Apparatus and method for high-frequency PWM with soft-start

[0001] The invention is related to regulators, and, in particular, to an apparatus and method for a boost regulator with soft-start for high-frequency pulse-width modulation of LED current.

[0002] Light-emitting diodes (LEDs) may be used for lighting in portable electronics applications, and the like. It is often desirable to provide a mechanism for adjusting the brightness of the LED. According to a first approach, an LED may be driven with a DC voltage, and the DC voltage may be adjusted to adjust the brightness of the LED. According to a second approach, an LED may be driven with a high peak current having a low duty cycle, and the duty cycle of the current may be adjusted to adjust the brightness of the LED. The second approach consumes less power at a given level of brightness than the first approach.

[0003] White LEDs may be used for back-lighting applications. Typically, several white LEDs are coupled in series. A boost switching regulator may be employed to provide a voltage across the white LEDs.

[0013] Various embodiments of the present invention will be described in detail with reference to the drawings, where like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the invention, which is limited only by the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the claimed invention.

[0014] Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context clearly dictates otherwise. The meanings identified below are not intended to limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” includes plural reference, and the meaning of “in” includes “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. The term “coupled” means at least either a direct electrical connection between the items connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. The term “signal” means at least one current, voltage, charge, temperature, data, or other signal.

[0015] Briefly stated, the invention is related to a boost regulator circuit with soft-start that is arranged for high-frequency pulse width modulation of LED current. When a shutdown signal is asserted, all circuitry in the boost regulator circuit is disabled, except for circuitry needed for a shutdown delay timer. Also, when the shutdown signal is asserted, the shutdown delay timer begins. If the shutdown signal is de-asserted before the shutdown delay timer expires, the boost regulator circuit resumes normal operation and soft-start is not employed. If the timer expires before the shutdown signal is de-asserted, a soft-start timer is reset, and the entire boost regulator circuit is disabled. When the shutdown signal is de-asserted after the soft-start timer is reset, soft-start is employed.

[0016] FIG. 1 shows a block diagram of an embodiment of regulator circuit 100, which may include error circuit 110, pulse modulation circuit 120, switch circuit 130, shutdown delay circuit 140, and soft-start circuit 150. In one embodiment, regulator circuit 100 is a current mode boost regulator. In another embodiment, regulator circuit 100 is a voltage mode boost regulator. In one embodiment, regulator circuit 100 is an asynchronous switching regulator that employs an external diode. In another embodiment, regulator circuit 110 is a synchronous switching regulator that employs a synchronous switch (not shown).

[0017] Error circuit 110 is arranged to provide error signal Err based on feedback signal FB and reference signal Ref. In one embodiment, regulator circuit 100 is a current regulator, and signal FB is a current sense signal that is based on a regulated output current. In another embodiment, regulator circuit 100 is a voltage regulator, and signal FB is a feedback voltage that is based on a regulated output voltage. In one embodiment, both current and voltage feedback may be employed in regulator circuit 100.

[0018] Pulse modulation circuit 120 is arranged to provide switch control signal SC such that a parameter of switch control signal SC is modulated based on signal Err. Also, pulse modulation circuit 120 may be arranged to provide signal SC such that signal SC is inactive if signal SHDN is active.

[0019] Switch circuit 130 is arranged to provide signal SW. Also, switch circuit 130 is configured to open and close based on the logic level of signal SC.

[0020] Soft-start circuit 150 is configured to enable soft-starting. In one embodiment, soft-start circuit 150 is configured to enable soft-starting of signal Ref such that, if a reference value associated with signal Ref is less than a threshold value that is associated with signal Thresh, the reference value increases until the reference value is substantially equal to the threshold value. The reference value is reset if signal Reset is asserted.

[0021] In one embodiment, shutdown delay circuit 150 includes a counter circuit and a digital-to-analog converter (DAC), as described below with regard to FIG. 8. In another embodiment, soft-start circuit 150 includes an analog timer including a current source and a capacitor, as described below with regard to FIG. 5. In yet another embodiment, soft-start circuit 150 provides current to an external capacitor to implement a timer.

[0022] Shutdown delay circuit 140 is configured to enable resetting of soft-start circuit 150 if signal SHDN is asserted for an amount of time that is greater that a timer delay td. In one embodiment, shutdown delay circuit 140 asserts signal Reset if signal SHDN is asserted for an amount of time that is greater than timer delay td. In another embodiment, shutdown delay circuit 140 asserts signal T₂ if signal SHDN is asserted for an amount of time that is greater that timer delay td, and other circuitry (not shown in FIG. 1) asserts signal Reset responsive to signal T₂.

[0023] In one embodiment, shutdown delay circuit 140 includes a digital timer circuit. In another embodiment, shutdown delay circuit 140 includes an analog timer including a current source and a capacitor, as described below with regard to FIG. 5. In yet another embodiment, shutdown delay circuit 140 provides current to an external capacitor to implement a timer.

[0024] FIG. 2 illustrates a block diagram of an embodiment of LED circuit 202, which may include boost regulator 201, capacitors C_IN and C_OUT, resistor R1, and LEDs 260. Boost regulator 201 may include inductor L1, diode D1, and regulator circuit 200. Regulator circuit 100 of FIG. 1 may be employed as an embodiment of regulator circuit 200. Boost regulator 201 is arranged to regulate an output signal. In one embodiment boost regulator 201 regulates output voltage V_out. In another embodiment, boost regulator 201 regulates current I_LED.

[0025] LEDs 260 are arranged to provide light with brightness that may be adjusted by adjusting the duty signal of signal SHDN. Further, LEDs 260 are coupled in series with voltage VOUT-VFB across LEDs 260. Boost regulator 201 enables the voltage across LEDs 260 to be greater than supply voltage VIN, which may be provided by a battery, and the like.

[0026] Additionally, regulator circuit 200 may be arranged to enter a low-current state if signal SHDN is asserted. Regulator circuit 200 may be further arranged to shut down completely if signal SHDN is asserted longer than delay time td. If signal SHDN is asserted for longer than delay time td, then when signal SHDN is re-asserted, LED current I_LED and inductor current I_L are soft-started. If signal SHDN is asserted for less than delay time td, then when signal SHDN is re-asserted, soft-start is not employed. This way, high-frequency pulse width modulation may be employed for LED current I_LED.

[0027] FIG. 2 illustrates an embodiment of boost regulator 201 in which boost regulator 201 is an asynchronous, inductive-based boost regulator. However, in other embodiments, boost regulator 201 may be synchronous or asynchronous, and may be inductive-based or switched capacitor-based. Also, as previously described, embodiments of boost regulator 201 may provide current regulation, voltage regulation, or both.

[0028] FIG. 3 shows a block diagram of an embodiment of the regulator circuit 300. Components in regulator circuit 300 may operate in a substantially similar manner to similarly-named components in regulator circuit 100 of FIG. 1, and may operate in a different manner in some ways. Regulator circuit 300 further includes shutdown logic circuit 390.

[0029] In operation, shutdown delay circuit 340 asserts signal T₂ if signal SHDN is asserted for an amount of time that is greater that a timer delay td. Shutdown logic circuit 390 is arranged to provide signals SD1, SD2, and Reset based on signals SHDN and T₂. Shutdown logic circuit 390 is arranged to assert signal SD2 if signal SHDN is asserted. Also, shutdown logic circuit 390 is arranged to assert signals Reset and SD1 if signals SHDN and T₂ are asserted.

[0030] When signal SD2 is asserted, regulator circuit 300 enters a low-current mode to save power. In the low-current mode, substantially all components in regulator circuit 300, except for shutdown delay circuit 340 and any circuitry necessary for the proper operation of shutdown delay circuit 340, are disabled. If signal SD2 is asserted and signal SD1 remains unasserted, shutdown delay circuit 340 remains enabled. In one embodiment, shutdown delay circuit 340 and soft-start circuit 350 share circuitry, and soft-start circuit 350 remains enabled when shutdown delay circuit 340 is enabled. Also, in one embodiment, a bandgap reference circuit, a bias circuit, and an oscillator (not shown) remain enabled when signal SD2 is asserted and signal SD1 is unasserted.

[0031] When signal SD1 is asserted, regulator circuit 300 is fully shut down.

[0032] In the embodiment illustrated in FIG. 3, pulse modulation circuit 320 and error circuit 310 are enabled and disabled responsive to signal SD2, and soft-start circuit 350 and shutdown delay circuit 340 are enabled and disabled responsive to signal SD1.

[0033] FIG. 4 illustrates a block diagram of an embodiment of regulator circuit 400. Regulator circuit 400 is a current mode boost regulator. Components in regulator circuit 400 may operate in a similar manner to similarly-named components in regulator circuit 100, and may operate in a different manner in some ways. Error circuit 410 includes current limit comparator 411. Feedback signal FB includes current sense signal CS. Reference signal Ref includes current limit signal CL. Additionally, pulse modulation circuit 420 includes PWM control circuit 421. Switch circuit 430 includes transistor M1. Also, regulator circuit 400 further includes driver DRV1 and current sense circuit 460.

[0034] Current sense circuit 460 is arranged to sense current ISW, where current ISW is the current across transistor M1. Current sense circuit 460 may be further arranged to provide current sense signal CS such that current sense signal CS is substantially equal to current ISW times an on-resistance associated with transistor M1.

[0035] Additionally, current limit comparator circuit 411 is arranged to provide signal Err such that the logic level of signal Err is based on a comparison of current sense signal CS and current limit signal CL. PWM control circuit 421 is arranged to modulate the duty cycle of signal SC based on signal Err. Also, driver DRV1 is arranged to provide signal DRV based on signal SC for driving transistor M1.

[0036] FIG. 5 shows a block diagram of portion 505 of an embodiment of regulator circuit 500, where embodiments of the shutdown delay circuit and the soft-start circuit are shown in greater detail. Portion 505 includes timer logic 580, current limit comparator 511, shutdown logic 590, shutdown delay circuit 540, soft-start circuit 550, and transistor M2. Shutdown delay circuit 540 includes current source I2, capacitor C2, and comparator circuit Comp3. Soft-start circuit 550 includes comparator circuits Comp1 and Comp2, current source I1, capacitor C1, and transistor M3.

[0037] In operation, timer logic circuit 580 provide signals OS₁ based, in part, on signal T₁, and provides signal OS₂ based, in part, on signal T₂. In one embodiment, a high pulse is provided in signal OS₁ if signal T₁ changes to a logic high, and a high pulse is provided in signal OS₂ if signal T₂ is changed to a logic high.

[0038] Also, switch circuit S1 is arranged to open and close responsive to signal OS₁, and switch circuit S2 is arranged to open and close responsive to signal OS₂. Further, switch circuit S1 is arranged to provide current from current source I1 to capacitor C1 if the soft-start switch is closed, such that voltage V1 ramps upward if switch circuit S1 is closed. Similarly, switch circuit S2 is arranged to provide current from current source I2 to capacitor C2 if the shutdown delay switch is closed, such that voltage V2 ramps upward if switch circuit S2 is closed.

[0039] Comparator circuit Comp1 is arranged to compare voltage V1 with signal Thresh and to provide signal T₁ in response to the comparison. Similarly, comparator circuit Comp3 is arranged to compare voltage V2 to reference signal Ref2, and to provide signal T₂ based on the comparison. Comparator circuit Comp2 is arranged to operate as a buffer.

[0040] Additionally, transistor M3 is arranged to discharge capacitor C1 if signal Reset is asserted. Transistor M2 is arranged to operate as a switch that opens if signal SHDN is asserted.

[0041] FIG. 6 illustrates a block diagram of an embodiment of timer logic circuit 680. Timer logic circuit 680 may operate in a substantially similar manner as timer logic circuit 580 of FIG. 5, and may operate in a different manner in some ways. Timer logic circuit 580 includes NAND gates NAND1–3, inverters INV1–INV2, NOR gate NOR1, one-shot timer 635, and flip-flops 636.

[0042] FIG. 7 shows a diagram of an embodiment of shutdown logic circuit 790. Shutdown logic circuit 790 may operate in a substantially similar manner to shutdown logic circuit 390 of FIG. 3 and shutdown logic circuit 590 of FIG. 5, and may operate in a different manner in some ways. Signal SD2 may include signals Noff and Poff. Signal SD1 may include signals NSD and PSD.

[0043] In the embodiment shown, signal SHDN is an active low signal. Shutdown logic circuit 790 provides signals Noff and Poff based on signal SHDN. When signal SHDN is asserted (e.g. changes to logic low), signal Noff changes to a high logic level, and signal Poff changes to a low logic level. Signal Noff may be provided to the gate of n-type transistors, and signal Poff may be provided to the gate of p-type transistors.

[0044] When signal T₂ and SHDN are both asserted, signals NSD and Reset both change to a high logic level, and signal PSD changes to a low logic level. Signal NSD may be provided to the gate of n-type transistors, and signal PSD may be provided to the gate of p-type transistors.

[0045] FIG. 8 illustrates a block diagram of an embodiment of soft-start circuit 850. Soft-start circuit 850 may operate in a substantially similar manner to soft-start circuit 150 of FIG. 1, and may operate in a different manner in some ways. Soft-start circuit 850 includes counter circuit 852, DAC 854, and comparator circuit Comp4.

[0046] In operation, comparator circuit Comp4 compares signals Ref and Thresh, and provides signal Comp_out in response to the comparison. If the reference value is less than the threshold value, signal Comp_out corresponds to a first logic level, and if the reference value is greater than the threshold value, signal Comp_out corresponds to a second logic level.

[0047] Also, counter circuit 852 is arranged to increment a count value if a triggering edge of signal CLK2 is received and signal Comp₁₃ out corresponds to the first logic level. Further, counter circuit 852 is arranged to reset the count value (e.g. to zero) if signal Reset is asserted. Counter circuit 852 is also arranged to provide signal Count such that signal Count corresponds to the count value. Also, DAC 854 is arranged to perform an analog-to-digital conversion to provide signal Ref from signal Count.

[0048] The above specification, examples and data provide a description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention also resides in the claims hereinafter appended.