Patent Publication Number: US-2012044601-A1

Title: Startup protection circuit and electronic device using the same

Description:
BACKGROUND 
     1. Technical Field 
     The disclosed embodiments relate to electronic devices, and more particularly to an electronic device with a startup protection circuit. 
     2. Description of Related Art 
     Generally, an electronic device includes an initialization circuit and a processing unit, and the electronic device is powered by a supply voltage from a power source. The initialization circuit is used for initializing the processing unit, when the processing unit has initialized completely, the processing unit is ready for use. 
     However, sometimes, the supply voltage may be too low. In this situation, the processing unit may still start up and try to operate in some way. In such situation, the processing unit may malfunction because of trying to operate with low voltage. 
     Therefore, there is room for improvement in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout two views. 
         FIG. 1  is a block diagram of an electronic device in accordance with an exemplary embodiment. 
         FIG. 2  is a circuit diagram of the electronic device of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an electronic device  100  includes a startup protection circuit  200 , a processing unit  300 , and a power source  400 . The power source  400  provides a supply voltage to the startup protection circuit  200  and the processing unit  300 . In other embodiments, the power source  400  is not included in the electronic device  100 . 
     The startup protection circuit  200  includes an initialization unit  10  and a protection unit  20 . The initialization unit  10  is connected to the processing unit  300 . The initialization unit  10  generates a first level signal when initially receiving the supply voltage which is larger than or equal to a predetermined voltage. After the first level signal is generated for a predetermined time period, the initialization unit  10  generates a second level signal. 
     The processing unit  300  is initialized according to the first level signal, during the predetermined time period, the processing unit  300  may execute built-in software programs. After the predetermined time period, the processing unit  300  has initialized completely. Therefore, the processing unit  300  is ready for use according to the second level signal. In this embodiment, the first level signal is a low level voltage signal, and the second level signal is a high level voltage signal. 
     The protection unit  20  is connected to the processing unit  300 . The protection unit  20  generates the first level signal when the supply voltage is smaller than the predetermined voltage, and stops generating the first level signal when the supply voltage is larger than or equal to the predetermined voltage. Therefore, when the supply voltage is smaller than a predetermined voltage, the processing unit  300  cannot receive the second level signal from the initialization unit  10 , thus the processing unit  300  cannot be activated and cannot perform error operations. 
     Referring to  FIG. 2 , the initialization unit  10  includes a first resistor R 1 , a first capacitor C 1 , and a second capacitor C 2 . One end of the first resistor R 1  is connected to the power source  400 , the other end of the first resistor R 1  is grounded through the first capacitor C 1 . The processing unit  300  includes an initializing pin  302  for receiving the first level signal and the second level signal, the initializing pin  302  is connected between the first resistor R 1  and the first capacitor C 1 . One end of the second capacitor C 2  is connected to the initializing pin  302 , the other end of the second capacitor C 2  is grounded. 
     The protection unit  20  includes a second resistor R 2 , a third resistor R 3 , a fourth resistor R 4 , a fifth resistor R 5 , a first transistor Q 1  and a second transistor Q 2 . The second resistor R 2  and the third resistor R 3  are connected in series between the power source  400  and ground. A base of the first transistor Q 1  is connected between the second resistor R 2  and the third resistor R 3  through the fifth resistor R 5 , an emitter of the first transistor Q 1  is grounded, a collector of the first transistor Q 1  is connected to the power source  400  through the fourth resistor R 4 . The base of the second transistor Q 2  is connected to the collector of the first transistor Q 1 , the emitter of the second transistor Q 2  is grounded, the collector of the second transistor Q 2  is connected to the initializing pin  302 . In this embodiment, the first transistor Q 1  and the second transistor Q 2  are npn type transistors. 
     The principle of the electronic device  100  is as follows: 
     When the electronic device  100  initially receives the supply voltage from the power source  400 , the base of the first transistor Q 1  is pulled high, and the first transistor Q 1  is turned on. The base of the second transistor Q 2  is pulled low, and the second transistor Q 2  is turned off. The initializing pin  302  receives a low level voltage signal, and is initialized. The supply voltage charges up the first capacitor C 1  and the second capacitor C 2  through the first resistor R 1 , the processing unit  300  has initialized completely after a predetermined time period T, T is calculated by the formula: T=R 1 ×(C 1 +C 2 ). The initializing pin  302  receives a high level voltage signal, and the processing unit  300  is ready for use. 
     When the supply voltage from the power source  400  is smaller than the predetermined voltage, the base of the first transistor Q 1  is pulled low, and the first transistor Q 1  is turned off. The base of the second transistor Q 2  is pulled high, and the second transistor Q 2  is turned on. The initializing pin  302  also receives a low level voltage signal, and is initialized. Because the initializing pin  302  receives the low level voltage signal when the supply voltage is smaller than the predetermined voltage, the processing unit  300  cannot be activated. 
     Further alternative embodiments will become apparent to those skilled in the art without departing from the spirit and scope of what is claimed. Accordingly, the present invention should be deemed not to be limited to the above detailed description, but rather only by the claims that follow and equivalents thereof.