Voltage boosting system with slew rate control and method thereof
A system includes a voltage controlled oscillator, a charge pump, and a current regulator circuit. The voltage controlled oscillator has a control input and a clock output that provides a clock signal at a clock frequency that is variable. The charge pump is coupled to the clock output and has an output that provides a boosted output voltage. The current regulator circuit is coupled to the control input of the voltage controlled oscillator to adjust the clock frequency based on a simulation of a rate of change of the boosted output voltage. This allows for a controlled slew rate for the output of the charge pump.
1. A system, comprising:
a voltage controlled oscillator having a control input and a clock output that provides a clock signal at a clock frequency that is variable;
a charge pump coupled to the clock output having an output that provides a boosted output voltage; and
a current regulator circuit coupled to the control input of the voltage controlled oscillator to adjust the clock frequency based on a simulation of a rate of change of the boosted output voltage.
2. The system of claim 1 , further comprising a clock generator coupled between the voltage controlled oscillator and the charge pump which generates a pair of complementary clocks in response to the clock signal.
3. The system of claim 1 , further comprising a low pass filter coupled between the current regulator circuit and the control input of the voltage controlled oscillator.
4. The system of claim 1 , wherein the charge pump comprises a diode-connected transistor and a charging capacitor coupled for use in generating the boosted output voltage and the current regulator circuit comprises a tracking capacitor and a tracking transistor for simulating a response of the diode-connected transistor and the charging capacitor to the clock signal.
5. The system of claim 4 , wherein the current regulator circuit further comprises a switching transistor, wherein:
the tracking transistor has a first current electrode coupled to a power supply terminal, a control electrode coupled to a control electrode of the diode-connected transistor, and a second current electrode;
the tracking capacitor has a first terminal coupled to a reference terminal and a second terminal; and
the switching transistor is coupled between the second terminal of the tracking capacitor and the second current electrode.
6. The system of claim 5 , further comprising:
a clock generator coupled between the voltage controlled oscillator and the charge pump which generates a first clock and a second clock in response to the clock signal, wherein the first clock and the second clock are complementary; and
a discharge transistor;
wherein:
the discharge transistor has first current electrode coupled to the reference terminal, a control electrode for receiving the first clock, and a second current electrode coupled to the second terminal of the tracking capacitor.
7. The system of claim 6 , wherein the switching transistor is coupled by having a first current electrode coupled to the second current electrode of the tracking transistor, a control electrode for receiving the second clock, and a second current electrode coupled to the second terminal of the tracking capacitor.
8. The system of claim 7 , wherein the current regulator circuit further comprises:
a second switching transistor having a first current electrode coupled to the second current electrode of the tracking transistor, and a second current electrode; and
a second tracking capacitor having a first terminal coupled to the reference terminal and a second terminal coupled to the second current electrode of the second switching transistor.
9. The system of claim 8 further comprising a second discharge transistor having a first current electrode coupled to the reference terminal in which the reference terminal is a ground connection, a control electrode for receiving the first clock, and a second current electrode coupled to the second terminal of the second tracking capacitor.
10. The system of claim 5 wherein the current regulator circuit further comprises a comparator having a first input coupled to the second current electrode of the tracking transistor, a second input coupled to a voltage reference, and an output coupled to the control input of the voltage controlled oscillator.
11. The system of claim 10 , wherein the current regulator circuit further comprises a sample and hold circuit coupled between the first input of the comparator and the second current electrode of the tracking transistor.
12. A method of obtaining a boosted voltage, comprising:
generating the boosted voltage from a power supply voltage in response to a clock signal having a clock frequency that is variable;
simulating a rate of change of the boosted voltage to provide a simulated rate of change value;
comparing the simulated rate of change value to a reference value; and
changing the clock frequency of the clock signal in response to the comparing.
13. The method of claim 12 , wherein the step of changing is further characterized as decreasing the clock frequency if the comparing determines that simulated rate of change indicates that rate of change of the boosted voltage is higher than a desired rate of change.
14. The method of claim 13 , wherein the step of changing is further characterized as increasing the clock frequency if the step of comparing determines that simulated rate of change indicates that rate of change of the boosted voltage is lower than the desired rate of change.
15. The method of claim 12 , wherein:
the step of generating is further characterized by the boosted voltage being generated by a charge pump comprising a diode-connected transistor and a charging capacitor; and
the step of simulating is further characterized as using a tracking transistor and a tracking capacitor, wherein the tracking transistor and the diode-connected transistor have a predetermined gain relationship and the charging capacitor and the tracking capacitor have a predetermined capacitance relationship.
16. A system, comprising:
a voltage controlled oscillator having a control input and a clock output that provides a clock signal at a clock frequency that is variable;
a clock generator that provides a first clock and a second clock at the clock frequency in response to the clock signal, wherein the first clock and the second clock are complementary;
a charge pump that provides a boosted output voltage relative to a power supply voltage in response to the first and second clocks; and
a current regulator circuit coupled to the control input of the voltage controlled oscillator to adjust the clock frequency based on a simulation of a rate of change of the boosted output voltage using the first and second clocks.
17. The system of claim 16 , further comprising a low pass filter coupled between the current regulator circuit and the control input of the voltage controlled oscillator.
18. The system of claim 16 , wherein the current regulator circuit comprises:
a first transistor having a first current electrode and a control electrode coupled to a power supply node for receiving the power supply voltage and a second current electrode; and
a first capacitor having a first terminal selectively coupled to the second current electrode of the first transistor and a second terminal coupled to a reference terminal.
19. The system of claim 16 , wherein:
the current regulator circuit comprises a first transistor and a first capacitor that are used in providing a simulated rate of change value; and
the charge pump comprises a second transistor and a second capacitor used in providing the boosted output voltage.
20. The system of claim 19 , wherein the current regulator circuit compares the simulated rate of change value to a reference value to determine if the clock frequency should be increased or decreased.