Open loop LED driver system
AssigneeNational Semiconductor Corporation
InventorJohn Patrick Fairbanks
Life of this patent
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Abstract
A switching regulator circuit is arranged to provide a constant current to a load. The switching regulator circuit is operated in discontinuous current mode such that an inductor stores energy in a first part of an oscillation cycle, and discharges in a second part of the cycle. The trigger mechanism for the oscillator is disabled when the charged inductor couples energy to the load, and enabled after the inductor is detected as discharged. The energy stored in the inductor is proportional to the square of the on-time associated with switching regulator. Constant voltage load devices such as LEDs for a display can be driven by the switching regulator in an open-loop mode such that the current in the load devices is a linear function of the on-time.
Claims20
A switching regulator that is arranged to provide current to a load circuit, comprising:
- a first resistor that is coupled between a first node and a second node, wherein the first node is associated with an input voltage;
- a second resistor that is coupled between the second node and a circuit ground;
- an inductor that is coupled between the first node and a third node;
- a third resistor that is coupled between the third node and a fourth node;
- a fourth resistor that is coupled between the fourth node and the circuit ground;
- a diode that is coupled between the third node and the load circuit;
- a capacitor that is coupled in parallel with the load circuit;
- a switch circuit that is arranged to selectively couple the third node to the circuit ground in response to a pulsed gate signal;
- and
- a voltage controlled oscillator that is responsive to a frequency signal from the third node and arranged to provide a pulsed gate signal, wherein the voltage controlled oscillator is:
- disabled by a first signal from the fourth node when the first signal is above a predetermined reference level, enabled when the first signal is below the predetermined reference level, and arranged to provide the pulsed gate signal when enabled such that the switch circuit is activated for a time interval that is associated with a pulse width that is determined by a second signal from the second node.
A switching regulator that is arranged to generate an output voltage (VOUT) from an input voltage (VIN) by coupling a current (ILOAD) to a load circuit, the switching regulator comprising:
- an inductor that is coupled between a first node and a second node, wherein the first node is associated with the input voltage, wherein the inductor has a value that corresponds to L;
- a diode that is coupled between the second node and the load circuit;
- a sense means that is arranged to monitor a sense voltage from the second node;
- an oscillator means that is arranged to selectively provide a gate signal when enabled, wherein a pulse width associated with the gate signal is responsive to the input voltage (VIN);
- a switch means that is arranged to selectively couple the second node to a circuit ground when the gate signal is asserted;
- and
- a disable means that is arranged to disable the oscillator means in response to the sense voltage when the inductor is discharging until the sense voltage collapses below a predetermined reference level such that the switching regulator is operated in a discontinuous current node.
A method of generating an output voltage (VOUT) for a load circuit from an input voltage (VIN) with a switching regulator, the method comprising:
- coupling an inductor to a circuit ground such that the inductor is charged during a first cycle, wherein the inductor has a value corresponding to L;
- discharging the inductor after the first cycle by coupling energy from the inductor to the load circuit through a diode during a second cycle, wherein the second cycle is different from the first cycle;
- sensing when the inductor is discharged;
- restarting the first cycle when the inductor is determined to be discharged such that the switching regulator is operated in a discontinuous current rode;
- and
- adjusting the period associated with the first cycle in response to the input voltage (VIN) such that a load current that is provided to the load circuit is a linear function of the input voltage (VIN).
Dependencies are inferred from the claim text.