Buck-boost based high linearity driver with separately controllable duty cycle for energizing and de-energizing phases
A variable de-energizing phase duty cycle D d is generated based on an input signal to optimize for a low duty cycle in a freewheeling phase. A variable energizing phase duty cycle D e is generated based on a buck-boost control loop tracking the input signal. A tri-state buck-boost converter is controlled using the variable energizing phase duty cycle D e and the variable de-energizing phase duty cycle D d .
1. A tri-state buck-boost system having a freely changeable de-energizing duty cycle, comprising:
a tri-state buck-boost converter configured to operate in an energizing phase in which energy of an inductor is increased, a de-energizing phase in which the energy of the inductor is decreased, and a freewheeling phase in which the energy of the inductor is allowed to recirculate, the tri-state buck-boost converter configured to produce a voltage output based on a power supply voltage, a variable energizing phase duty cycle D e for operation in the energizing phase, and a variable de-energizing phase duty cycle D d for operation in the de-energizing phase; and
a buck-boost control loop, configured to:
generate the variable de-energizing phase duty cycle D d based on an input signal to optimize for a low duty cycle in the freewheeling phase,
generate the variable energizing phase duty cycle D e based on the control loop tracking the input signal, and
control the tri-state buck-boost converter using the variable energizing phase duty cycle D e and the variable de-energizing phase duty cycle D d .
2. The tri-state buck-boost system of claim 1 , where D e is set based on D d scaled by a ratio of the input signal to the power supply voltage, plus a feedback signal.
3. The tri-state buck-boost system of claim 1 , where D d is set according to a ratio of the power supply voltage to an absolute value of the input signal plus the power supply voltage.
4. The tri-state buck-boost system of claim 3 , where the ratio is scaled by a factor based on a desired freewheeling duty cycle.
5. The tri-state buck-boost system of claim 4 , wherein the desired freewheeling duty cycle is a constant.
6. The tri-state buck-boost system of claim 1 , where D d is set according to a ratio of the power supply voltage to a quantity determined as: a sum of a square root of the input signal squared plus a real coefficient squared, plus the power supply voltage.
7. The tri-state buck-boost system of claim 6 , where the real coefficient is a constant.
8. The tri-state buck-boost system of claim 6 , where the real coefficient is variable and set to a low pass filtered version of the input signal.
9. The tri-state buck-boost system of claim 6 , where the real coefficient is variable and set to the derivative of the input signal scaled by a scaling factor.
10. The tri-state buck-boost system of claim 1 , wherein the tri-state buck-boost converter includes a plurality of switches, such that in the energizing phase the switches are set such that current flows from the power supply voltage through the inductor such that current in the inductor rises, in the de-energizing phase the switches are set such that the current flows from the inductor to the voltage output, and in the freewheeling phase the switches are set such that the current recirculates through the inductor.
11. The tri-state buck-boost system of claim 1 , wherein the buck-boost control loop comprises:
a subtracter configured to determine a difference between the input signal and the voltage output to use as an error signal;
a gain equalizer configured to receive the error signal and D d , and compute an output indicative of D e to achieve a constant unity gain bandwidth;
a loop filter configured to suppress non-linearity and to stabilize the buck-boost control loop; and
an input feedforward configured to provide D d , the input signal, and the power supply voltage as additional inputs to inform D e .
12. The tri-state buck-boost system of claim 11 , wherein the gain equalizer is configured to compute the output indicative of D e by multiplying the error signal by a factor of K/D d .
13. The tri-state buck-boost system of claim 11 , wherein the buck-boost control loop comprises:
a digital-to-analog converter (DAC) configured to receive the input signal in digital form and provide an analog input signal, such that the subtracter operates in the analog domain; and
an analog-to-digital converter (ADC) configured to provide a digital filter output to the gain equalizer.
14. The tri-state buck-boost system of claim 13 , wherein the loop filter is implemented in the analog domain between the subtracter and the ADC.
15. The tri-state buck-boost system of claim 14 , wherein the loop filter is implemented as a low pass filter.
16. The tri-state buck-boost system of claim 15 , wherein the low pass filter is a high-order low pass filter.
17. The tri-state buck-boost system of claim 11 , wherein the loop filter is implemented in the digital domain after the gain equalizer.
18. The tri-state buck-boost system of claim 11 , wherein the buck-boost control loop comprises:
an analog-to-digital converter (ADC) configured to provide a digital output of the voltage output,
wherein the subtracter operates in the digital domain.
19. A method for operation of a tri-state buck-boost system, comprising:
generating a variable de-energizing phase duty cycle D d based on an input signal to optimize for a low duty cycle in a freewheeling phase;
generating a variable energizing phase duty cycle D e based on a buck-boost control loop tracking the input signal; and
controlling a tri-state buck-boost converter using the variable energizing phase duty cycle D e and the variable de-energizing phase duty cycle D d .
20. The method of claim 19 , further comprising setting D e according to D d scaled by a ratio of the input signal to power supply voltage, plus a feedback signal.
21. The method of claim 19 , further comprising setting D d according to a ratio of power supply voltage to an absolute value of the input signal plus the power supply voltage.
22. The method of claim 21 , further comprising scaling the ratio by a factor based on a desired freewheeling duty cycle.
23. The method of claim 22 , wherein the desired freewheeling duty cycle is a constant.
24. The method of claim 19 , further comprising setting D d according to a ratio of power supply voltage to a quantity determined as: a sum of a square root of the input signal squared plus a real coefficient squared, plus the power supply voltage.
25. The method of claim 24 , where the real coefficient is a constant.
26. The method of claim 24 , where the real coefficient is variable and further comprising setting the real coefficient to a low pass filtered version of the input signal.
27. The method of claim 24 , where the real coefficient is variable and further comprising setting the real coefficient to the derivative of the input signal scaled by a scaling factor.