Multi-level envelope tracking systems with separate DC and AC paths

1. A mobile device comprising:

a transceiver configured to generate a radio frequency signal for transmission;

a front end circuit including a power amplifier configured to amplify the radio frequency signal and to receive power from a power amplifier supply voltage; and

a power management circuit including an envelope tracker that includes a DC-to-DC converter configured to output a plurality of regulated voltages, a modulator including a modulator output and a plurality of switches each coupled between the modulator output and a corresponding one of the plurality of regulated voltages, a modulator control circuit configured to control activation of the plurality of switches based on an envelope of the radio frequency signal, a first filter coupled between the modulator output and the power amplifier supply voltage, and a second filter coupled between a DC voltage and the power amplifier supply voltage.

2. The mobile device of claim 1 wherein the DC-to-DC converter is further configured to generate the DC voltage.

3. The mobile device of claim 1 further comprising a battery that provides a battery voltage, the DC-to-DC converter configured to generate the plurality of regulated voltages and the DC voltage based on providing DC-to-DC conversion of the battery voltage.

4. The mobile device of claim 1 wherein the first filter includes at least one series inductor and at least one shunt capacitor.

5. The mobile device of claim 1 wherein the envelope tracker further includes a controllable delay circuit configured to control a relative delay between an AC path through the first filter and a DC path through the second filter.

6. The mobile device of claim 5 wherein the controllable delay circuit includes a plurality of controllable delay buffers each configured to control a delay of a respective one of plurality of regulated voltages.

7. The mobile device of claim 1 wherein each of the plurality of regulated voltages has a different voltage level.

8. An envelope tracking system comprising:

a power amplifier configured to amplify a radio frequency signal and to receive power from a power amplifier supply voltage; and

an envelope tracker including a DC-to-DC converter configured to output a plurality of regulated voltages, a modulator including a modulator output and a plurality of switches each coupled between the modulator output and a corresponding one of the plurality of regulated voltages, a modulator control circuit configured to control activation of the plurality of switches based on an envelope of the radio frequency signal, a first filter coupled between the modulator output and the power amplifier supply voltage, and a second filter coupled between a DC voltage and the power amplifier supply voltage.

9. The envelope tracking system of claim 8 wherein the DC-to-DC converter is further configured to generate the DC voltage.

10. The envelope tracking system of claim 8 wherein each of the plurality of regulated voltages has a different voltage level.

11. The envelope tracking system of claim 8 wherein the first filter includes at least one series inductor and at least one shunt capacitor.

12. The envelope tracking system of claim 11 wherein the first filter further includes a DC blocking capacitor in series between the modulator output and the power amplifier supply voltage.

13. The envelope tracking system of claim 8 wherein the second filter includes at least one series inductor and at least one shunt capacitor.

14. The envelope tracking system of claim 8 wherein the envelope tracker further includes a controllable delay circuit configured to control a relative delay between an AC path through the first filter and a DC path through the second filter.

15. The envelope tracking system of claim 14 wherein the controllable delay circuit includes a plurality of controllable delay buffers each configured to control a delay of a respective one of the plurality of regulated voltages.

16. The envelope tracking system of claim 8 wherein a DC path through the second filter carries at least seventy five percent of the energy provided from the envelope tracker to the power amplifier.

17. A method of envelope tracking, the method comprising:

amplifying a radio frequency signal using a power amplifier that is powered by a power amplifier supply voltage from an envelope tracker;

generating a plurality of regulated voltages using a DC-to-DC converter of the envelope tracker;

controlling activation of a plurality of switches of a modulator of the envelope tracker based on an envelope of the radio frequency signal, the plurality of switches each coupled between a modulator output and a corresponding one of the plurality of regulated voltages;

controlling the power amplifier supply voltage using a first filter that is between the modulator output and the power amplifier supply voltage; and

controlling the power amplifier supply voltage using a second filter that is between a DC voltage and the power amplifier supply voltage.

18. The method of claim 17 further comprising generating the DC voltage using the DC-to-DC converter.

19. The method of claim 17 further comprising providing DC blocking using a series DC blocking capacitor of the first filter.

20. The method of claim 17 further comprising controlling a relative delay between an AC path through the first filter and a DC path through the second filter using a controllable delay circuit.