Voltage selecting circuit capable of successfully turning on internal transistor even if the difference between input voltages is less than threshold voltage of internal transistor

A voltage selecting circuit (100;

• 200), comprising:
• a first input terminal (102), arranged to operably receive a first input voltage (Vi1);
• a second input terminal (104), arranged to operably receive a second input voltage (Vi2);
• a voltage output terminal (106), arranged to operably provide an output voltage (Vout);
• a first transistor (112;
• 212), wherein a first terminal of the first transistor (112;
• 212) is coupled with the first input terminal (102) while a second terminal of the first transistor (112;
• 212) is coupled with the voltage output terminal (106);
• a second transistor (114;
• 214), wherein a first terminal of the second transistor (114;
• 214) is coupled with the second input terminal (104) while a second terminal of the second transistor (114;
• 214) is coupled with the voltage output terminal (106);
• a first resistor (122), positioned on 6a first signal path between the first input terminal (102) and a control terminal of the second transistor (114;
• 214);
• a second resistor (124), positioned on a second signal path between the second input terminal (104) and a control terminal of the first transistor (112;
• 212);
• a first current adjusting circuit (132;
• 232), coupled with the control terminal of the first transistor (112;
• 212);
• a second current adjusting circuit (134;
• 234), coupled with the control terminal of the second transistor (114;
• 214);
• a comparing circuit (140), coupled with the first input terminal (102) and the second input terminal (104), arranged to operably compare the first input voltage (Vi1) with the second input voltage (Vi2);
• and
• a control circuit (150), coupled with the first current adjusting circuit (132;
• 232), the second current adjusting circuit (134;
• 234), and the comparing circuit (140), arranged to operably control the first current adjusting circuit (132;
• 232) and the second current adjusting circuit (134;
• 234) according to a comparison result of the comparing circuit (140),
• wherein the output voltage (Vout) is equal to one of the first input voltage (Vi1) and the second input voltage (Vi2) when the control circuit (150) turns on the first current adjusting circuit (132;
• 232) and turns off the second current adjusting circuit (134;
• 234) or when the control circuit (150) turns off the first current adjusting circuit (132;
• 232) and turns on the second current adjusting circuit (134;
• 234).