Advanced constant off-time control for four-switch buck-boost converter
A buck-boost converter comprises a first high-side switch and a first low-side switch connected in series between an input voltage terminal and ground, a second high-side switch and a second low-side switch connected in series between an output voltage terminal and ground, an inductor connected to a common node of the first high-side switch and the first low-side switch, and a common node of the second high-side switch and the second low-side switch and a control apparatus configured to generate gate drive signals, wherein the control apparatus comprises a first timer for setting a turn-off time of the first low-side switch, a second timer for setting a turn-off time of the second high-side switch and a peak current mode control device for setting a turn-off time of the first high-side switch and a turn-off time of the second low-side switch.
1. An apparatus comprising:
a first timer configured to determine a turn-off edge of a gate drive signal applied to a low-side switch of a buck converter portion of a buck-boost converter, wherein the first timer comprises a first input configured to receive a first ramp, a second input configured to receive a first threshold voltage, and wherein the first ramp is generated by a first current source having a current level proportional to an input voltage of the buck-boost converter, and the first threshold voltage is proportional to a difference between the input voltage of the buck-boost converter and an output voltage of the buck-boost converter; and
a second timer configured to determine a turn-off edge of a gate drive signal applied to a high-side switch of a boost converter portion of the buck-boost converter, wherein the second timer comprises a first input configured to receive a second ramp, a second input configured to receive a second threshold voltage, and wherein the second ramp is generated by a second current source having a current level proportional to the output voltage of the buck-boost converter, and the second threshold voltage is proportional to the input voltage of the buck-boost converter.
2. The apparatus of claim 1 , further comprising:
a first offset voltage source connected between the first input of the first timer and the first ramp, wherein the first offset voltage source is configured such that a positive voltage is applied to the first input of the first timer when the first ramp is reset; and
a second offset voltage source connected between the first input of the second timer and the second ramp, wherein the second offset voltage source is configured such that a negative voltage is applied to the first input of the second timer when the second ramp is reset.
3. The apparatus of claim 1 , wherein:
a turn-off edge of a gate drive signal applied to a high-side switch of the buck converter portion of the buck-boost converter and a turn-off edge of a gate drive signal applied to a low-side switch of the boost converter portion of the buck-boost converter are determined by a comparator, and wherein the comparator has a first input configured to receive an output voltage of an error amplifier and a second input configured to receive a signal proportional to a current flowing through an inductor of the buck-boost converter.
4. The apparatus of claim 3 , wherein:
the error amplifier has a first input connected to a predetermined reference and a second input configured to detect the output voltage of the buck-boost converter.
5. The apparatus of claim 4 , wherein the signal proportional to the current flowing through the inductor of the buck-boost converter is generated by a current sensing device comprising:
a resistor-capacitor network connected between a first switching node and a second switching node of the buck-boost converter; and
a current sense amplifier connected to the resistor-capacitor network.
6. The apparatus of claim 5 , wherein the resistor-capacitor network comprises:
a first resistor and a first capacitor connected in series between the first switching node and ground; and
a second resistor and a second capacitor connected in series between the second switching node and ground, and wherein:
a common node of the first resistor and the first capacitor is connected to a first input of the current sense amplifier; and
a common node of the second resistor and the second capacitor is connected to a second input of the current sense amplifier.
7. The apparatus of claim 5 , wherein the resistor-capacitor network comprises:
a first resistor and a first capacitor connected in series between the first switching node and ground; and
a second resistor and a second capacitor connected in series between the second switching node and ground;
a third capacitor connected between a first input and a second input of the current sense amplifier, and wherein:
a common node of the first resistor and the first capacitor is connected to the first input of the current sense amplifier; and
a common node of the second resistor and the second capacitor is connected to the second input of the current sense amplifier.
8. The apparatus of claim 5 , wherein the resistor-capacitor network comprises:
a first resistor connected between the first switching node and a first input of the current sense amplifier;
a second resistor connected between the second switching node and a second input of the current sense amplifier; and
a first capacitor connected between the first input and the second input of the current sense amplifier.
9. The apparatus of claim 5 , wherein the resistor-capacitor network comprises:
a first resistor and a first capacitor connected in series between the first switching node and ground;
a second resistor and a second capacitor connected in series between the second switching node and ground;
a third resistor and a fourth resistor connected in parallel with the first capacitor and the second capacitor respectively; and
a third capacitor connected between a first input and a second input of the current sense amplifier, and wherein:
a common node of the first resistor and the first capacitor is connected to the first input of the current sense amplifier; and
a common node of the second resistor and the second capacitor is connected to the second input of the current sense amplifier.
10. The apparatus of claim 5 , wherein the resistor-capacitor network comprises:
a first resistor and a third resistor connected in series between the first switching node and ground;
a second resistor and a fourth resistor connected in series between the second switching node and ground; and
a first capacitor connected between a first input and a second input of the current sense amplifier, and wherein:
a common node of the first resistor and the third resistor is connected to the first input of the current sense amplifier; and
a common node of the second resistor and the fourth resistor is connected to the second input of the current sense amplifier.
11. A method comprising:
generating a first ramp using a first current source having a current level proportional to an input voltage of a buck-boost converter;
generating a second ramp using a second current source having a current level proportional to an output voltage of the buck-boost converter;
generating a first threshold voltage proportional to a difference between the input voltage and the output voltage of the buck-boost converter;
generating a second threshold voltage proportional to the input voltage of the buck-boost converter;
comparing the first threshold voltage with a sum of the first ramp and a first predetermined offset voltage using a first comparator;
comparing the second threshold voltage with a sum of the second ramp and a second predetermined offset voltage using a second comparator;
terminating a gate drive signal of a low-side switch of a buck converter portion of the buck-boost converter based upon a comparing result generated by the first comparator; and
terminating a gate drive signal of a high-side switch of a boost converter portion of the buck-boost converter based upon a comparing result generated by the second comparator.
12. The method of claim 11 , further comprising:
generating a current sense signal proportional to a current flowing through an inductor of the buck-boost converter;
comparing a detected output voltage of the buck-boost converter with a predetermined reference using an error voltage amplifier;
comparing the current sense signal with an output voltage of the error voltage amplifier using a comparator; and
terminating an on-time of a high-side switch of the buck converter portion of the buck-boost converter based upon a comparing result generated by the comparator.
13. The method of claim 12 , further comprising:
terminating an on-time of a low-side switch of the boost converter portion of the buck-boost converter based upon the comparing result generated by the comparator.
14. The method of claim 11 , further comprising:
configuring the buck-boost converter to operate in a buck operating mode when the input voltage of the buck-boost converter is greater than a first operating mode threshold;
configuring the buck-boost converter to operate in a boost operating mode when the input voltage of the buck-boost converter is less than a second operating mode threshold, wherein the first operating mode threshold is greater than the second operating mode threshold; and
configuring the buck-boost converter to operate in a buck-boost operating mode when the input voltage of the buck-boost converter is between the first operating mode threshold and the second operating mode threshold.
15. The method of claim 14 , further comprising:
comparing the input voltage of the buck-boost converter with the first operating mode threshold and the second operating mode threshold through a first comparator and a second comparator respectively; and
determining an operating mode of the buck-boost converter based upon output results of the first comparator and the second comparator.
16. A converter comprising:
a first high-side switch and a first low-side switch connected in series between an input voltage terminal and a ground;
a second high-side switch and a second low-side switch connected in series between an output voltage terminal and the ground;
an inductor connected to a common node of the first high-side switch and the first low-side switch, and a common node of the second high-side switch and the second low-side switch; and
a control apparatus configured to generate gate drive signals, wherein the control apparatus comprises:
a first timer for setting a turn-off time of the first low-side switch, wherein the turn-off time of the first low-side switch is generated based on a first threshold voltage proportional to a difference between an input voltage of the converter and an output voltage of the converter, wherein the first timer comprises a first input configured to receive a first ramp and a second input configured to receive the first threshold voltage, and wherein the first ramp is generated by a first current source having a current level proportional to the input voltage of the converter;
a second timer for setting a turn-off time of the second high-side switch, wherein the turn-off time of the second high-side switch is generated based on a second threshold voltage proportional to the input voltage of the converter, wherein the second timer comprises a first input configured to receive a second ramp and a second input configured to receive the second threshold voltage, and wherein the second ramp is generated by a second current source having a current level proportional to the output voltage of the converter; and
a peak current mode control device for setting a turn-off time of the first high-side switch and a turn-off time of the second low-side switch.
17. The converter of claim 16 , wherein:
the converter is a buck-boost converter, and wherein:
a turn-off edge of a gate drive signal applied to a high-side switch of a buck converter portion of the buck-boost converter and a turn-off edge of a gate drive signal applied to a low-side switch of a boost converter portion of the buck-boost converter are determined by an output of a comparator, and wherein the comparator has a first input configured to receive an output voltage of an error amplifier and a second input configured to receive a signal proportional to a current flowing through an inductor of the buck-boost converter.
18. The converter of claim 17 , wherein:
the signal proportional to a current flowing through the inductor of the buck-boost converter is generated by a dc resistance (DCR) current sensing device.
19. The converter of claim 18 , wherein:
an error amplifier has a first input connected to a predetermined reference and a second input configured to detect an output voltage of the buck-boost converter.