Dynamic load current adjustment to avoid PFM frequency in audio range
In an embodiment of the techniques presented herein, a universal serial bus power delivery (USB-PD) power adaptor includes a USB port, and a USB controller configured to deliver power to the USB port, wherein the USB controller includes a voltage regulator configured to generate a power supply voltage based on a pulse frequency modulation (PFM) signal, and a variable current bleed unit configured to generate a variable current bleed load on the voltage regulator based on a frequency of the PFM signal to maintain the frequency of the PFM signal above an audible frequency range.
1 . A universal serial bus power delivery (USB-PD) power adaptor comprising:
a USB port; and
a USB controller configured to deliver power to the USB port, wherein the USB controller comprises:
a voltage regulator configured to generate a power supply voltage dependent on a load current of the USB controller based on a pulse frequency modulation (PFM) signal having a switching frequency; and
a variable current bleed unit comprising a control unit configured to generate a variable current bleed load on the voltage regulator based on the switching frequency and an audible frequency threshold to add a bleed current not dependent on the load current of the USB controller to the load current to maintain a frequency of the PFM signal above an audible frequency range.
2 . The USB-PD power adaptor of claim 1 , wherein:
the variable current bleed unit comprises a digital variable current bleed unit.
3 . The USB-PD power adaptor of claim 2 , wherein the control unit in the digital variable current bleed unit comprises:
a frequency calculator configured to determine the frequency of the PFM signal;
a current digital to analog converter configured to generate the variable current bleed load based on a current code; and
a controller configured to generate the current code based on the frequency of the PFM signal.
4 . The USB-PD power adaptor of claim 1 , wherein:
the variable current bleed unit comprises an analog variable current bleed unit.
5 . The USB-PD power adaptor of claim 4 , wherein the control unit in the analog variable current bleed unit comprises:
a frequency-to-voltage converter configured to generate a control voltage as a function of the frequency of the PFM signal; and
a voltage-to-current converter configured to generate the variable current bleed load based on the control voltage.
6 . The USB-PD power adaptor of claim 5 , wherein the frequency-to-voltage converter comprises:
a pulse generator configured to generate a pulse based on a time interval between PFM pulses in the PFM signal;
a rising edge reset unit configured to generate a reset signal for the pulse generator to terminate the pulse based on a rising edge of the PFM signal; and
a filter configured to generate the control voltage based on the pulse.
7 . The USB-PD power adaptor of claim 6 , wherein the pulse generator comprises:
a current source;
a capacitor connected to the current source;
a switch controlled by the reset signal to discharge the capacitor; and
a comparator configured to generate the pulse based on a reference voltage and a voltage on the capacitor.
8 . The USB-PD power adaptor of claim 5 , wherein the voltage-to-current converter comprises a transconductance amplifier.
9 . A method for operating a universal serial bus power delivery (USB-PD) power adaptor comprising:
delivering power to a USB port using a USB controller;
generating a power supply voltage dependent on a load current of the USB controller in a voltage regulator based on a pulse frequency modulation (PFM) signal having a switching frequency; and
generating a variable current bleed load on the voltage regulator based on the switching frequency and an audible frequency threshold to add a bleed current not dependent on the load current of the USB controller to the load current to maintain a frequency of the PFM signal above an audible frequency range.
10 . The method of claim 9 , wherein generating the variable current bleed load comprises:
configuring a digital variable current bleed unit based on the frequency of the PFM signal.
11 . The method of claim 10 , wherein configuring the digital variable current bleed unit comprises:
determining the frequency of the PFM signal in a frequency calculator;
generating the variable current bleed load in a current digital to analog converter based on a current code; and
generating the current code based on the frequency of the PFM signal.
12 . The method of claim 9 , wherein generating the variable current bleed load comprises:
configuring an analog variable current bleed unit based on the frequency of the PFM signal.
13 . The method of claim 12 , wherein configuring the analog variable current bleed unit comprises:
generating a control voltage in a frequency-to-voltage converter as a function of the frequency of the PFM signal; and
generating the variable current bleed load in a voltage-to-current converter based on the control voltage.
14 . The method of claim 13 , wherein generating the control voltage in the frequency-to-voltage converter comprises:
generating a pulse in a pulse generator based on a time interval between PFM pulses in the PFM signal;
generating a reset signal for the pulse generator based on a rising edge of the PFM signal to terminate the pulse; and
generating the control voltage based on the pulse.
15 . The method of claim 14 , wherein generating the pulse in the pulse generator comprises:
charging a capacitor with a current source;
generating the pulse based on a reference voltage and a voltage on the capacitor; and
controlling a switch based on the reset signal to discharge the capacitor to terminate the pulse.
16 . The method of claim 13 , wherein generating the variable current bleed load in the voltage-to-current converter comprises:
generating the variable current bleed load in a transconductance amplifier.
17 . A universal serial bus power delivery (USB-PD) power adaptor comprising:
a USB port;
a power switch connected between a voltage input terminal and the USB port; and
a USB controller configured to control the power switch to deliver power to the USB port from the voltage input terminal, wherein the USB controller comprises:
a voltage regulator configured to generate a power supply voltage dependent on a load current of the USB controller based on a pulse frequency modulation (PFM) signal having a switching frequency; and
a variable current bleed unit comprising a control unit configured to generate a variable current bleed load on the voltage regulator based on the switching frequency and an audible frequency threshold to add a bleed current not dependent on the load current of the USB controller to the load current to maintain the frequency of a PFM signal above an audible frequency range.
18 . The USB-PD power adaptor of claim 17 , wherein the variable current bleed unit comprises a digital variable current bleed unit, and the control unit comprises:
a frequency calculator configured to determine the frequency of the PFM signal;
a current digital to analog converter configured to generate the variable current bleed load based on a current code; and
a controller configured to generate the current code based on the frequency of the PFM signal.
19 . The USB-PD power adaptor of claim 17 , wherein the variable current bleed unit comprises an analog variable current bleed unit, and the control unit comprises:
a frequency-to-voltage converter configured to generate a control voltage as a function of the frequency of the PFM signal; and
a voltage-to-current converter configured to generate the variable current bleed load based on the control voltage.
20 . The USB-PD power adaptor of claim 19 , wherein:
the frequency-to-voltage converter comprises:
a pulse generator configured to generate a pulse based on a time interval between PFM pulses in the PFM signal;
a rising edge reset unit configured to generate a reset signal for the pulse generator to terminate the pulse based on a rising edge of the PFM signal; and
a filter configured to generate the control voltage based on the pulse; and
the voltage-to-current converter comprises a transconductance amplifier.