VOLTAGE / CURRENT REGULATOR FOR CONTROLLED CURRENT SUPPLY WITH PVT-ADJUSTED POWER RESERVE

Description

BACKGROUND

[0001] Touch screen smart phones and tablets commonly use white-LED (WLED) backlighting power supplies. An example is an inductor-based backlighting driver, which is a type of switching power supply.

[0002] Because -40% of a tablet's power consumption is used in the WLED backlight driver and the LEDs, optimizing system efficiency to increase battery life is advantageous.

[0003] WLED driver losses fall into three categories, which are: external component losses, boost losses and LED losses. The LED losses are a function of LED current and LED headroom. Significant effort has been spent to minimize headroom voltage, while still ensuring that the LED driver has sufficient headroom to supply the expected current. US 2012 / 126712 A1 discloses an LED driving circuit and a display having the same. US 2012 / 280632 A1 discloses an LED driving apparatus and a method for driving the same. US 2012 / 0081016 A1 discloses a nLED driver with adaptive dynamic headroom voltage control.SUMMARY

[0004] The present invention provides an LED driver circuit according to claim 1 and a method adaptable for controlling illumination of an LED backlight system according to claim 9. Embodiments of the apparatus and the method are defined in the dependent claims. In described examples of controlling illumination of an LED backlight system that includes multiple strings of LEDs, the methodology includes controlling a power supply to regulate string voltage VOUT, and includes: (a) controlling ILED string current through a current control transistor MLED and a sense resistor RSENSE, including generating an MLED gate control signal based on comparing a reference voltage VREF to a sense voltage VSENSE from RSENSE, such that ILED corresponds to VREF / RSENSE; and (b) controlling the power supply to regulate VOUT to supply the controlled ILED with a controlled headroom voltage VHDRM, including adjusting VHDRM for PVT operating conditions based on a reference voltage PVT REF.

[0005] The methodology further includes generating the reference voltage PVT REF by: (a) generating a reference current ILED / RATIO proportional to the ILED string current based on comparing VREF to a replica sense voltage R VSENSE using (i) a replica current control transistor MLED / RATIO that is a replica of MLED, and (ii) a replica resistor RSENSE*RATIO that is a replica of RSENSE, such that (iii) the reference current ILED / RATIO is sourced by MLED / RATIO through RSENSE*RATIO generating R VSENSE; and then (b) generating the reference voltage PVT_REF by (i) mirroring ILED / RATIO through a PVT_MLED / RATIO transistor that is a replica of MLED, and a PVT_RSENSE*RATIO resistor that is a replica of RSENSE, and (ii) generating PVT_REF based on the saturation voltage of PVT_MLED / RATIO, corresponding to a VOUT string voltage that can supply the ILED string current, accounting for PVT operating conditions.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 illustrates an example embodiment of an LED backlight system for controlling illumination from multiple LED strings, including an LED driver and a boost power supply, the LED driver controlling ILED string current and VOUT string voltage, including dynamically adjusting string voltage headroom for PVT operating conditions. FIG. 2 functionally illustrates an LED backlight system including an LED driver represented by an LED string and ILED string driver that includes an NMOS current control transistor (MLED) that controls ILED string current (gate drive amplifier based on a reference voltage VREF), with the LED driver providing boost control for regulated string voltage VOUT, including headroom voltage VHDRM. FIG. 3 illustrates an example embodiment of an LED driver that controls ILED string current, and incorporates PVT head room control for dynamically adjusting string voltage headroom for PVT operating conditions, based on generating a reference / replica string current ILED / RATIO with a replica MLED / RATIO transistor (replicating the ILED current control transistor MLED), which is mirrored through a second replica MLED / RATIO transistor that saturates at a reference voltage PVT_REF corresponding to a minimum voltage that can supply the required ILED string current (as represented by the reference current ILED / RATIO), accounting for PVT operating conditions. FIG. 4 illustrates an example alternate embodiment of a PVT headroom circuit configured with PMOS replica transistors. DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0007] Example embodiments are directed to voltage / current regulators supplying a controlled current to a load, and generally to the problem of minimizing voltage headroom supplied by the voltage / current regulator in supplying the controlled current. An example application is LED backlight illumination, including controlling string voltage to minimize string voltage headroom in supplying a controlled ILED string current. Example embodiments include apparatus and methods suitable for supplying a controlled current based on a regulated voltage with headroom adjusted for PVT operating conditions.

[0008] Example embodiments and system applications illustrate various features and advantages of a voltage / current regulator supplying controlled current with PVT adjusted headroom. Headroom is dynamically adjusted for PVT operating conditions.

[0009] Example embodiments are implemented as an LED driver circuit suitable for use in an LED backlight system, controlling ILED current through multiple LED strings, including controlling string voltage VOUT to supply ILED with voltage headroom VHDRM.

[0010] In brief overview, for the example application, an LED backlight driver controls ILED string current, and controls a power supply (boost regulator) to regulate VOUT string voltage to supply the ILED string current with sufficient headroom voltage VHDRM, adjusted for PVT operating conditions. The LED driver includes PVT headroom circuitry that generates a replica / reference current ILED / RATIO (proportional to ILED string current), which is mirrored to a replica MLED / RATIO transistor (replicating the MLED current control transistor used for controlling ILED) that saturates at a PVT_REF reference voltage corresponding to a minimum voltage that can supply the required ILED current, accounting for PVT operating conditions.

[0011] In example embodiments, an LED driver controls ILED current through an LED string, including controlling a power supply to regulate string voltage VOUT. The LED driver includes: (a) ILED control circuitry configured to control ILED string current; and (b) PVT headroom circuitry configured to generate reference voltage PVT_REF to adjust VHDRM for PVT operating conditions. VOUT control circuitry is configured to provide VOUT_control to the power supply to regulate VOUT to supply the controlled ILED with a controlled headroom voltage VHDRM, including adjusting VHDRM for PVT operating conditions.

[0012] In example embodiments, the ILED control circuitry is configured to control ILED string current, and can include: (a) a current control transistor MLED that controls ILED based on an MLED gate control signal; (b) a sense resistor RSENSE that generates a sense voltage VSENSE based on ILED through the current control transistor; and (c) an MLED gate control amplifier that compares a reference voltage VREF to VSENSE, and generates the MLED gate control signal, such that ILED corresponds to VREF / RSENSE.

[0013] In example embodiments, the PVT headroom circuitry is configured to generate the reference voltage PVT_REF, and can include: (a) an ILED reference circuit that generates an ILED reference current ILED / RATIO; and (b) a PVT reference circuit configured to generate reference voltage PVT_REF. The ILED reference circuit can include: (i) a replica current control transistor MLED / RATIO that is a replica of MLED, (ii) a replica resistor RSENSE*RATIO that is a replica of RSENSE, (iii) a replica gate control amplifier configured to generate a replica gate control signal by comparing VREF to a replica sense voltage R_VSENSE from RSENSE*RATIO, and (iv) in response to the replica gate control signal, the replica MLED / RATIO sources a reference current ILED / RATIO proportional to the ILED string current through RSENSE*RATIO generating R_VSENSE. The PVT reference circuit can include: (i) a PVT_MLED / RATIO transistor that is a replica of MLED; (ii) a PVT_RSENSE*RATIO resistor that is a replica of RSENSE; and (iii) a current mirror that mirrors the ILED / RATIO current through PVT_MLED / RATIO and PVT_RSENSE*RATIO. The PVT reference circuit is configured, such that PVT_MLED / RATIO saturates at the reference voltage PVT_REF corresponding to a VOUT string voltage that can supply the ILED string current, accounting for PVT operating conditions.

[0014] FIG. 1 illustrates an example embodiment of an LED backlight system 10 that includes a boost power supply 50 and an LED backlight driver 100. LED driver 100 controls ILED string current through LED strings 150, including providing boost control for string voltage VOUT.

[0015] As described in connection with FIG. 3, the example LED backlight driver 100 incorporates headroom control to dynamically adjust string voltage headroom for PVT operating conditions.

[0016] LED backlight system 10 is illustrated with six LED driver channels for six LED strings LED1-LED6. For example, each LED string can have eight LEDs, each with a forward voltage of approximately 3V, resulting in a VOUT string voltage of around 25V.

[0017] Backlight system 10 uses a boost converter 50 to provide LED string voltage VOUT. As illustrated, boost converter 50 includes an external boost inductor L1 and flyback diode D1. Boost converter 50 is coupled to the LED string load 150 through an output filter capacitor Cout.

[0018] LED driver 100 incorporates a boost controller that controls the power transfer switch node SW in boost converter 50. LED driver 100 controls boost VOUT to ensure that the LED drivers (ILED string driver circuits) have sufficient headroom to supply the required ILED string current. A boost controller includes boost headroom control that monitors / senses LED string headroom voltage VHDRM on each of the LED channel pins LED1-LED6. The boost controller controls VOUT to maintain the required VHDRM above an internal reference.

[0019] FIG. 2 illustrates elements of the example LED backlight system 10, with the LED backlight system represented by LED string 251. For each LED string, including LED string 251, the LED Driver (FIG. 1, 100) includes an ILED string driver 201. ILED string driver 201 is coupled to the bottom of LED string 251, at an associated LED channel pin 201_1.

[0020] ILED string driver 201 includes a gate drive amplifier 211 controlling a current control transistor (MLED) 221 (NMOS), and a sense resistor Rsense. Boost converter 50 is controlled to supply string voltage VOUT sufficient to supply a controlled string current ILED through LED string 251 with sufficient headroom voltage VHDRM at the LED channel pin 201_1.

[0021] Gate drive amplifier 211 and MLED 221 are configured as a source follower, regulating ILED string current through MLED 221 by comparing the voltage Vsense at the top of Rsense to a reference voltage VREF. ILED through the LEDs is therefore VREF / Rsense.

[0022] As the voltage on Rsense increases, ILED becomes easier to regulate accurately, but requires a higher headroom voltage on the LED string / channel pin (VHDRM) to ensure correct ILED string current. A typical ILED string current range is 0.1mA through 25mA.

[0023] Referring also to FIG. 1, the boost controller in LED driver 100 (FIG. 1) monitors the VHDRM voltage at the LED string / channel pins, and regulates the boost voltage VOUT to keep LED pin voltage above VREF (based on gate control amplifier comparing VREF to Vsense). The reference voltage VREF can be selected to ensure proper headroom requirements over operating process, voltage and temperature (PVT), for the required ILED string current (such as 800mV).

[0024] Headroom control is useful to adjust VHDRM (by regulating VOUT) based on ILED string current requirements. For example, maximum headroom (such as 800mV) can be specified for a maximum ILED current based on slow process, low voltage and high temperature, and then headroom can be adjusted lower for lower ILED operating currents. A maximum headroom approach enables backlight driver 100 to operate over specified temperature and voltage ranges, and over process variations, so that LED string headroom is set higher to account for these variations. However, under "nominal" PVT conditions, the required headroom can be significantly less than a maximum, worst case design constraint (such as < 300mV).

[0025] FIG. 3 illustrates an example embodiment of an LED driver 300 that controls ILED string current through LED strings LED1-LED4, and incorporates PVT head room circuitry 350 to dynamically adjust string voltage headroom (bottom of the LED string) for PVT operating conditions.

[0026] For each of the LED1-LED4 strings, an ILED string driver controls ILED string current. The ILED string drivers include respective gate control amplifiers 311-314 and string current control transistors (MLED) 321-323 (NMOS), together with associated sense resistors, and function to control ILED string current as described in connection with FIG. 2 (based on a reference voltage VREF).

[0027] PVT headroom circuitry 350 in effect replicates the ILED string drivers, including generating a reference current ILED / RATIO (based on VREF), and outputs a PVT_REF voltage (based on the ILED / RATIO) used by the boost controller in regulating VOUT to adjust string voltage headroom VHDRM to account for PVT operating conditions.

[0028] PVT headroom circuitry 350 includes a replica gate control amplifier 351 that replicates the gate control amplifiers 311-314, and is referenced to the VREF reference voltage. PVT headroom circuitry 350 includes an ILED reference circuit 352 that generates a reference / replica current ILED / RATIO proportional to the string ILED current (based on VREF) by the RATIO parameter, and a PVT reference circuit 356 that generates (based on ILED / RATIO) a PVT_REF reference voltage that tracks PVT.

[0029] Accordingly, the reference voltage PVT_REF tracks variations in process, voltage and temperature, and tracks ILED variation. PVT reference circuit 356 measures the minimum headroom voltage required to ensure accurate ILED current regulation based on PVT operating conditions.

[0030] ILED reference circuit 352 includes a replica current control transistor (MLED / RATIO) 353 that replicates the current control MLEDs 321-324, and a replica sense resistor (RSENSE*RATIO). Replica gate control amplifier 351 provides gate control to MLED / RATIO 353 based on a comparison of VREF and a sense voltage R_VSENSE from RSENSE*RATIO, sourcing the reference / replica current ILED / RATIO, represented by current source 354. The reference / replica current ILED / RATIO is mirrored to PVT reference circuit 356.

[0031] PVT reference voltage circuit 356 includes a replica current control transistor MLED / RATIO 357 that replicates the current control MLEDs 321-324, and a replica sense resistor RSENSE*RATIO. The reference current ILED / RATIO from replica ILED circuit 352 is mirrored to PVT reference circuit 356, as represented by current source 358. The mirrored ILED / RATIO reference / replica current is sourced through replica MLED / RATIO 357 and RSENSE*RATIO, and generates the reference voltage PVT_REF.

[0032] MLED / RATIO 357 and RSENSE*RATIO are designed, such that MLED / RATIO 357 saturates at a minimum voltage (PVT_REF) that can supply the required ILED current (as represented by the reference / replica current ILED / RATIO), accounting for PVT operating conditions. Accordingly, the PVT_REF (saturation) voltage generated by PVT reference voltage circuit 356 tracks PVT.

[0033] The PVT_REF reference voltage from the PVT reference circuit 356 (PVT headroom circuit 350), corresponding to the saturation voltage for MLED / RATIO 357, is provided to headroom detection circuitry 305. Headroom detection circuitry 305 receives the VHDRM headroom voltages from the LED strings LED1-LED4, supplying headroom information to the boost controller (FIG. 1, LED driver 100). Based on the PVT_REF reference voltage from PVT headroom circuit 350, the boost converter (FIG. 1, 50) regulates VOUT to supply the required VHDRM, adjusted for PVT operating conditions. Accordingly, VHDRM is adjusted in response to PVT_REF to track PVT, but remaining high enough for the required ILED string current.

[0034] FIG. 4 illustrates an example embodiment of a PVT headroom circuit 450 configured with PMOS replica transistors. PVT headroom circuit 450 includes ILED reference circuit 452 with PMOS MLED / RATIO 453, and PVT reference circuit 456 with PMOS MLED / RATIO 457. As with the NMOS embodiment of FIG. 3, PVT headroom circuit outputs a PVT_REF reference voltage that corresponds to the saturation voltage for MLED / RATIO 457 (based on the reference current ILED / RATIO generated with PMOS MLED / RATIO 453 and mirrored to the PVT reference circuit 456.

[0035] Advantages of the LED driver with PVT adjusted LED string headroom voltage (adjusted for PVT operating conditions) include improved system efficiency and lower power losses, resulting in longer battery life. Headroom control is adapted for process, supply voltage and temperature. Accounting for PVT variation can result in additional headroom adjustment in the range of 500mV, which can result in system savings in the range of 2% (a reduction in losses in the range of 16-20+%). Additional embodiments / applications for voltage / current regulation with PVT_adjusted headroom control include battery charging.

[0036] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.

Claims

1. An LED driver circuit suitable for use in controlling a string current through an LED string, including controlling a power supply to regulate a string voltage (VOUT), comprising: string current control circuitry configured to control the string current, including: a current control transistor (321-323) configured to control the string current based on a gate control signal thereof; a boost converter (50); a sense resistor to generate a sense voltage based on the string current through the current 10 control transistor (321-323); and a gate control amplifier (311-314) configured to compare a reference voltage (VREF) to the sense voltage and generate the gate control signal, such that the string current corresponds to a ratio of the reference voltage to a resistance of the sense resistor (RSENSE); string voltage control circuitry configured to provide string voltage control to the power 15 supply to regulate the string voltage (VOUT) to supply a controlled string current with a controlled headroom voltage, including adjusting a headroom voltage for process, voltage and temperature operating conditions based on a reference voltage (VREF), the string voltage control circuitry including a boost controller; and headroom circuitry (350) configured to generate a further reference voltage (PVT_REF), 20 including: a string current reference circuit (352) to generate a reference string current (ILED / RATIO), including: a replica current control transistor (353) that is a replica of the current control transistor (321-323); 25 a replica sense resistor that is a replica of the sense resistor; and a replica gate control amplifier (351) configured to generate a replica gate control signal by comparing the reference voltage (VREF) to a replica sense voltage from the replica sense resistor; wherein, in response to the replica gate control signal, the replica current control transistor (353) is configured to source a reference current (ILED / RATIO) 30 proportional to the string current through the replica sense resistor generating the replica sense voltage; and a reference circuit (356) configured to generate the further reference voltage (PVT _REF), including: a second replica current control transistor (357) that is a replica of the current control transistor (321-323); a second replica sense resistor that is a replica of the sense resistor; and a current mirror (354) configured to mirror (358) the reference current (ILED / RATIO) through the replica current control transistor (353) and the replica sense resistor of the string current 5 reference circuit (352) to the reference circuit (356); wherein the reference circuit (356) is configured such that the second replica current control transistor (357) saturates at the further reference voltage (PVT_REF) corresponding to the string voltage (VOUT) that can supply the string current accounting for the operating conditions, wherein the reference circuit (356) is configured to output the generated further reference voltage (PVT_REF) to the boost converter (50); wherein, based on the generated further reference voltage (PVT_REF), the boost converter (50) is configured to regulate the string voltage (VOUT) to adjust string voltage headroom (VHDRM) to account for the operating conditions of the LED string; wherein the second replica current control transistor (357) is a low side NMOS source follower, with a source connected to the second replica sense resistor at a node of the sense voltage and to the gate control amplifier (311-314).

2. The LED driver circuit of claim 1, wherein the replica current control transistor (353) and the current control transistor (321-323) are NMOS transistors.

3. The LED driver circuit of claim 1, wherein the LED driver circuit is a component of an LED backlight system with multiple strings of LEDs, and the LED driver circuit includes an LED driver channel per LED string, and includes respective string current control circuitry and headroom circuitry (350) per LED driver channel.

4. The LED driver circuit of claim 3, wherein the LED driver circuit is an integrated circuit configured with multiple LED driver channels / pins, each configured to connect to a respective LED string, and further comprising: multiple string current control circuits, each configured to control a respective string current for a respective LED string connected to a respective LED driver channel / pin; and, per string current control circuit, a headroom circuit configured to generate a respective reference voltage; the string voltage control circuitry configured to provide string voltage control to the power supply to regulate the string voltages for the multiple LED strings, including adjusting the headroom voltage for operating conditions based on the multiple reference voltages from respective headroom circuits (350).

5. A system for LED backlight illumination, comprising: multiple LED strings; a power supply configured to supply to each LED string a regulated string voltage and string current; an LED driver according to claim 1, and comprising, per LED string: an LED channel / pin connected to a respective LED string; and string current control circuitry configured to control the string current, wherein the current control transistor (321-323) is a low side NMOS source follower, with a source connected to the sense resistor at a node of the sense voltage and to the gate control amplifier (311-314).

6. The system of claim 5, wherein the replica current control transistor (353) and the current control transistor (321-323) are NMOS transistors.

7. The system of claim 5, wherein the power supply includes a boost converter, and the string voltage control circuitry includes a boost controller.

8. The system of claim 5, wherein the LED driver circuit is an integrated circuit configured with multiple LED driver channels / pins, each configured to connect to a respective LED string, and further comprising: multiple string current control circuits, each configured to control a respective string current for a respective LED string connected to a respective LED driver channel / pin; and, per string current control circuit, a headroom circuit (350) configured to generate a respective reference voltage; the string voltage control circuitry configured to provide string voltage_control to the power supply to regulate the string voltage for the multiple LED strings, including adjusting the headroom voltage for operating conditions based on the multiple reference voltages from respective headroom circuits (350).

9. A method adaptable for controlling illumination of an LED backlight system that includes multiple strings of LEDs, including controlling a power supply to regulate string voltage, the power supply including a boost converter (50) and a string voltage control circuitry, wherein the string voltage control circuitry includes a boost controller, comprising: controlling string current through a current control transistor (321-323) and a sense resistor, including generating a gate control signal of the current control transistor (321-323) based on comparing a reference voltage to a sense voltage from the sense resistor, such that the string current corresponds to a ratio of the reference voltage to a resistance of the sense resistor; controlling the power supply to regulate the string voltage to supply the controlled string current with a controlled headroom voltage, including adjusting the headroom voltage for process, voltage and temperature operating conditions based on a reference voltage (VREF); and generating a further reference voltage (PVT _REF), including generating a reference current proportional to the string current based on comparing the reference voltage (VREF) to a replica sense voltage using: a replica current control transistor (353) that is a replica of the current control transistor (321-323), and a replica sense resistor that is a replica of the sense resistor (RSENSE); the reference current sourced by the replica current control transistor (353) through the replica sense resistor generating the replica sense voltage R_VSENSE; and generating the further reference voltage (PVT_REF), including: mirroring, by a current mirror (354), the reference current through the replica current control transistor (353) and the replica sense resistor to a reference circuit (356), the reference circuit (356) including a second replica current control transistor (357) of the power supply that is a replica of the current control transistor (321-323), and a second replica sense resistor that is a replica of the sense resistor; generating, by means of the second replica current control transistor (357), the further reference voltage (PVT_REF) wherein the second replica current control transistor (357) saturates at the further reference voltage (PVT_REF) corresponding to the string voltage (VOUT) that can supply the string current accounting for the operating conditions; outputting, by the reference circuit (356) of the power supply, the generated further reference voltage (PVT_REF) to the boost converter (50); and regulating, by the boost converter (50), based on the generated further reference voltage (PVT_REF), the string voltage (VOUT) to adjust string voltage headroom (VHDRM) to account for the operating conditions of the LED string, wherein the second replica current control transistor (357) is a low side NMOS source follower, with a source connected to the second replica sense resistor at a node of the sense voltage and to a gate control amplifier (311-314) of the power supply.

10. The method of claim 9, wherein the current control transistor (321-323) is a low side NMOS source follower, with a source connected to the sense resistor.

11. The method of claim 10, wherein the replica current control transistor (353) and the current control transistor (321-323) are NMOS transistors.

12. The method of claim 9, performed by an LED driver integrated circuit configured with multiple LED driver channels / pins, each configured to connect to a respective LED string, and with multiple respective string current drivers, each configured to control a string current through a respective current control transistor (321-323), and to generate the reference voltage, including: generating a respective reference current proportional to the string current based on the reference voltage using a respective replica current control transistor (353) that is a replica of the current control transistor (321-323); and generating the respective reference voltage by mirroring the reference current through a current control transistor (353) that is a replica of the current control transistor (321-323), and generating the reference voltage based on the saturation voltage of the current control transistor (321-323).