Ultra low power servo-controlled single clock ramp generator with amplitude independent to clock frequency
A low power servo-controlled single clock ramp generator ( 100 ) includes a fast switched comparator ( 102 ), charge pump ( 110 ) and voltage-to-current converter ( 120 ) connected to provide a feedback control mechanism under control of a pulse comparison clock signal (pulse_comp) and a reset pulse clock signal (rst_pulse) that are generated from a single input clock signal (clkin) so that there are well defined time intervals between pulses in the pulse comparison clock signal and the reset pulse clock signal, thereby providing a ramp signal (Vramp_out) having a stable, frequency-independent amplitude that is not limited by the reference voltage.
1. A ramp generator circuit for generating a ramped voltage at an output node, comprising:
a pulse generator circuit for generating a pulse comparison clock signal and a reset pulse clock signal from a single input clock signal so that there are well defined time intervals between pulses in the pulse comparison clock signal and the reset pulse clock signal;
a switched comparator circuit comprising a first input coupled to receive a reference signal, a second input coupled to the output node to receive the ramped voltage, and a comparator output node, wherein the switched comparator circuit is configured to produce, in response to the pulse comparison clock signal, a comparison signal at the comparator output node which indicates a difference between the ramped voltage and the reference signal;
a charge pump circuit comprising a charge pump capacitor which is charged or discharged to develop a control voltage in response to the comparison signal;
a voltage-to-current converter circuit for generating a ramp current in response to the control voltage;
a charging capacitor which is coupled to receive the ramp current so as to develop the ramped voltage at the output node; and
a clamping circuit coupled across the charging capacitor for discharging the charging capacitor in response to the reset pulse clock signal.
2. The ramp generator circuit of claim 1 , where the pulse generator circuit generates the pulse comparison clock signal by generating a pulse at each falling edge of the single input clock signal, and generates the reset pulse clock signal by generating a pulse at each rising edge of the single input clock signal.
3. The ramp generator circuit of claim 1 , where the single input clock signal has a fifty percent duty cycle.
4. The ramp generator circuit of claim 1 , where the pulse generator circuit generates the pulse comparison clock signal by generating a pulse in each cycle of the single input clock signal to occur substantially midway through the charging of the charging capacitor by the ramp current.
5. The ramp generator circuit of claim 1 , further comprising ramp control logic module for generating one or more control signals in response to the comparison signal, where the one or more control signals control the charge pump circuit.
6. The ramp generator circuit of claim 5 , where the charge pump circuit comprises:
a first charging transistor for connecting the charge pump capacitor to a first reference voltage in response to a first control signal; and
a second discharging transistor for connecting the charge pump capacitor to a second reference voltage in response to a second control signal.
7. The ramp generator circuit of claim 5 , where the one or more control signals control the charge pump circuit to adjust the control voltage so that the ramp current is correspondingly adjusted, thereby adjusting the ramped voltage to a predetermined maximum amplitude over a plurality of cycles in the single input clock signal.
8. The ramp generator circuit of claim 1 , where the charge pump circuit charges or discharges the charge pump capacitor only during pulses in the reset pulse clock signal.
9. A method for generating a ramped voltage having controlled maximum amplitude, comprising:
generating a pulse comparison clock signal and a reset pulse clock signal having well defined time intervals between pulses in the pulse comparison clock signal and the reset pulse clock signal; and
periodically charging and discharging a ramp capacitor to generate the ramped voltage, comprising:
monitoring the ramped voltage with a switched comparator by comparing the ramped voltage with a reference voltage during each pulse in the comparison clock signal to develop a ramp correction signal that is captured during each pulse in the reset pulse clock signal, and
correcting the ramped voltage with each pulse in the reset pulse clock signal by discharging the ramp capacitor during each pulse in the reset pulse clock signal and then charging the ramp capacitor with a ramp current that is generated in response to the ramp correction signal.
10. The method of claim 9 , where generating a pulse comparison clock signal and a reset pulse clock signal comprises:
generating pulses for the pulse comparison clock signal at each falling edge of a single input clock signal, and
generating pulses for the reset pulse clock signal at each rising edge of the single input clock signal.
11. The method of claim 9 , where monitoring the ramped voltage with a switched comparator occurs substantially midway through the charging the ramp capacitor with the ramp current.
12. The method of claim 9 , where discharging the ramp capacitor comprises connecting a clamping circuit across the ramp capacitor in response to each pulse in the reset pulse clock signal.
13. The method of claim 9 , where charging the ramp capacitor with a ramp current that is generated in response to the ramp correction signal comprises:
charging a charge pump capacitor to a control voltage in response to the ramp correction signal;
generating the ramp current in response to the control voltage; and
charging the ramp capacitor with the ramp current so as to develop the ramped voltage.
14. The method of claim 9 , monitoring the ramped voltage with a switched comparator comprises feeding back the ramped voltage to an inverting input of the switched comparator for comparison with the reference voltage with is applied to a non-inverting input of the switched comparator.
15. The method of claim 9 , where periodically charging and discharging the ramp capacitor to generate the ramped voltage further comprises generating one or more control signals in response to the ramp correction signal during each pulse in the reset pulse clock signal.
16. The method of claim 15 , where charging the ramp capacitor with a ramp current that is generated in response to the ramp correction signal comprises:
charging a charge pump capacitor to a control voltage in response to the one or more control signals;
generating the ramp current in response to the control voltage; and
charging the ramp capacitor with the ramp current so as to develop the ramped voltage.
17. The method of claim 15 , where charging the charge pump capacitor to a control voltage in response to the one or more control signals comprises:
charging the charge pump capacitor to a first reference voltage in response to a first control signal; and
discharging the charge pump capacitor to a second reference voltage in response to a second control signal.
18. A circuit for generating a ramped voltage having a controlled maximum amplitude over a plurality of cycles in an input clock signal, comprising:
a pulse generator for generating a pulse comparison clock signal and a reset pulse clock signal from a single input clock signal so that pulses in the pulse comparison clock signal are substantially centered between pulses in the reset pulse clock signal;
a switched comparator having a first input coupled to receive a reference signal, a second input coupled to receive the ramped voltage in feedback, a comparator output node, and an enable node coupled to receive a pulse comparison clock signal, wherein the switched comparator is configured to produce a comparison signal which indicates a difference between the ramped voltage and the reference signal in response to being enabled by the pulse comparison clock signal;
a correction circuit for generating a target control voltage at an internal charging capacitor in response to the comparison signal;
a voltage-to-current converter circuit for generating a ramp current in response to the target control voltage;
a ramp capacitor which is periodically charged and discharged in a controlled fashion by receiving the ramp current so as to develop the ramped voltage having a controlled maximum amplitude at the output node over a plurality of cycles of the single input clock signal, where the ramp capacitor is discharged at each pulse of the reset pulse clock signal.
19. The circuit of claim 18 , where the correction circuit comprises:
a ramp logic module coupled to receive the comparison signal from the comparator output node, wherein the ramp logic module is configured to generate one or more control signals which indicate a difference between the ramped voltage and a target voltage in response to being enabled by a reset pulse clock signal; and
a charge pump circuit for charging the internal charging capacitor to a target control voltage in response to the one or more control signals.
20. The circuit of claim 18 , where the pulse generator comprises:
a first subcircuit for generating pulses for the pulse comparison clock signal at each falling edge of a single input clock signal, and
a second subcircuit for generating pulses for the reset pulse clock signal at each rising edge of the single input clock signal.