Current drive circuit for light emitting diode

• US 8,492,982 B2
• Filed: 04/20/2011
• Issued: 07/23/2013
• Est. Priority Date: 04/23/2010
• Status: Active Grant

• Current Assignee: ROHM Co., Ltd.
• Original Assignee: ROHM Co., Ltd.
• Inventors: Hagino, Junichi, Haruta, Shingo
• Primary Examiner(s): Chang, Daniel D
• Application Number:
  US13/090,357
• Publication Number:
  US 20110279043A1
• Time in Patent Office: 825 Days
• Field of Search: 315/185.R, 315/192, 315/191, 315/193, 315/315, 315/317
• US Class Current: 315/185.R
• CPC Class Codes: G09G 2320/064 by time modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2330/02 Details of power systems and of start or stop of display operation
• CPC Class Codes: G09G 3/342 using several illumination sources separately controlled corresponding to different display panel areas, e.g. along one dimension such as lines
• CPC Class Codes: H05B 45/38 using boost topology
• CPC Class Codes: H05B 45/382 with galvanic isolation between input and output
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines

Abstract

A pulse width modulated dimming pulse signal PWM is input to a control input terminal P5. A standby terminal P6 receives a standby signal STB that indicates switching between a standby state and an operating state of a current drive circuit 8. A burst dimming terminal BS is provided to each of eight respective channels. Each burst dimming terminal BS receives a voltage at one terminal (cathode) of a corresponding LED string 6. When the voltage level of the standby signal STB is included in a first voltage range, a burst controller 9 is set to an all channel common mode, and when it is included in a second voltage range, the mode is set to a phase shift mode φ_SHIFT. The burst controller 9 set to the phase shift mode φ_SHIFT automatically sets the phase shift angle according to the number of connected LED strings 6.

Claims

1. A current drive circuit configured to allow a maximum of eight channels of light emitting diode strings to be connected, and to drive the light emitting diode strings thus connected, the current drive circuit comprising:

a control input terminal configured to receive a pulse width modulated dimming pulse signal;

a standby terminal configured to receive a standby signal which is an instruction to switch the state of the current drive circuit between a standby state and an operating state;

eight burst dimming terminals respectively provided to the channels, and each configured to receive a voltage that occurs at one terminal of the corresponding diode string; and

a controller configured such that, when the voltage level of the standby signal is included in a first voltage range, the operating mode is set to an all channel common mode in which the light emitting diode strings of the respective channels are each driven using a corresponding driving current having the same phase, and when the voltage level of the standby signal is included in a second voltage range, the mode is set to a phase shift mode in which the light emitting diode strings of the respective channels are each driven using a corresponding driving current having a shifted phase,

wherein the controller set to the phase shift mode performs a driving operation such that, when the electric potentials at the burst dimming terminals of the fifth through eighth channels are all lower than a predetermined second threshold voltage in a judgment period, the mode is set to a 90-degree phase shift mode in which the light emitting diode strings of the first through fourth channels are driven in a driving period after the judgment period using respective driving currents the phases of which are shifted from one another by ¼

the period of the dimming pulse signal, and, when the electric potentials at the burst dimming terminals of the seventh and eighth channels are all lower than the predetermined second threshold voltage in the judgment period, the mode is set to a 60-degree phase shift mode in which the light emitting diode strings of the first through sixth channels are driven in the driving period after the judgment period using respective driving currents the phases of which are shifted from one another by ⅙

the period of the dimming pulse signal, and, in cases other than the foregoing, the mode is set to a 45-degree phase shift mode in which the light emitting diode strings of the first through eighth channels are driven in the driving period after the judgment period using respective driving currents the phases of which are shifted from one another by ⅛

the period of the dimming pulse signal.

2. A light emitting apparatus comprising:

at least one light emitting diode string;

a switching power supply configured to supply a driving voltage to the aforementioned at least one light emitting diode string; and

a current drive circuit configured to control the driving current that flows through the aforementioned at least one light emitting diode string, wherein the current drive circuit is configured to allow a maximum of eight channels of light emitting diode strings to be connected, and to drive the light emitting diode strings thus connected, and wherein the current drive circuit comprises;

a control input terminal configured to receive a pulse width modulated dimming pulse signal;

a standby terminal configured to receive a standby signal which is an instruction to switch the state of the current drive circuit between a standby state and an operating state;

eight burst dimming terminals respectively provided to the channels, and each configured to receive a voltage that occurs at one terminal of the corresponding diode string; and

a controller configured such that, when the voltage level of the standby signal is included in a first voltage range, the operating mode is set to an all channel common mode in which the light emitting diode strings of the respective channels are each driven using a corresponding driving current having the same phase, and when the voltage level of the standby signal is included in a second voltage range, the mode is set to a phase shift mode in which the light emitting diode strings of the respective channels are each driven using a corresponding driving current having a shifted phase,

wherein the controller set to the phase shift mode performs a driving operation such that, when the electric potentials at the burst dimming terminals of the fifth through eighth channels are all lower than a predetermined second threshold voltage in a judgment period, the mode is set to a 90-degree phase shift mode in which the light emitting diode strings of the first through fourth channels are driven in a driving period after the judgment period using respective driving currents the phases of which are shifted from one another by ¼

the period of the dimming pulse signal, and, when the electric potentials at the burst dimming terminals of the seventh and eighth channels are all lower than the predetermined second threshold voltage in the judgment period, the mode is set to a 60-degree phase shift mode in which the light emitting diode strings of the first through sixth channels are driven in the driving period after the judgment period using respective driving currents the phases of which are shifted from one another by ⅙

the period of the dimming pulse signal, and, in cases other than the foregoing, the mode is set to a 45-degree phase shift mode in which the light emitting diode strings of the first through eighth channels are driven in the driving period after the judgment period using respective driving currents the phases of which are shifted from one another by ⅛

the period of the dimming pulse signal.

3. An electronic device comprising:

a liquid crystal panel; and

a light emitting apparatus arranged as a backlight of the liquid crystal panel, the light emitting apparatus comprising;

at least one light emitting diode string;

a switching power supply configured to supply a driving voltage to the aforementioned at least one light emitting diode string; and

a current drive circuit configured to control the driving current that flows through the aforementioned at least one light emitting diode string, wherein the current drive circuit is configured to allow a maximum of eight channels of light emitting diode strings to be connected, and to drive the light emitting diode strings thus connected, and wherein the current drive circuit comprises;

a control input terminal configured to receive a pulse width modulated dimming pulse signal;

a standby terminal configured to receive a standby signal which is an instruction to switch the state of the current drive circuit between a standby state and an operating state;

eight burst dimming terminals respectively provided to the channels, and each configured to receive a voltage that occurs at one terminal of the corresponding diode string; and

a controller configured such that, when the voltage level of the standby signal is included in a first voltage range, the operating mode is set to an all channel common mode in which the light emitting diode strings of the respective channels are each driven using a corresponding driving current having the same phase, and when the voltage level of the standby signal is included in a second voltage range, the mode is set to a phase shift mode in which the light emitting diode strings of the respective channels are each driven using a corresponding driving current having a shifted phase,

wherein the controller set to the phase shift mode performs a driving operation such that, when the electric potentials at the burst dimming terminals of the fifth through eighth channels are all lower than a predetermined second threshold voltage in a judgment period, the mode is set to a 90-degree phase shift mode in which the light emitting diode strings of the first through fourth channels are driven in a driving period after the judgment period using respective driving currents the phases of which are shifted from one another by ¼

the period of the dimming pulse signal, and, when the electric potentials at the burst dimming terminals of the seventh and eighth channels are all lower than the predetermined second threshold voltage in the judgment period, the mode is set to a 60-degree phase shift mode in which the light emitting diode strings of the first through sixth channels are driven in the driving period after the judgment period using respective driving currents the phases of which are shifted from one another by ⅙

the period of the dimming pulse signal, and, in cases other than the foregoing, the mode is set to a 45-degree phase shift mode in which the light emitting diode strings of the first through eighth channels are driven in the driving period after the judgment period using respective driving currents the phases of which are shifted from one another by ⅛

the period of the dimming pulse signal.

4. A driving mode setting method for a maximum of eight channels of light emitting diode strings, the setting method comprising:

setting the mode, when the voltage level of a standby signal that indicates switching between a standby state and an operating state is included in a predetermined first voltage range, to an all channel common mode in which the light emitting diode strings of the respective channels are each driven using a corresponding driving current having the same phase; and

setting the mode, when the voltage level of the standby signal is included in a second voltage range, to a phase shift mode in which the light emitting diode strings of the respective channels are each driven using a corresponding driving current having a shifted phase,

wherein setting the mode to the phase shift mode comprises;

setting the mode, when judgment is made in a judgment period that no light emitting diode string is connected to the fifth through eighth channels, to a 90-degree phase shift mode in which the light emitting diode strings of the first through fourth channels are driven using respective driving currents the phases of which are shifted from one another by ¼

the period of the dimming pulse signal;

setting the mode, when judgment is made in the judgment period that no light emitting diode string is connected to the seventh and eighth channels, to a 60-degree phase shift mode in which the light emitting diode strings of the first through sixth channels are driven using respective driving currents the phases of which are shifted from one another by ⅙

the period of the dimming pulse signal; and

setting the mode, in cases other than the foregoing, to a 45-degree phase shift mode in which the light emitting diode strings of the first through eighth channels are driven using respective driving currents the phases of which are shifted from one

another by ⅛

the period of the dimming pulse signal.