detection enabling circuit thereof, especially to a voltage detection enabling circuit which determines whether or not to enable a function circuit according to a voltage parameter of a transistor.

2. Description of the Related Art

Because a power supply provided to devices inside an electronic circuit is not stable enough during the beginning when the electronic circuit is enabled, it may frequently cause erroneous operation during the period that the electronic circuit starts to operate. For avoiding the occurrence of the erroneous operation on account of unstable power supply during the beginning, a predetermined voltage value is generally set to determine whether the driving voltage is stable enough or not. The electronic circuit is then enabled only if the driving voltage reaches the predetermined voltage value.

Please refer toFIG. 1, which is the circuit diagram of under voltage lockout (UVLO) circuit of the prior art. The UVLO circuit includes a voltage divider which has resisters RD1and RD2, a reference voltage generator VG, a comparator COM, and an AND gate AND. In which the voltage divider is coupled with a driving voltage VCC in order to generate a divided voltage signal V1.

A non-inverting input terminal of the comparator COM receives the divided voltage signal V1. An inverting input terminal of the comparator COM receives a reference voltage signal VREF generated by the reference voltage generator VG. A comparison signal CMP is generated when the divided voltage signal V1is higher than the reference voltage signal. The AND gate AND receives the comparison signal CMP and a delay signal DELAY. When both the comparison signal CMP and the delay signal DELAY are high, an enabling signal UVLO is outputted for enabling an electronic circuit formally.

Please refer now toFIG. 2, which shows the timing diagram of the under voltage lockout circuit inFIG. 1. After the circuit is enabled, the driving voltage VCC increases, so the divided voltage signal V1increases accordingly. Because the reference voltage generator VG is also driven by the driving voltage VCC, the reference voltage generator VG starts to generate the reference voltage signal VREF when the driving voltage VCC increases and reaches the potential level which is capable to drive the reference voltage generator VG. At time point t1, the reference voltage generator VG starts to generate the reference voltage signal VREF. However, the reference voltage signal VREF reaches a steady value only when the driving voltage VCC increases and reaches a higher potential level.

As the time the reference voltage generator VG is driven and enabled, the comparator COM is also enabled. At the moment. Because the reference voltage signal VREF is lower then the divided voltage signal V1during the beginning of the reference voltage generator VG, the potential of outputted comparison signal CMP is high. At time point t3, the potential level of the reference voltage signal VREF rises and becomes higher than the divided voltage signal V1, the comparator COM stops outputting the comparison signal CMP since the potential of the comparison signal CMP is low.

For avoiding the comparator COM from performing wrong comparison because of the unstable reference voltage signal VREF during the beginning of the reference voltage generator VG, a delay signal DELAY is provided. The delay signal DELAY is set to generate a delay time Td after the circuit is enabled.

The delay signal DELAY is adopted to avoid the erroneous judgment. However, the rising speed of driving voltage VCC in different power supply is different, the comparison may still err for the slow rising speed of the driving voltage VCC. As shown in the present case, the time point t2of the delay signal DELAY generating comes before the time point t3, thus during the period between t2and t3, the enabling signal UVLO may still provide error output. Until the time point t4, the divided voltage signal V1becomes higher than the stable reference voltage signal VREF again, so the enabling signal UVLO is outputted correctly.

As described above, the prior under voltage lockout circuit may still possibly make wrong determination, trigger error actions, and further cause circuit damage. Therefore, the problem is what should be overcome.

SUMMARY OF THE INVENTION

In view of above-described issues, a controller and its voltage detection enabling circuit of the present invention is used to determine the state of the reference voltage generating unit according to the parameter of transistor, for properly postponing the judgment for determining whether or not to enable of the electronic circuit.

In order to achieve the aforementioned purposes, the present invention provides the voltage detection enabling circuit, which includes a reference voltage generating unit, an enabling protection unit, and an enabling judgment unit.

In which, the reference voltage generating unit is coupled with a driving voltage, and is for generating a reference voltage signal. The enabling protection unit receives the reference voltage signal, and outputs an enabling judgment signal when the reference voltage signal is higher than a voltage parameter representative of a transistor parameter. In addition, the enabling judgment unit is for determining whether to generate an enabling signal or not according to the enabling judgment signal and the driving voltage.

The present invention also discloses another voltage detection enabling circuit, which includes a reference voltage generating unit, an enabling judgment unit, and an enabling protection unit.

Wherein the reference voltage generating unit is coupled with a driving voltage, and is for generating a reference voltage signal. The enabling judgment unit is for determining whether or not to generate an enabling signal according to a potential signal representative of the driving voltage. The enabling protection unit is coupled with the enabling judgment unit, in which the enabling protection unit is for determining whether to stop the enabling judgment unit from generating the enabling signal according to the reference voltage signal.

Furthermore, the present invention provides a controller, which includes a voltage detection enabling circuit and a function circuit.

Wherein, the voltage detection enabling circuit is coupled with a driving voltage for generating a reference voltage signal, and for determining whether to generate an enabling signal or not according to the driving voltage when the reference voltage signal is higher than a voltage parameter representative of a transistor parameter. Additionally, the function circuit is coupled with the driving voltage to receive power for operating and converts at least one input signal into at least one output signal according to a predetermined function when the function circuit receives the enabling signal.

For further understanding of the invention, reference is made to the following detailed description illustrating the embodiments and examples of the invention. The description is only for illustrating the invention, not for limiting the scope of the claim.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Please refer toFIG. 3, which is a circuit diagram of a preferred embodiment of a controller with voltage detection enabling circuit. The controller includes a voltage detection enabling circuit and a function circuit140. The voltage detection enabling circuit has an enabling protection unit110, an enabling judgment unit120, and a reference voltage generating unit130. In which, the reference voltage generating unit130is coupled with a driving voltage VCC, and generates a reference voltage signal VBG.

The enabling protection unit110includes a build-in electronic component, which has some component parameters. Further, the enabling protection unit110receives reference voltage signal VBG, and outputs an enabling judgment signal EN when the reference voltage signal VBG is higher than a voltage parameter representative of the component parameter. When receiving the enabling judgment signal EN, the enabling judgment unit120starts to determine whether the driving voltage VCC reaches a predetermined voltage potential value. If the driving voltage VCC reaches the predetermined voltage potential value, the enabling judgment unit120then generates an enabling signal UVLO.

Additionally, the function circuit140is coupled with the driving voltage VCC in order to receive power for operating. The function circuit140starts to operate when receiving the enabling signal UVLO, for converting an input signal Sin to an output signal Sout according to a predetermined function.

Wherein, the function circuit140can be any kind of circuit capable of processing and calculating. The function circuit140is used for generating at least one output signal with predetermined functions according to the signals provided by other circuits or according to the feedback signals. For example, the function circuit can be an AC to DC converting control circuit, a DC to DC converting control circuit, a DC to AC converting control circuit, an analog to digital converter circuit, a digital to analog converter circuit, LED driving control circuit, or battery charging control circuit, etc.

Next, please refer toFIG. 4, which shows a circuit diagram of a first preferred embodiment of a voltage detection enabling circuit. The voltage detection enabling circuit includes an enabling protection unit210, an enabling judgment unit220, and a reference voltage generating unit230. In which, the reference voltage generating unit230is coupled with a driving voltage VCC, and is used for generating a reference voltage signal VBG.

The enabling protection unit210has an inverter212, a transistor component214, and a resistor216. Wherein the transistor component214can be a MOS transistor, which has a control end, a first end, and a second end. In which, the control end is coupled with the reference voltage generating unit230, and receives reference voltage signal VBG. The first end is coupled with the resistor216, and is coupled with the enabling judgment unit220through the inverter212. The second end is grounded.

Further, the other side of the resistor216is coupled with the driving voltage VCC for generating a voltage signal215and sends the voltage signal215to the inverter212. Then, the inverter212inverts the voltage signal215into an enabling judgment signal EN.

The enabling judgment unit220includes a voltage divider235, a comparator240, and an AND gate245. Wherein the voltage divider235is coupled with the driving voltage VCC and generates a driving divided voltage signal236. The non-inverting input terminal of comparator240receives the driving divided voltage signal236, and the inverting input terminal of the comparator240receives the reference voltage signal VBG. The comparator240compares the driving divided voltage signal236with the reference voltage signal VBG and then generates a comparison signal241. Further, the AND gate245receives the comparison signal241and the enabling judgment signal EN, and outputs the enabling signal UVLO.

Please refer toFIGS. 5a˜5f, which show several timing diagrams of the voltage detection enabling circuit as shown inFIG. 4. As shown inFIG. 5a, the driving voltage VCC increases gradually after the circuit starts, and the driving divided voltage signal236also increases simultaneously. At time point t5, when the driving voltage VCC increases and reaches the potential level which is able to activate the reference voltage generating unit230. The reference voltage generating unit230starts to generate the reference voltage signal VBG. Since the driving voltage VCC still increases gradually, the reference voltage signal VBG eventually becomes higher than the driving voltage VCC at time point t6.

Now referring toFIG. 5b, the comparator240also activates around the activated timing of the reference voltage generating unit230. At the moment, the reference voltage signal VBG is lower than the driving divided voltage signal236, so the comparison signal241is high until the time point t6. At the time point t6, the reference voltage signal VBG is higher than the driving divided voltage signal236, thus the comparison signal241stops outputting, i.e.: the comparison signal241is low.

Referring toFIG. 5c, the reference voltage signal VBG is lower than the threshold voltage Vth of the transistor component214before time point t7and so the transistor component214is in cut-off state during the period. Therefore, the potential level of the voltage signal215is equal to the driving voltage VCC before the time point t7. When the reference voltage signal VBG equals to the threshold voltage Vth of the transistor component214at time point t7, the transistor component214is set conductive, and the potential level of the voltage signal215is low (ground).

SeeFIG. 5d, at the moment, the voltage signal215is inverted by the inverter212to generate the high-level enabling judgment signal EN. Then please refer toFIG. 5f, the comparator240outputs high-level comparison signal241during the period between time points t5and t6. But because the reference voltage signal VBG is still lower than the threshold voltage Vth of the transistor component214and the voltage signal215becomes high. Next, the enabling judgment signal EN inverted from the voltage signal215is low. Therefore, the enabling signal UVLO maintains low level.

During the period between the time points t7and t8, the reference voltage signal VBG is higher than the threshold voltage Vth of the transistor component214and makes the enabling protection unit210generate high-level enabling judgment signal EN. But at the moment, because the reference voltage signal VBG is higher than the driving divided voltage signal236and outputs low-level comparison signal241, so the enabling signal UVLO still keeps at low level.

At time point t8, the comparison signal241and the enabling judgment signal EN are high at the same time, thus the enabling signal UVLO turns to high level for enabling the operation of the function circuit.

Therefore, as described above, the present invention uses the component parameter, such as a parameter of transistor, which is not influenced by application condition of the circuit, for determining whether or not the reference voltage signal generated by the reference voltage generating unit is proper to treat as a comparison standard of potential level. By doing this, the problem of wrongly determining the potential of driving voltage can be avoided.

Please refer toFIG. 6, which shows a circuit diagram of a second preferred embodiment of voltage detection enabling circuit. The voltage detection enabling circuit includes an enabling protection unit310, an enabling judgment unit320, and a reference voltage generating unit330. In which the reference voltage generating unit330is coupled with a driving voltage VCC and generates a reference voltage signal VBG.

The enabling protection unit310has a transistor switch311, two inverters312and318, a transistor component314, and a resistor316. In which the transistor component314includes a control end, a first end, and a second end. The control end is coupled to the reference voltage generating unit330for receiving the reference voltage signal VBG. The first end is coupled to the resistor316, and the second end is grounded.

Additionally, the other end of the resistor316is coupled to the driving voltage VCC for generating a voltage signal315. The voltage signal315is then inverted by the inverter316and318for enforcing the driving capability, and is converted into a driving signal319.

Further, the drain node of the transistor switch311is coupled with a voltage divider335of the enabling judgment unit320, for providing an enabling judgment signal EN. The source node of the transistor switch311is grounded, and the gate node of the transistor switch311receives the driving signal319.

The enabling judgment unit320has the voltage divider335and a comparator340. Wherein the voltage divider335is coupled to the driving voltage VCC and generates a driving divided voltage signal336at the voltage dividing point of the voltage divider335. The non-inverting input terminal of the comparator340receives the driving divided voltage signal336, and the inverting input terminal receives the reference voltage signal VBG. Then an enabling signal UVLO is generated after comparing the driving divided voltage signal336with the reference voltage signal VBG. In which, the enabling signal UVLO provided by the comparator340has a default state of low level, and in practice, it achieves by coupling a resistor (not shown) with the output terminal of the comparator340while the other side grounded.

Then please referFIGS. 7a˜7d, which are the timing diagrams of the voltage detection enabling circuit as shown inFIG. 6. SeeFIG. 7a, the driving voltage VCC increases gradually after the circuit is started. When the driving voltage VCC reaches the potential level which is enough to drive the reference voltage generating unit330. The reference voltage generating unit330then starts to generate the reference voltage signal VBG, and the potential of the reference voltage signal VBG also increases gradually according to the driving voltage VCC.

Please refer toFIG. 7bandFIG. 7c, when the potential of the reference voltage signal VBG rises above the threshold voltage Vth of the transistor component314and makes the transistor component314become conductive state from cut-off state, the potential of the voltage signal315is changed from the potential level of the driving voltage VCC to zero (grounded).

Next, please checkFIG. 7d, when the reference voltage signal VBG is lower than the threshold voltage Vth of the transistor component314, the voltage signal315is at high level for making the transistor switch311conduct. At the moment, referring toFIG. 7a, the driving divided voltage signal336(enabling judgment signal EN) is low (zero) and lower than the reference voltage signal VBG, so the comparator340stops outputting the enabling signal UVLO.

When the reference voltage signal VBG is higher than the threshold voltage Vth of the transistor component314, the voltage signal315is at low level. The transistor switch311is then at cut-off state. The voltage divider335starts to work for outputting the driving divided voltage signal336according to the driving voltage VCC (in the embodiment, the driving divided voltage signal336and the enabling judgment signal EN are coupled to the same node, so the two are the same signal). At the moment, the reference voltage generating unit330already generates the reference voltage signal VBG which is higher than the driving divided voltage signal336, so the comparator still stops outputting the enabling signal UVLO.

Then, only when the driving voltage VCC further increases to proper potential level which makes the driving divided voltage signal336higher than the reference voltage signal VBG. The comparator340starts to generate the enabling signal UVLO to enable the operation of the function circuit. This can avoid the function circuit from erroneous operation due to the driving voltage VCC having too low potential level.

Please refer toFIG. 8, which is a circuit diagram of a third preferred embodiment of voltage detection enabling circuit. The voltage detection enabling circuit has an enabling protection unit410, an enabling judgment unit420, a reference voltage generating unit430, and a hysteresis enabling unit450. Wherein the reference voltage generating unit430is coupled with a driving voltage VCC and generates a reference voltage signal VBG.

The enabling protection unit410includes a transistor switch411, a bipolar junction transistor switch414, a resistor416, and a voltage divider418. In which the voltage divider418receives the reference voltage signal VBG for generating a divided voltage to the base of the bipolar junction transistor switch414. The bipolar junction transistor switch414is biased in the forward active region when the divided voltage is higher than the forward biased voltage of the bipolar junction transistor switch414.

In other words, the combination of the bipolar junction transistor switch414and the voltage divider418is for comparing the reference voltage signal VBG with a voltage parameter VTH′ representative of the forward biased voltage of the bipolar junction transistor switch414(that is, the voltage parameter VTH′ is the value which is the forward biased voltage divided by the division ratio of the voltage divider418). When the reference voltage signal VBG is higher than the reference parameter VTH′, the voltage signal415is then pulled down to generate an high-level enabling judgment signal EN through the transistor switch411(the drain of the transistor switch411is coupled to the voltage dividing point of a voltage divider435in the enabling judgment unit420).

Additionally, the enabling judgment unit420has the voltage divider435, a capacitor437, and a comparator440. In which the voltage divider435is coupled to the driving voltage VCC, and generates a driving divided voltage signal436at the voltage dividing point. The capacitor437is coupled to the voltage dividing point of the voltage divider435, for filtering out the high frequency noises which may be generated while the transistor switch411switching. The capacitor437can also slow down the potential increasing rate at the voltage dividing point, for further enhancing the capability of avoiding the electrical circuit from incorrect operation.

Moreover, the non-inverting input terminal of the comparator440receives the driving divided voltage signal436, and the inverting input terminal receives the reference voltage signal VBG. After comparison, a comparison signal441is then generated by the comparator440accordingly.

Furthermore, the hysteresis enabling unit450includes an inverter452and a NOR gate454, and has the initial potential level of the enabling signal UVLO at low potential, in order to prevent the circuit from erroneous operation because of initial state being uncertain. Wherein, the inverter452receives the comparison signal441and then sends the comparison signal441to the NOR gate454. The NOR gate454receives the output of the inverter452and the voltage signal415of the enabling protection unit410, and outputs an enabling signal UVLO by performing “NOR” operation.

Additionally, the enabling signal UVLO is also provided for enabling a hysteresis function of the comparator440. For example, the comparator440can be a comparator when the enabling signal UVLO is at low level, and be a hysteresis comparator when the comparator440receives the high-level enabling signal UVLO. This can prevent the comparator440from operating incorrectly because of the potential variation of the driving divided voltage signal436and/or the reference voltage signal VBG, which are induced by the noises of the circuit.

Alternatively, the comparator440can be a hysteresis comparator, in which the range of hysteresis voltage is small when the enabling signal UVLO is low and the range of hysteresis voltage of the comparator440is enlarged after receiving the high-level enabling signal UVLO, for increasing the anti-noise capability of the circuit.

Besides, because the hysteresis enabling unit450receives the voltage signal415at the same time, it is confirmed that the enabling signal UVLO is outputted only when the voltage signal415changes to low level. The enabling signal UVLO is avoided from outputting due to any improper operation of the enabling judgment unit420.

Next, refer toFIGS. 9a˜9d, are the timing diagram of the voltage detection enabling circuit as shown inFIG. 8. SeeFIG. 9a, the driving voltage VCC increases gradually after the circuit is activated. When the driving voltage VCC increases to the potential level which is enough to enable the reference voltage generating unit430, the reference voltage generating unit430starts to generate the reference voltage signal VBG. Then, the reference voltage signal VBG increases with the driving voltage VCC increasing.

Please refer toFIG. 9b, when the reference voltage signal VBG is higher than the voltage parameter VTH′ which represents the forward biased voltage of the bipolar junction transistor switch414. The bipolar junction transistor switch414becomes conductive state from cut-off state, thus the potential level of the voltage signal415decreases to zero from the driving voltage VCC.

Next, please refer toFIG. 9d, when the reference voltage signal VBG is lower than the voltage parameter VTH′, the voltage signal415is high which makes the transistor switch411conducts. SeeFIG. 9a, at the moment, the driving divided voltage signal436(enabling judgment signal EN) is low so as to be lower than reference voltage signal VBG. This time, the comparator440stops outputting the comparison signal441.

When the reference voltage signal VBG is higher than the voltage parameter VTH′, which makes the voltage signal415be low and cut off the transistor switch411, the voltage divider435starts to work. Therefore, the capacitor437starts to be charged and a voltage level of the driving divided voltage signal436increases gradually. At the time, the reference voltage signal VBG of reference voltage generating unit430is still higher than the driving divided voltage signal436, thus the comparator440still stops outputting the enabling signal UVLO (i.e.: low level).

Until the driving voltage VCC further increases to proper potential level which makes the driving divided voltage signal436higher than the reference voltage signal VBG. The comparator440then starts to output the high-level comparison signal441. The generated comparison signal441is then inverted to low level signal by the inverter452. At the moment, the voltage signal415is also low, so the NOR gate454generates the high-level enabling signal UVLO, which enables the operation of the function circuit, and further enables the comparator440as a hysteresis comparator or adjusts the range of the hysteresis voltage of the comparator440.

Then please refer toFIG. 10, which is the circuit diagram of a forth preferred embodiment of voltage detection enabling circuit. The voltage detection enabling circuit includes an enabling protection unit510, an enabling judgment unit520, a reference voltage generating unit530, a hysteresis enabling unit550, and a delay circuit560. The reference voltage generating unit530is coupled with a driving voltage VCC and is for generating a reference voltage signal VBG.

Additionally, the enabling protection unit510includes a transistor switch511, a voltage parameter generator512, and a comparator514. In which, the non-inverting input terminal of the comparator514receives a voltage parameter TH generated by the voltage parameter generator512, and the inverting input terminal of the comparator514receives the reference voltage signal VBG. The comparator514compares the two inputted signal and outputs a voltage signal515, which controls the switching of the transistor switch511for generating an enabling judgment signal EN. Wherein the voltage signal515outputted by the comparator514is initially at high level, practically, it can be achieved by providing a resistor (not shown) which is coupled to the output terminal of the comparator514at one end, while the other end is coupled to the driving voltage VCC.

Further, the enabling judgment unit520includes a voltage divider535, a capacitor537, and a comparator540. In which, the voltage divider535is coupled with driving voltage VCC, and is used for generating a driving divided voltage signal536at the voltage dividing point. The capacitor537is coupled to the voltage divider535at the voltage dividing point, in order to filter out the high frequency noises which are generated when the transistor switch511is switching, and slow down the increasing rate of the potential level at the voltage dividing point for further enhancing the capability of avoiding the electrical circuit from erroneous enabling.

The comparator540receives the driving divided voltage signal536by the non-inverting input terminal, receives the reference voltage signal VBG by the inverting input terminal, and outputs a comparison signal541by the output terminal.

Moreover, the hysteresis enabling unit550includes an inverter552and a NOR gate554. In which the inverter552receives the comparison signal541, and outputs the inverted signal to the NOR gate554. The NOR gate554receives the signal outputted by the inverter552and the voltage signal515of the enabling protection unit and outputs an enabling signal UVLO. The outputting of the enabling signal UVLO is then delayed for a predetermined time by the delay circuit560.

The enabling signal UVLO is also used to adjust the range of the hysteresis voltage of the comparator540. For example, the comparator540can be a normal comparator when the enabling signal UVLO is low and be a hysteresis comparator when the enabling signal UVLO is high. This is for avoiding wrong determination of the comparator540caused by the variation of the driving divided voltage536and the reference voltage signal VBG induced by the electrical noises among the circuits.

Alternatively, the comparator540can be a hysteresis comparator, which has smaller range of hysteresis voltage when the enabling signal UVLO is low and has larger range of hysteresis voltage when the enabling signal UVLO is high. This is for enhancing the anti-noise capability of the circuits.

In addition, because the hysteresis enabling unit550also receives voltage signal515at the same time, thus it's for sure that the enabling signal UVLO is outputted only when the voltage signal515turns into low level. So the enabling signal UVLO can be avoided outputting due to any erroneous operation of the enabling judgment unit520. Besides, the probability of erroneous operation may be further reduced by using the delay circuit560to delay the enabling signal UVLO.

Next, please refer toFIGS. 11a˜11e, which is the signal timing diagram of the voltage detection enabling circuit as shown inFIG. 10. SeeFIG. 11a, the driving voltage VCC increases gradually after the circuit is activated. When the driving voltage VCC increases to the level that is enough to drive the reference voltage generating unit530, the reference voltage generating unit530starts to generate the reference voltage signal VBG, in which the potential level of the reference voltage signal VBG increases with the potential level of the driving voltage VCC increasing.

Then please refer toFIG. 11band11c, when the reference voltage signal VBG is lower than the voltage parameter TH, the comparator514outputs high-level voltage signal515. When the reference voltage signal VBG increases and reaches the level which is higher than the voltage parameter TH, the voltage signal515is then turned into low level.

Referring toFIG. 11aagain, when the voltage signal515is high, the transistor switch511is conducted, so the enabling judgment signal EN and the driving divided voltage signal536at the voltage dividing point are grounded in order to restrain the voltage divider535from working. When the voltage signal515is turned into low, the transistor switch511is cut off. Then the voltage divider535starts to work, so the enabling judgment signal EN and the driving divided voltage signal536start to increase.

And as shown inFIG. 11d, only when the driving divided voltage signal536increases to the potential level which is higher than the reference voltage signal VBG, the comparator540outputs the high-level comparison signal541. Then the comparison signal541is inverted to low level signal by the inverter552. At the moment, the voltage signal515is low, thus the NOR gate554outputs the high-level enabling signal UVLO. In the meanwhile, the comparator440is enabled as a hysteresis comparator or the range of hysteresis voltage of the comparator440is adjusted, and the enabling signal UVLO is delayed to be outputted for a predetermined time Td.

As described in the above embodiments, the component parameter of the electronic components such as MOSFET and bipolar junction transistor will not be influenced by the application conditions of the electrical circuits. Therefore, they are suitable for determining the status of the reference voltage generating unit, in order to delay the enabling time of the circuit for avoiding the problems in prior techniques.

The description above only illustrates specific embodiments and examples of the invention. The invention should cover various modifications and variations made to the structures and operations described herein, and they still fall within the scope of the invention as defined in the following appended claims.