PFM scheme for boost and flyback converter in LED backlight application

Patent metadata

Field	Value
Publication number	US-10009968-B2
Application number	US-201615069724-A
Country	US
Kind code	B2
Filing date	Mar 14, 2016
Priority date	May 22, 2015
Publication date	Jun 26, 2018
Grant date	Jun 26, 2018

Abstract

This application relates to systems, methods, and apparatus for controlling a switching frequency of a boost or flyback converter to be above an audible frequency range when operating the boost or flyback converter in a pulse frequency modulation (PFM) mode. The boost or flyback converter uses one or more switches for converting power for a display panel. In order to boost the switching frequency when operating in the PFM mode, the boost or flyback converter can selectively implement certain current and/or voltage limits for pulses that are generated as a result of the switching. The current and/or voltage limits can be set according to a load of the boost or flyback converter, and a correspondence between the current and/or voltage limits and the loads can be stored in a lookup table accessible to the boost or flyback converter.

First claim

What is claimed is:

1. A boost converter for a display panel, the boost converter comprising:
a logic circuit configured to switch the boost converter into a pulse width modulation (PWM) mode when a load of the boost converter is above a load threshold and a pulse frequency modulation mode (PFM) mode when the load of the boost converter is equal to or below the load threshold; and
a PFM controller configured to monitor a current output of the boost converter and limit the current output to one of a plurality of predetermined current limits for the PFM mode, each of the predetermined current limits associated with a load range of the boost converter and each configured to maintain the current output for the associated load range to a current associated with a switching frequency above a predetermined frequency threshold, wherein the predetermined frequency threshold is greater than or equal to 20 kilohertz.

2. The boost converter of claim 1 , wherein the PFM controller is further configured to select the one of the plurality of predetermined current limits according to a brightness input to the boost converter.

3. The boost converter of claim 1 , wherein each of the plurality of predetermined current limits comprises an upper current limit stored at the boost converter.

4. The boost converter of claim 1 , wherein each of the predetermined current limits is a range of current values.

5. The boost converter of claim 1 , wherein the PFM controller is configured to select a variable voltage output or static output voltage for the boost converter.

6. The boost converter of claim 1 , wherein the PFM controller is configured to disconnect an oscillator of the boost converter when the load of the boost converter is equal to or below the load threshold.

7. A method for operating a boost converter above a predetermined frequency, the method comprising:
by the boost converter:
switching from a pulse width modulation mode (PWM) to a pulse frequency modulation (PFM) mode when a load of the boost converter is below a predetermined load threshold;
outputting a series of pulses from the boost converter; and
throttling the series of pulses when the series of pulses cause a voltage output of the boost converter to reach a voltage threshold, wherein the voltage threshold is configured to maintain a frequency of the series of pulses equal to or above 20 kilohertz.

8. The method of claim 7 , wherein a voltage output of the boost converter is throttled when the series of pulses corresponds to an upper voltage limit of the voltage threshold.

9. The method of claim 8 , further comprising:
comparing a sensed inductor current to a current limit accessible to the boost converter.

10. The method of claim 9 , further comprising:
selecting the current limit from a plurality of current limits accessible to the boost converter, wherein the current limit is selected based on a correspondence between the current limit and the load of the boost converter.

11. The method of claim 10 , wherein the correspondence between the current limit and the load of the boost converter is embodied as a lookup table that is accessible to the boost converter.

12. The method of claim 7 , wherein switching from the PWM mode to the PFM mode includes disabling an oscillator of the boost converter.

13. The method of claim 7 , further comprising:
selecting the voltage threshold from a plurality of voltage thresholds accessible to the boost converter.

Assignees

• Apple Inc

Inventors

• Chen Jingdong
• Hussain Asif
• Mohtashemi Behzad
• Pandya Manisha P
• Navabi-Shirazi Mohammad J

Classifications

• G09G3/3406 Primary

  Control of illumination source (illumination devices structurally associated with liquid crystal cells G02F1/1336) · CPC title

• G09G2330/06

  Handling electromagnetic interferences [EMI], covering emitted as well as received electromagnetic radiation · CPC title

• H05B33/0815 Primary

  Electricity · mapped topic

• H05B33/086

  Electricity · mapped topic

• H05B45/385

  using flyback topology · CPC title