Patent Publication Number: US-6661258-B1

Title: Low voltage detecting circuit for detecting input power of a modem

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to a low voltage detecting circuit of an ADSL modem, and more specifically, to a dying gasp circuit for generating a signal when the input power of the modem drops below a predetermined voltage. 
     2. Description of the Prior Art 
     A low voltage detecting circuit is used in DSL modems to detect an input power voltage level. More specifically, the low voltage detecting circuit is used for detecting input voltage to generate a low active signal to be sent to a DMT (Discrete Multi-tone) chip in the DSL modem when the input voltage falls below a threshold level. The ADSL modem will inform the central office to ask for disconnection, and the service is then terminated. 
     Please refer to FIG.  1 . FIG. 1 is a low voltage detecting circuit  10  of an ADSL modem according to the prior art. The low voltage detecting circuit  10  comprises a comparator  24  for detecting sufficient input voltage. Non-inverting and inverting input terminals of the comparator  24  are connected to node  16  and node  12 , respectively. The low voltage detecting circuit  10  is fed by three voltage sources, which include an input voltage source Vinp and two reference voltage sources Vref 1  and Vref 2 . Resistors R 1  and R 2  form a voltage divider between the reference voltage source Vref 1  and ground, and provide a threshold voltage to the inverting input terminal of the comparator  24  at node  12 . Likewise resistors R 3 , R 4  and R 7 , R 8  form a voltage divider between the input voltage source Vinp and ground, and provide a fraction of the input voltage to the non-inverting input terminal of the comparator  24  at node  16 . An output terminal of the comparator  24  is connected to a discrete multi-tone (DMT) chip  22  at node  20 . 
     The low voltage detecting circuit  10  performs a simple function. The comparator  24  is used to compare the threshold voltage at node  12  to the fraction of the input voltage from input voltage source Vinp at node  16 . If the input voltage drops, such that voltage at node  16  is less than the threshold voltage at node  12 , then the comparator  24  outputs a low voltage signal at node  20 . When the DMT chip  22  receives this low voltage signal at node  20 , it sends the information to the central office (CO) and the ADSL modem will then be disconnected. 
     Although simple in function, the low voltage detecting circuit  10  uses the comparator  24  to generate the low voltage signal that is used to request disconnection of the ADSL modem. The comparator  24  is a complicated circuit that takes many logic gates to realize. Thus, the use of the comparator  24  adds size and expense of the low voltage detecting circuit  10 . 
     SUMMARY OF INVENTION 
     It is therefore a primary objective of the claimed invention to provide a low voltage detecting circuit that uses two transistors instead of a comparator for detecting a drop of an input power voltage of a modem below a predetermined voltage. 
     According to the claimed invention, a low voltage detecting circuit comprises a first transistor having a first electrode electrically coupled to a reference voltage, a control electrode electrically coupled to the input power voltage, and a second electrode. The low voltage detecting circuit also has a second transistor having a first electrode electrically coupled to a logic high voltage, a control electrode electrically coupled to the second electrode of the first transistor, and a second electrode electrically coupled to a logic low voltage. When the input power voltage drops below the predetermined voltage, the voltage at the control electrode of the first transistor drops below the reference voltage to turn on the first transistor and consequently the second transistor so that a logic low signal is output from the first electrode of the second transistor. 
     It is an advantage of the claimed invention that the low voltage detecting circuit uses two transistors instead of a comparator in order to make the low voltage detecting circuit smaller and less expensive. 
    
    
     These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a low voltage detecting circuit of an ADSL modem according to the prior art. 
     FIG. 2 is a low voltage detecting circuit of an ADSL modem according to the present invention. 
     FIG. 3 is another low voltage detecting circuit of an ADSL modem according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     Please refer to FIG.  2 . FIG. 2 is a low voltage detecting circuit  40  of an ADSL modem according to the present invention. The low voltage detecting circuit  40  uses two transistors Q 1  and Q 2  to perform the same function as the comparator  24  of the prior art low voltage detecting circuit  10 . Transistors Q 1  and Q 2  are preferably bi-polar junction transistors (BJTs), but can also be MOS transistors. As shown in FIG. 2, transistor Q 1  is a pnp BJT and transistor Q 2  is a npn BJT. 
     The low voltage detecting circuit  40  is fed by three voltage sources, which include an input voltage source Vinp and two reference voltage sources Vref 1  and Vref 2 . Reference voltage source Vref 2  is used as a logic high voltage source, and can have the same voltage value as the reference voltage source Vref 1 . A diode D 1  is connected between reference voltage source Vref 1  and node  41  for preventing the reverse current from passing from Vinp to voltage source Vref 1  through resistors R 21  and R 20  to thereby protect the reference voltage source Vref 1 , and node  41  is also connected to an emitter of the transistor Q 1 . A resistor R 20  is connected between the emitter of the transistor Q 1  at node  41  and a base of the transistor Q 1  at node  48 . Furthermore, resistors R 21  and R 22  form a voltage divider between the input voltage source Vinp and ground, and provide a fraction of the input voltage to the base of transistor Q 1  at node  48 . 
     A collector of the transistor Q 1  is connected to a resistor R 23  at node  42 , and the resistor R 23  is also connected to a base of transistor Q 2  at node  44 . A resistor R 24  is connected between the base of the transistor Q 2  at node  44  and an emitter of the transistor Q 2  at node  46 , which is grounded. A collector of the transistor Q 2  is connected to a discrete multi-tone (DMT) chip  52  at node  50 . Finally, a resistor R 25  is connected between the reference voltage source Vref 2  at node  50 . 
     The advantage of the low voltage detecting circuit  40  is that only two transistors Q 1  and Q 2  are needed to properly detect a low input voltage coming from the input voltage source Vinp. The reference voltage source Vref 1  provides a reference voltage to the emitter of transistor Q 1  at node  41 . Furthermore, the voltage divider made up of resistors R 21  and R 22  provides a reduced value of the voltage from the input voltage source Vinp to the base of the transistor Q 1  at node  48 . Therefore, when the voltage level at node  48  is less than the voltage level at node  41 , transistor Q 1  turns on. When transistor Q 1  turns on, this signifies that the input voltage source Vinp is providing a voltage that is too low for operation of the ADSL modem. Transistor Q 1  turning on causes the transistor Q 2  to turn on as well. Then, the collector of the transistor Q 2  outputs a logic low signal to node  50 . Finally, the DMT chip  52  receives this logic low signal at node  50 , and uses this information to ask for disconnection of the ADSL modem. 
     Compared to the prior art, the low voltage detecting circuit  40  uses only two transistors Q 1  and Q 2  to perform the same function as the comparator  24  of the prior art low voltage detecting circuit  10 . Thus, the low voltage detecting circuit  40  is realized with simpler logic, and is smaller and less expensive to manufacture. 
     Please refer to FIG.  3 . As is well known in the art, MOS transistors can replace BJT transistors Q 1  and Q 2 . In this case, a PMOS transistor Q 31  can replace transistor Q 1 , and an NMOS transistor Q 32  can replace transistor Q 2 . Operation of the low voltage detecting circuit  40  remains the same with BJT or MOS transistors. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.