Power supply circuit having reset circuit

• US 5,332,960 A
• Filed: 08/02/1993
• Issued: 07/26/1994
• Est. Priority Date: 01/30/1990
• Status: Expired due to Fees

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Kudoh, Kazuhiro
• Primary Examiner(s): Skudy, R.
• Assistant Examiner(s): BERHANE, ADOLF D
• Application Number:
  US08/100,187
• Time in Patent Office: 358 Days
• Field of Search: 323/303, 323/901, 323/299, 363/49
• US Class Current: 323/299
• CPC Class Codes: H03K 17/223 in field-effect transistor switches
• CPC Class Codes: Y10S 323/901 Starting circuits

Abstract

A power supply circuit includes a power supply stabilizing circuit, a reset circuit, and a switching circuit. The power supply stabilizing circuit stabilizes a power supply voltage from a power supply and applying the voltage to a main circuit. The reset circuit outputs a reset signal to the main circuit when the power supply voltage rises. The switching circuit is connected to the power supply stabilizing circuit. The switching circuit stops supply of power to the reset circuit when the voltage applied to the main circuit becomes a value not more than a predetermined value after supply of power is interrupted.

Claims

1. A power supply circuit comprising:

a power supply stabilizing circuit, receiving a power supply voltage from a power supply, for stabilizing the power supply voltage;

a main circuit, receiving the power supply voltage without a substantial voltage drop, connected in parallel with the power supply stabilizing circuit;

a reset circuit for outputting a reset signal to the main circuit when the power supply voltage rises; and

reset circuit control means, receiving said power supply voltage for interrupting supply of power from the power supply to said reset circuit when the power supply voltage applied to said main circuit decreases to a value not more than a predetermined value after supply of power is interrupted.

2. A circuit according to claim 1, wherein said power supply stabilizing circuit includes a capacitor, and said reset circuit control means interrupts supply of power to said reset circuit when a voltage of said capacitor decreases a value not more than a predetermined value.

3. A circuit according to claim 1, wherein said reset circuit control means includes a field effect transistor which is connected to receive the power supply voltage and which operates to interrupt the supply of power to said reset circuit when the power supply voltage drops below the predetermined value.

4. A power supply circuit comprising:

a main circuit, connected between an external power supply and a ground receiving a power supply voltage without a substantial voltage drop from the external power supply, the main circuit being initialized in response to a first signal changing from low-to high-level states;

a power supply stabilizing circuit having a capacitor connected in parallel with a main circuit between the power supply and the ground and receiving the power supply voltage;

a reset circuit having a first predetermined time constant for outputting said first signal to said main circuit when the stabilized power supply voltage rises; and

reset circuit control means, connected to said power supply and receiving the power supply voltage, for interrupting supply of power from said power supply voltage to said reset circuit to produce said low-level state when the power supply voltage applied to said main circuit decreases to a value not more than a predetermined value when supply of power is interrupted.

11. The power supply circuit as recited in claim 4 wherein the power supply voltage varies with the second predetermined time constant, the second predetermined time constant being greater than the first predetermined time constant.

5. A power supply circuit comprising:

a power supply stabilizing circuit for stabilizing a power supply voltage from a power supply and applying the power supply voltage to a main circuit without a substantial voltage drop;

a reset circuit supplied with said power supply voltage for outputting a reset signal to the main circuit when said power supply changes from a first level to a second level; and

reset circuit control means, receiving said power supply voltage, for changing said power supply voltage to said reset circuit to said first level when the power supply voltage applied to said main circuit decreases to a predetermined value.

6. A circuit according to claim 5, wherein said first level<

said second level.

7. A reset circuit comprising:

an integrated circuit including;

an external power supply input terminal,

an external ground input terminal,

an external reset input terminal,

a main circuit connected between the external power supply input terminal and the external ground input terminal and being reset by an input received from the external reset input terminal,

an external voltage output terminal,

a transistor having a first terminal directly connected to the external power supply input terminal, a second terminal connected to the external voltage output terminal, and a control terminal coupled to a ground; and

an external reset circuit, coupled to the integrated circuit, connected between the external voltage output terminal and the external reset input terminal, receiving a voltage supplied from the external power supply input terminal, and coupling the voltage received to the external reset input terminal.

8. The reset circuit as recited in claim 7 wherein the external reset circuit includes a resistor and capacitor, coupled to the external reset input terminal, causing a voltage at the external reset input to vary with a first time constant rate.

9. The reset circuit as recited in claim 8 including a capacitor, connected between the external power supply input terminal of the integrated circuit and the ground, causing a voltage at the external power supply input terminal to vary with a second time constant.

10. The reset circuit as recited in claim 9 wherein the second time constant is greater than the first time constant.

12. The reset circuit as recited in claim 7 wherein the integrated circuit includes a plurality of metal oxide semiconductor field effect transistors MOSFETs forming the main circuit and the transistor, wherein each of the plurality of MOSFETs have a threshold voltage that is substantially the same, and wherein a minimum operating voltage of the integrated circuit is substantially the same as the threshold voltage, wherein the transistor is activated when a voltage, substantially equal to the threshold voltage, is applied to the external power supply input terminal.