Fast start-up circuit for low power current mirror

• US 9,312,747 B1
• Filed: 11/22/2014
• Issued: 04/12/2016
• Est. Priority Date: 11/20/2014
• Status: Active Grant

• Current Assignee: Dialog Semiconductors (UK) Limited
• Original Assignee: Dialog Semiconductors (UK) Limited
• Inventors: Svorc, Jindrich
• Primary Examiner(s): Laxton, Gary L
• Application Number:
  US14/550,924
• Time in Patent Office: 507 Days
• Field of Search: 363/49, 323/315, 323/901
• US Class Current: 1/1
• CPC Class Codes: G05F 3/26 Current mirrors
• CPC Class Codes: G05F 3/262 using field-effect transistors only
• CPC Class Codes: H02M 1/36 Means for starting or stopping converters

Abstract

A fast startup circuit delivers current to a number of loads, comprising a P-channel current mirror circuit that provides current to an N-channel current mirror circuit that distributes current to a plurality of circuit networks. A first capacitor and a second capacitor are charged to two different voltages when the circuit is disabled and the two voltages are equalized when the circuit is enabled creating an initial operational starting point that has a steady state operating voltage that is the same as that during continuous operations. Thus there is no waiting for the fast startup circuit to start from an off condition and build up exponentially to a steady state operational level.

Claims

1. A fast startup circuit, comprising:

a) a first capacitor charged to zero volts when a disable switch is closed

b) a second capacitor charged to a bias voltage when the disable switch is closed; and

c) a current mirror circuit enabled in which the disable switch is opened and a enable switch is closed, wherein the current mirror circuit comprises the first and second capacitors, and wherein charge on the first and second capacitors equalize causing current in the current mirror circuit to start at a steady state operating value.

2. The circuit of claim 1, wherein the first capacitor is split to form the second capacitor, wherein the sum of the capacitance of the first and second capacitor is equal to the capacitance of the first capacitor prior to being split.

3. The circuit of claim 1, wherein the disable switch when closed shorts the first capacitor to ground, charging the first capacitor to zero volts.

4. The circuit of claim 1, wherein the enable switch is open when the disable switch is closed, which separates a bias source from the current mirror circuit.

5. The circuit of claim 1, wherein the enable switch and the disable switch are mutually exclusive.

6. The circuit of claim 1, wherein said current mirror circuit when enabled produce a voltage on the base of the N channel transistor of the current mirror current source, which is a result of the combined charge on the first and second capacitors, that is equal to steady state voltage during continuous operations, thereby starting circuit operations at normal steady state conditions.

7. The circuit of claim 1, wherein the first capacitor is connected between the base of the current source transistor of the N channel current mirror circuit and circuit ground.

8. The circuit of claim 1, wherein the second capacitor is connected between a current load circuit of a P channel current mirror circuit and circuit ground.

9. The circuit of claim 1, wherein said first and said second capacitors provide a capability to filter noise on the present current mirror circuit.

10. The circuit of claim 1, wherein said N-channel current mirror circuit drives a plurality of current loads.

11. The circuit of claim 1, wherein the second capacitor has a capacitance that is larger than a ratio of VddNbnss, then a startup bias current boost peak is generated.

12. The circuit of claim 11, wherein the current boost peak helps speed up the startup of other circuitry.

13. A method of starting a current mirror circuit at steady state conditions, comprising:

a) splitting a capacitor into two capacitors having the same total capacitance prior to being split;

b) connecting the first of the two capacitors between the base terminal of an NMOS transistor and circuit ground, and charging the first of the two capacitors to zero volts when the current mirror circuit is disabled by a closed disable switch and an open enable switch;

c) connecting the second of the two capacitors between a P-channel transistor connected to circuit bias and circuit ground, and charging the second of the two capacitors to circuit bias voltage when the current mirror circuit is disabled; and

d) enabling the NMOS current mirror circuit, wherein the charge on the two capacitors normalizes causing current in the current mirror circuit to start at a steady state operating value.

14. The circuit of claim 13, wherein the disable switch and the enable switch are mutually exclusive and cannot be on, or off, at the same time.

15. The method of claim 13, wherein the NMOS transistor is a current source for the NMOS current mirror circuit.

16. The method of claim 13, wherein the base terminal of the NMOS transistor is shorted to ground when the current mirror circuit is disabled.

17. The method of claim 13, wherein the P-channel transistor is the current load transistor of a P-channel current mirror, and is connected to circuit bias.

18. The method of claim 13, wherein the second capacitor is connected directly to circuit bias with a second disable switch and if the second capacitor has a capacitance that is larger than a ratio of VddNbnss, then a startup bias current boost peak is generated.

19. The circuit of claim 18, wherein the current boost peak helps speed up the startup of other circuitry.

20. The circuit of claim 18, wherein the voltage Vdd is circuit bias voltage and Vbss is the steady state voltage on the net connected to the base terminals of the NMOS transistors.