(1) Technical Field
This disclosure relates generally to the field of white LED drivers and relates more specifically to white LED drivers with improved efficiency.
(2) Background
White light emitting diodes (WLED) are used most often in notebooks and desktop screens, and in virtually all mobile LCD screens. A WLED is typically a blue LED with broad-spectrum yellow phosphor to give the impression of white light. WLEDs are often used for backlighting LCD displays. For such an application WLED drivers have to generate constant current required for a constant luminance.
Charge pumps or inductive converters are usually used as WLED drivers, generating high bias voltages from a single low-voltage supply, such as a battery.
It is a challenge for engineers designing WLED drivers to optimize efficiency of the drivers.
A principal object of the present disclosure is to optimize the efficiency of a WLED driver.
A further object of the disclosure is to minimize switching losses associated with the switching activity of a boost power converter used.
A further object of the disclosure is to minimize conduction losses associated with the current flowing in the boost converter and mainly depending on the resistance of the elements in the regulation loop.
A further object of the disclosure is to reduce to a minimum the regulated voltage at a node between a programmable current source and a string of WLEDs allowing the boost converter working at smaller duty cycles.
A further object of the disclosure is to utilize a very low voltage and accurate programmable current source.
A further object of the disclosure is to use a programmable reference voltage for an error amplifier.
A further object of the disclosure is to use a size programmable NFET power switch with constant current limit for optimization of switching losses and conduction losses.
A further object of the disclosure is to use a PWM generator with programmable clock frequency.
Moreover an object of the disclosure is to maximize efficiency of the WLED driver in a region where the boost converter is operating in Discontinuous mode.
In accordance with the objects of this disclosure a method to optimize efficiency of white LED driver has been achieved. The method disclosed comprises, firstly, the following steps: (1) providing a device comprising an arrangement of one or more LEDs in series, a programmable iDAC current source, a boost converter comprising a size programmable power switch, a PWM generator with programmable clock frequency, and a programmable reference voltage generator for an error amplifier stage, (2) identifying a number of configuration windows specifying configuration of the LED driver and mode of modulation, wherein the configuration windows and modulation mode depend on current iDAC required through the LED arrangement, and (3) defining clock frequency, size of power switch and said programmable reference voltage once the current iDAC is known. Furthermore the method disclosed comprises (4) storing characteristics of all configuration windows selected, and (5) implementing configuration of a specific LED driver for a specific LED arrangement according to the corresponding configuration window providing optimum efficiency in regard of correspondent characteristics.
In accordance with the objects of this disclosure a circuit for a LED driver having optimized efficiency has been disclosed. The circuit disclosed comprises, firstly, a digital core comprising: a current selection block to select a current generated by a programmable current source, an OTP memory to store profiles of operation windows, a digital comparator, and a means for a frequency divider, wherein an output of the digital block comprises a digital word prog setting a selected value of the current generated by the programmable current source, a clock signal driving a PWM generator, a reference voltage for a regulation loop, and a size of a size programmable power switch of a boost converter. Furthermore the circuit disclosed comprises the boost converter comprising: a port for an input voltage, an inductor connected between a first terminal of the port for the input voltage and a node LX, and a rectifying means connected between the node LX and an output voltage of the boost converter. Furthermore the boost converter disclosed comprises: a capacitor connected between output ports of the boost converter, said size programmable power switch connected between the node LX and a second terminal of a sense resistor, wherein the power switch is controlled by a signal from a regulation loop, said sense resistor, wherein a second terminal of the sense resistor is connected to a second terminal of said port for the input voltage, said PWM generator driving via said regulation loop the power switch, wherein the PWM generator receives said clock signal, and said regulation loop to control an output voltage an output voltage of the programmable current source using said reference voltage, being connected between a second terminal of a programmable current source and a gate of said power switch. Moreover the driver disclosed comprises: one or more LEDs connected in series wherein a first terminal of the one or more LEDs is connected to a first output port of the boost converter and a second terminal of the one or more LEDs is connected to the second terminal of the programmable current source, and said programmable current source to deliver a bias current to the one or more LEDs, wherein a second terminal of the current source is connected to the second terminal of said port for the input voltage.
In the accompanying drawings forming a material part of this description, there is shown:
a illustrates a process to define the reference voltage for regulation.
b depicts a block diagram of a digital core selecting a profile of the driver dependent on the different operating windows.
a presents a detailed circuitry of the programmable current source Idac providing current for the LEDs.
b shows a diagram of the output currents of the programmable current source Idac depending on the IDAC output voltage FBK.
Methods and circuits for driving white LEDs (WLED) are disclosed. In a preferred embodiment a string of WLEDS is powered by the driver, e.g. for backlighting a display LCD.
It should be noted that the methods and circuits disclosed could be used for any LED drivers based on inductive boost converter.
Furthermore
The status of the programmable IDAC 6 is used to know the load current and get a specific profile of for the boost voltage. It should be noted that the principle of the disclosure can be used to any system that uses the same approach as the disclosure to drive any LEDs.
The integrated regulation loop comprises an error amplifier EA 7 with fixed voltage gain, a programmable voltage reference generator 8 of one fixed current source and variable resistor Rref 9, a PWM comparator 10, and a saw tooth generator (PWM) 11 with programmable clock (clk) frequency.
The efficiency of the WLED system 100 can be defined by the ratio:
η=Pout/Pin, (1)
in other words the relation between output power and input power, wherein
Pout=(VBOOST−FBK)×iDAC,and (2)
Pin=VBAT×iBAT. (3)
Equation (2) is valid for a single string of WLEDs. For 2 or more (K) strings:
As Pout is the result of the luminance required by WLED 1, in order to maximize efficiency it is mandatory to reduce Pin or more specifically the losses.
The losses can be categorized as:
Since, as a general rule, switching losses SLoss become dominant at low load currents and the information of the output current required (iDAC) is always available, it makes sense to configure the boost in such a way it can operate always under the most efficient conditions.
The disclosed configurable solution is to address three areas where losses can be reduced:
As it will be explained later in this document, the different configurations for the boost will be defined by the programmed load current iDAC via digital control and stored in OTP registers during the trimming phase of the device.
The iDAC selection is an N-bits word to identify any of the 2 power of N levels of current for the WLED string (logarithmic scale). This N-bits word is the programming word “prog”. The digital comparator 32 reads “prog” and identifies the correspondent window for clk, vref and NFET.
In applications as e.g. backlight systems the IDAC current is programmed in logarithmic steps in order to compensate for human eye response.
As shown in
For this implementation, as non-limiting example, 4 different configuration windows (W1, W2, W3 and W4) have been identified, each covering an iDAC current range that can be extended or reduced at user's wish.
In windows W1-W2 the boost converter operates in DCM mode. In windows W3-W4 the boost converter operates in CCM mode. A purpose of the design in the disclosure presented is to maximize efficiency in a region where the boost converter is operating in DCM mode. Therefore, as shown in
A small hysteresis (also user programmable) guarantees smooth transitions between windows during the ramp up/down of the iDAC current (from 0 to 25 mA and vice versa) through all the iDAC programming codes.
As shown in
The way the VBOOST and FBK voltages, as shown in
FBK=f(clk,On-Resistance,Duty Cycle); (6)
Duty Cycle DCM=f(VBAT,VBOOST,iDAC); (7)
Duty Cycle CCM=f(VBAT,VBOOST); (8)
The procedure steps to define the reference voltage VREF for the error amplifier EA 7 is described in the
It is obvious that for the windows W2-W4 shown in
The voltage values for the windows selected as e.g. WLED1, WLED2, WLED3, and WLED4 can be identified during characterization at a testing site and stored in the OTP 31, as shown in
b shows a block diagram of a digital core 13 selecting a profile of the driver dependent on the different operating windows. The digital core 13 comprises a circuit 30 providing as output a digital word prog defining the value of the IDAC current selected. The digital word prog comprises information about a tap point for the reference resistor Rref 9, the scale factor for the power switch NFET 5, and the clock frequency of the PWM signal clk.
The digital programming word “prog” is an N-bit word to define any of the 2 power of N iDAC values between 0 and 25 mA decoded in the example of the preferred embodiment in a logarithmic scale. The iDAC selection block looks like a look-up table with currents vs. digital codes.
Furthermore the digital core 13 comprises an OTP memory 31 containing the profiles of all windows, e.g. in the preferred embodiment the profiles of windows W1-W4. Other numbers of windows are also possible. The digital word prog representing the IDAC current selected and the profile of operation windows are input of a digital comparator 32, i.e. the value of the PWM clock frequency clk, the size of the power switch NFET 5, and the resistance of the reference resistor Rref 9. These values are set by the comparator 32 according the operation window selected dependent on the IDAC current selected. The frequency clk is set via a programmable digital frequency divider 33 using in the preferred embodiment a base frequency of e.g. 3 MHz. Other base frequencies can be used as well.
The next
The circuit of
The output branch comprises transistor N246, transistor Nswitch 47 and output transistor Nout 40. The transistor NOUT 40 delivers the output current IDAC. Amplifier 41 controls the gate of transistor NOUT 40. The amplifier 41 together with transistor NOUT 40 provides a regulation that guarantees voltage Vdac equals voltage Vcasc. The size of transistors N1 and N2 is such that the saturation operation is guaranteed for very low drain-source voltages (<150 mV) and is the most important element to achieve efficiency of the boost operation.
Transistors N246 and N145 form a current mirror. The gate of transistor NSWITCH 47 is controlled by a voltage corresponding to a digital word prog, which is used to select the IDAC current. The adjustment of the transistor N246 via Nswitch 47 can be performed by e.g. by logarithmic steps or in another sequence.
There are actually a number of N2 devices in parallel, each with an Nswitch device on top. To increase the mirroring ratio, and reach the desired output current, a number of Nswitch devices are closed in sequence. The non-selected branches are with the Nswitch device open therefore that portion of the N2 device does not take part to the conduction. Due to the nature of the logarithmic response, the scaling ratio of the N2 devices is not binary weighted but logarithmic, meaning that for small codes (small currents) the ratio increases at slow rate, while when large codes (large currents) are selected the ratio is larger. Obviously the size of Nswitch device is proportional to the size of the N2 device underneath.
b shows a diagram of the output currents IDAC of the programmable current source versus the IDAC output voltage FBK. The IDAC values shown are for a maximum case 400 and typical case 401, wherein X2 stands for 2 strings of WLED supported.
The programming of VREF is explained in
The drivers 54 and 55 are simple buffer stages made of a series of MOS inverters (NFET+PFET) in order to scale up the driving strength and cope with the large capacitive load of gate A and gate B. Obviously the driving strength of driver A and B will be proportional to the gate size that they have to drive. When a gate is OFF, the input of the correspondent driver is set constantly to ground, while when a gate is ON, the correspondent driver is left to switch according to the PWM signal. Dedicated logic is used to prevent the overlapping of the rising and falling edges of the drivers and avoid cross conduction (which will cause loss in efficiency).
It should be noted that, as shown in
If for instance the ratio between NFET_A (52) and the total NFET area is 40%, the On-Resistance of NFET_A (52) will be 40% higher and the gate to source capacitance, which needs to be charged/discharged at every cycle, to be approximately 40% lower. The NFET_A ratio is used for low load configuration where SLoss dominate with respect to CLoss.
The current sensing (for slope compensation and current limiting) is performed using a scaled version (NFET_SensA 50 and NFET_SensB 51) of the main power switch (hence there is little contribution to the conduction of the boost current from the sense devices), which mirrors the main current into a sense resistor Rsense 12. The main point is that if power switch system NFET 5 changes its size according to current iDAC, the mirroring ratio stays constant hence the current gain (for slope compensation) and current limits (to prevent coil saturation) also remain constant.
The current gain is kept constant in regard of the NFET size selection; hence the following equation is valid:
wherein ILOAD is the current through inductor 2, NFET_SENSE/NFET represents the scale of the size of the scaled versions (50, 51) related to the selected size of the power switch (52, 53) and RSENSE is the resistance of the sense resistor 12.
The switching frequency clk selection is made through a programmable frequency divider for fine steps tuning (5 bits) in the range e.g. between 0.25 and 3 MHz. For low iDAC values, clk is reduced to minimize SLoss; the fine-tuning is needed to optimize the efficiency in respect to NFET and iDAC.
While the disclosure has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the disclosure.
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