Soft-start reference ramp and filter circuit

• US 7,573,252 B1
• Filed: 06/07/2004
• Issued: 08/11/2009
• Est. Priority Date: 06/07/2004
• Status: Active Grant

• Current Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Original Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Inventors: Griesert, Nathanael
• Primary Examiner(s): Ullah; Akm E
• Assistant Examiner(s): BEHM, HARRY RAYMOND
• Application Number:
  US10/862,225
• Time in Patent Office: 1,891 Days
• Field of Search: 323/49, 323/901, 323/315, 323/313, 323/314, 323/316, 323/317, 323/238, 323/321, 327/539, 327/554, 327/132
• US Class Current: 323/315
• CPC Class Codes: G05F 3/262 using field-effect transistors only
• CPC Class Codes: H02H 9/001 limiting speed of change of electric quantities, e.g. soft switching on or off progressive control of electronic switches for eliminating interferences H03K17/16
• CPC Class Codes: Y10S 323/901 Starting circuits

Abstract

A method and circuit for providing a controlled ramping and filtering to a reference voltage in a regulator circuit such that an inrush current is limited and noise coupling to an output voltage is reduced. In a typical regulator circuit a soft-start and filter circuit is inserted between a feedback amplifier and a charge transfer circuit. The soft-start and filter circuit comprises a pair of transistors that are arranged to operate as a current mirror, a biasing current source determining the current drawn from the current mirror, and a soft-start capacitor shunted by a third transistor providing a conditioned reference voltage. When reference voltage rises rapidly during power-up, charging of the soft-start capacitor with constant current provides a soft-sloped conditioned reference voltage. In steady state, soft-start capacitor and a resistance of the transistor provide a first order low pass filter reducing noise that couples to the reference voltage.

Claims

1. A circuit for providing controlled ramping and filtering, comprising:

a voltage regulator controller, including;

a current mirror circuit that is arranged to receive a reference voltage and to provide a charging current from a biasing current, wherein the current mirror circuit includes a first and a second transistor having sources receiving said reference voltage;

a soft-start capacitor circuit that is arranged to provide a conditioned reference voltage based, in part, on the charging current, wherein the soft-start capacitor circuit is coupled to the second transistor, and wherein the soft-start capacitor circuit is arranged such that the conditioned reference voltage is substantially stable during a steady state and such that the conditioned reference voltage substantially tracks the reference voltage during the steady state;

a switch circuit comprising a transistor, wherein the switch circuit is arranged to receive a switch control signal, and wherein the switch circuit is arranged to shunt the charging current away from the soft-start capacitor circuit, if the switch control signal corresponds to an inactive level, and further arranged to enable the soft-start capacitor circuit to be charged, if the switch control signal corresponds to an active level; and

a differential amplifier that is arranged to receive a feedback voltage that is based at least, in part, on a regulated output voltage;

to receive the conditioned reference voltage;

and to, during all operating states, compare the conditioned reference voltage with the feedback voltage.

2. The circuit of claim 1, wherein junction areas of the first and the second transistors have a ratio of

$\frac{W_1}{}$

L 1 W 2 ⁢

L 2 ,

and wherein the junction area ratio determines a ratio of the charging current to the biasing current.

3. The circuit of claim 2, wherein a value of the biasing current is I_B and a capacitance of the soft-start capacitor circuit is C_SS, and wherein a slope of the conditioned reference voltage is

$\frac{W_2}{}$

L 1 W 1 ⁢

L 2 ⁢

I B C SS .

4. The circuit of claim 1, further comprising a current source circuit that is arranged to provide the biasing current to the first transistor such that the charging current is provided to the soft-start capacitor circuit, if the switch control signal changes to an active level.

5. The circuit of claim 1, wherein the transistors include at least one of a PMOS FET and an NMOS FET.

6. The circuit of claim 1, further comprising a filtering resistor coupled between the soft-start capacitor circuit and the second transistor.

7. The circuit of claim 6, wherein a value of the filtering resistor ranges between about one kiloohms and about ten megaohms.

8. The circuit of claim 1, wherein the switch circuit is arranged to be opened and closed in response to a global enable signal.

9. The circuit of claim 1, wherein a resistance of the second transistor and a capacitance of the soft-start capacitor circuit are arranged to operate as a first-order low-pass filter in a steady state.

10. The circuit of claim 9, wherein a transconductance parameter of the second transistor is K, a width and a length of a junction area of the second transistor are W₂ and L₂, a saturation voltage of the second transistor V_SAT, a capacitance of the soft-start capacitor circuit is C_SS, and wherein a roll-off frequency of the low-pass filter is

$\frac{K}{}$

W 2 L 2 ⁢

V SAT 2 ⁢

π

⁢

⁢

C SS .

11. The circuit of claim 1, wherein the voltage regulator controller further includes:

a charge transfer circuit that is arranged to provide the regulated output voltage in response to an input voltage and a control voltage, wherein the control voltage is provided by the differential amplifier, and wherein the differential amplifier is an error amplifier; and

a reference source that is arranged to provide the reference voltage.

12. The circuit of claim 11, wherein the soft-start capacitor circuit is arranged such that a rise time of the regulated output voltage is determined based, in part, on a capacitance value that is associated with the soft-start capacitor circuit.

13. The circuit of claim 11, wherein the soft-start capacitor circuit is arranged such that a rise time of the conditioned reference voltage is determined, based in part, on the charging current from the current mirror circuit, and a width and a length of junction areas of the first and second transistors.

14. The circuit of claim 11, wherein the second transistor and the soft-start capacitor circuit are arranged to operate together as a low-pass filter in a steady state when the soft-start capacitor circuit is charged.

15. The circuit of claim 14, wherein the second transistor and the soft-start capacitor circuit are arranged such that a roll-off frequency of the low-pass filter is determined, in part, by a transconductance parameter and a junction area of the second transistor, and a capacitance of the soft-start capacitor circuit.

16. The circuit of claim 11, wherein the soft-start capacitor circuit includes at least one capacitor.

17. The circuit of claim 1, wherein the voltage regulator controller is arranged such that the switch control signal corresponds to an inactive level during a shutdown mode.

18. A circuit for soft-started ramping and filtering, comprising:

a means for providing a regulated input voltage at an output, including;

a current mirror that is arranged to receive a reference voltage at sources of first and second transistors of a current mirror and to provide a charging current;

a means for providing a conditioned reference voltage based, in part, on the charging current such that a rise time of the conditioned reference voltage is substantially greater than a rise time of the reference voltage, and such that a noise component of the reference voltage is filtered, such that the conditioned reference voltage is substantially stable during a steady state, and such that the conditioned reference voltage substantially tracks the reference voltage during the steady state;

a means for providing the reference voltage to the means for providing a conditioned reference voltage;

a means for providing the charging current to the means for providing a conditioned reference voltage, if a switch control signal changes to an active level, said charging current independent of the regulated input voltage at said output;

a means for filtering a noise component of the reference voltage in a steady state when the means for providing a conditioned reference voltage is charged; and

a means for comparing, during all operating states, the conditioned reference voltage with a feedback voltage, wherein the feedback voltage is based at least, in part, on the regulated input voltage at said output.

19. A method for providing controlled ramping and filtering, comprising:

regulating an output voltage, wherein regulating the output voltage includes;

receiving a reference voltage at sources of a first and second transistor of a current mirror;

providing a charging current from the current mirror;

if a switch control signal corresponds to an active level, charging a soft-start capacitor circuit with the charging current; and

if the switch control signal corresponds to an inactive level, shunting the charging current away from the soft-start capacitor circuit;

providing a conditioned reference voltage, such that a rise time of the conditioned reference voltage is greater than a rise time of the reference voltage, such that a noise component of the reference voltage is less than a noise component of the conditioned reference voltage, such that the conditioned reference voltage is substantially stable during a steady state, and such that the conditioned reference voltage substantially tracks the reference voltage during the steady state; and

comparing, during all operating states, the conditioned reference voltage with a feedback voltage, wherein the feedback voltage is based at least, in part, on a regulated output voltage.

20. The method of claim 19, wherein regulating the voltage further includes:

combining the conditioned reference voltage with a feedback voltage to provide an error voltage in a voltage regulator circuit, such that the error voltage is used to control the output voltage such that an inrush power-up current is reduced.