Apparatus and methods for envelope tracking in a mobile device

1. A mobile device comprising:

a power amplifier configured to amplify a radio frequency input signal to generate a radio frequency output signal, the power amplifier configured to receive power from a power amplifier supply voltage;

a DC-to-DC converter configured to control a voltage level of the power amplifier supply voltage based on a control voltage;

an error amplifier configured to adjust the voltage level of the power amplifier supply voltage based on an envelope of the radio frequency input signal;

a low pass filter configured to low pass filter the power amplifier supply voltage to generate a filtered voltage; and

a comparator configured to generate the control voltage based on comparing the filtered voltage to a reference voltage.

2. The mobile device of claim 1 wherein the DC-to-DC converter includes a buck converter.

3. The mobile device of claim 2 wherein an output of the buck converter is electrically connected to the power amplifier supply voltage via an inductor.

4. The mobile device of claim 1 wherein the error amplifier includes an input configured to receive an envelope signal that changes in relation to the envelope of the radio frequency input signal.

5. The mobile device of claim 4 further comprising a transceiver configured to generate the envelope signal and the radio frequency input signal.

6. The mobile device of claim 1 wherein the error amplifier includes an output that is electrically connected to the power amplifier supply voltage via an AC coupling capacitor.

7. The mobile device of claim 1 further comprising a reference voltage generator configured to generate the reference voltage, the reference voltage generator configured to receive data that controls a voltage level of the reference voltage over a serial peripheral interface.

8. A method of envelope tracking in a mobile device, the method comprising:

amplifying a radio frequency input signal using a power amplifier;

providing power to the power amplifier from a power amplifier supply voltage;

controlling a voltage level of the power amplifier supply voltage based on a control voltage using a DC-to-DC converter;

adjusting the voltage level of the power amplifier supply voltage based on an envelope of the radio frequency input signal using an error amplifier;

low pass filtering the power amplifier supply voltage to generate a filtered voltage using a low pass filter; and

generating the control voltage based on comparing the filtered voltage to a reference voltage using a comparator.

9. The method of claim 8 further comprising generating the radio frequency input signal and an envelope signal that changes in relation to the envelope of the radio frequency input signal using a transceiver, and providing the envelope signal as an input to the error amplifier.

10. The method of claim 8 wherein adjusting the voltage level of the power amplifier supply voltage based on the envelope of the radio frequency input signal using the error amplifier includes providing an error current from an output of the error amplifier to the power amplifier supply voltage via an AC coupling capacitor.

11. The method of claim 8 further comprising receiving data over a serial peripheral interface, and controlling a voltage level of the reference voltage based on the data using a reference voltage generator.

12. The method of claim 8 further comprising generating the reference voltage using a reference voltage generator, and dynamically changing a voltage level of the reference voltage between transmission slots of the power amplifier.

13. A phone board comprising:

a power management integrated circuit including a DC-to-DC converter configured to control a voltage level of a power amplifier supply voltage based on a control voltage; and

a power amplifier module including a power amplifier that provides amplification to a radio frequency input signal and that receives power from the power amplifier supply voltage, the power amplifier module further including an error amplifier configured to adjust the voltage level of the power amplifier supply voltage based on an envelope of the radio frequency input signal, a low pass filter configured to low pass filter the power amplifier supply voltage to generate a filtered voltage, and a comparator configured to generate the control voltage based on comparing the filtered voltage to a reference voltage.

14. The phone board of claim 13 wherein the DC-to-DC converter includes a buck converter.

15. The phone board of claim 14 wherein the power amplifier module further includes an inductor, the buck converter including an output that is electrically connected to the power amplifier supply voltage via the inductor.

16. The phone board of claim 13 wherein the error amplifier includes an input configured to receive an envelope signal that changes in relation to the envelope of the radio frequency input signal.

17. The phone board of claim 16 further including a transceiver integrated circuit configured to generate the radio frequency input signal and the envelope signal.

18. The phone board of claim 13 wherein the power amplifier module further includes an AC coupling capacitor, the error amplifier including an output that is electrically connected to the power amplifier supply voltage via the AC coupling capacitor.

19. The phone board of claim 13 wherein the power amplifier module further includes a reference voltage generator that generates the reference voltage.

20. The phone board of claim 19 wherein the power amplifier module further includes a serial peripheral interface that is electrically connected to the reference voltage generator, the serial peripheral interface configured to receive data that controls a voltage level of the reference voltage.