AS3688-T

Application Note DN[Document ID] Mobile Backlight Selection Guide For ams Lightning Management Units and Backlight Drivers ams Application Note [v1-02] 2014-Jul-18 Page 1 Document Feedback Mobile Backlight Selection Guide Content Guide 1 General Description ............................................................................................................. 4 2 Driving LEDs for Mobile Backlight Applications ................................................................... 4 3 LED Configuration Nomenclature ........................................................................................ 5 4 Product Overview ................................................................................................................. 6 5 Choosing the LED Congiguration ........................................................................................ 8 6 LED Configurations for Chargepump ................................................................................... 8 7 LED Configurations for DCDC ........................................................................................... 10 7.1 DCDC Regulation............................................................................................................... 12 7.1.1 AS3687xm/75/76 DCDC .................................................................................................... 12 7.1.2 AS3677 DCDC ................................................................................................................... 13 7.1.3 AS3490/AS3492 DCDC ..................................................................................................... 14 7.1.4 High Voltage Current Sink Overview ................................................................................. 15 7.2 General DCDC Application Component Selection ............................................................. 15 7.3 Single String Solutions ....................................................................................................... 16 7.3.1 Single String Solutions with higher Forward Voltages ....................................................... 18 7.4 Multiple String Solutions .................................................................................................... 20 7.4.1 Multiple Strings for Tablets ................................................................................................ 21 7.4.2 What is the best Configuration? ......................................................................................... 22 7.4.3 Increasing Efficiency in light load conditions ..................................................................... 24 7.5 Driving the DCDC with Serial Batteries.............................................................................. 25 7.6 Dimension of the external Components ............................................................................. 25 8 Conclusion ......................................................................................................................... 27 9 Design Examples ............................................................................................................... 28 9.1 Smartphone with 4.3" Display ............................................................................................ 28 9.2 Smartphone with 4.5" Display ............................................................................................ 30 9.3 Mid-range Smartphone, Backlight, Keypad, Torch and ALS ............................................. 31 9.4 10.1" Table with 7Sx6 Configuration (42 LEDs) + ALS ..................................................... 32 9.5 Smartphone backlight: 4.3~5" with higher Currents .......................................................... 32 9.6 Smartphone backlight: 4.3~5" + Button Backlight & ALS .................................................. 33 9.7 7" Tablet Backlight ............................................................................................................. 34 9.8 Headlamp for ebooks or e-ink devices............................................................................... 35 9.9 Feature phone - low size Backlight + 2x RGB LEDs ......................................................... 36 9.10 Feature phone - high efficiency backlight + 2x RGB LEDs................................................ 36 10 Contact Information ............................................................................................................ 37 ams Application Note [v1-02] 2014-Jul-18 Page 2 Document Feedback Mobile Backlight Selection Guide 11 Copyrights & Disclaimer ..................................................................................................... 38 12 Revision Information .......................................................................................................... 39 ams Application Note [v1-02] 2014-Jul-18 Page 3 Document Feedback Mobile Backlight Selection Guide 1 General Description This guide enables the determination which LED driver product, LED configuration, and external component selection will give the best output in terms of efficiency and solution size for mobile lightning applications. 2 Driving LEDs for Mobile Backlight Applications Depending on the application or display size, the amount of LEDs is different. For smartphone and feature phones the LED requirements are quite similar. LEDs are used in the current range of 2~30mA. The bigger the display gets the more LEDs are needed, also the more pixels the display contains for higher resolution the more LEDs are required to brighten up the display. Typically all LEDs inside the display are driven with the same amount of current. Figure 1: Market Research of several on the market available phones: display size vs. LED count ams Application Note [v1-02] 2014-Jul-18 Page 4 Document Feedback Mobile Backlight Selection Guide Figure 2: Example of a 4.3” smartphone display with 10 LEDs 3 LED Configuration Nomenclature The ams display nomenclature can be used for any kind of display or non-display LED configurations. The nomenclature explains the relation of serial LEDs in a string to the amount of parallel strings. First number is the amount of LEDs in series. Second number is the amount of LED strings in parallel For example: 6Sx2 = 6 LEDs in one string with 2 parallel strings Figure 3: ams Display Nomenclature Examples ams Application Note [v1-02] 2014-Jul-18 Page 5 Document Feedback Mobile Backlight Selection Guide 4 Product Overview Figure 4: Product Overview Part Number Keypad AS3490 AS3492 AS3675 AS3676 AS3677 AS3687 XM AS3688 AS3689 400mA 400mA 50mA 150mA 400mA 400mA 13 13 6 6 12 15 Lighting Inductive DCDC Chargepump Display # Current Sinks 3 5 ALS DLS Camera (2x) Flash Flash Current 150mA 150mA 900mA 150mA 1 1 2 1 Flash Timeout Indicator LED Supply LDOs Features LED Test LED Dimming RGB Pattern Audio Sync ams Application Note [v1-02] 2014-Jul-18 Page 6 Document Feedback Mobile Backlight Selection Guide Package Type WLP12 WLP12 WLP30 WLP30 WLP25 WLP20 QFN32 WLP36 Size in mm 1.8x1.4 1.8x1.4 3x2.5 3x2.5 2.2x2.2 2x2.5 5x5 3x3 Pitch in mm 0.4 0.4 0.5 0.5 0.4 0.5 ams Application Note [v1-02] 2014-Jul-18 Page 7 Document Feedback 0.5 Mobile Backlight Selection Guide 5 Choosing the LED Configuration The LED configuration should be considered after the number of LEDs given by the application, i.e. by the display and notification lights. As an adder to this, it is necessary to take external factors into account. A few of these are highlighted below: • LED forward voltage • LED max. forward voltage • DCDC maximum ratings and limitations • External component limits • Required display runtime / efficiency NOTE: The LED configuration could also be already fixed by the display module suppliers, which then makes the LED configuration easy. It is recommended for display applications to use the inductive DCDC boost converter due to a broader efficiency range. Although for small size applications or applications which have parallel LED configurations it is also possible to use the capacitive boost converter. LED configurations which need individual control like RGB LEDs or indicator LEDs should be connected to the capacitive charge-pump converter. It is not recommended to combine a larger number of serial connected LEDs with an shorter LED string in terms of forward voltage, due to the energy waste. 6 LED Configurations for Chargepump Figure 5: LED Configurations for Chargepump Configuration Product 1Sx4 AS3687XM, AS3676, AS3668 1Sx5 AS3676 1Sx6 AS3676 1Sx7 or higher AS3676 ams Application Note [v1-02] 2014-Jul-18 Page 8 Document Feedback Mobile Backlight Selection Guide Figure 6: Example for small solution size with the AS3687xm chargepump for display backlight and a RGB notification LED Figure 7: PCB placement example using the AS3687xm with its chargepump for 4 LED backlight in 1Sx4 configuration with a RGB notification LED ams Application Note [v1-02] 2014-Jul-18 Page 9 Document Feedback Mobile Backlight Selection Guide Figure 8: Example for the AS3668 as backlight driver for 4 LEDs backlight in a 1Sx4 configuration 7 LED Configurations for DCDC Figure 9: LED Configurations for DCDC LED Configuration Product Recommended (*2) 4Sx1 AS3687xm/AS3677/AS3676 AS3677 2Sx2 AS3490/ AS3687xm AS3490 5Sx1 AS3677/AS3676 AS3677 3Sx2 AS3687xm, AS3676, AS3677 AS3687xm(*1), AS3676, AS3677 AS3490, AS3687xm, AS3676,AS3677 AS3677 AS3676 AS3676 4 LEDs 5 LEDs 6 LEDs 6Sx1 2Sx3 AS3677 AS3490 7 LEDs 7Sx1 8 LEDs ams Application Note [v1-02] 2014-Jul-18 Page 10 Document Feedback Mobile Backlight Selection Guide 8Sx1 AS3676(*1) AS3676 4Sx2 AS3676, AS3677, AS3687 AS3677 2Sx4 AS3492 AS3492 9Sx1 AS3676(*1) AS3676 3Sx3 AS3676, AS3677 AS3677 10Sx1 AS3676(*1) AS3676 5Sx2 AS3676, AS3677 AS3677 2Sx5 AS3492 AS3492 12Sx1 AS3676(*1) AS3676 6Sx2 AS3676, AS3677 AS3676 4Sx3 AS3676, AS3677 AS3677 5Sx3 AS3676, AS3677(*5) AS3676 3Sx5 AS3676(*3), AS3677(*6) AS3676 6Sx3 AS3676, AS3677(*5) AS3676 3Sx6 AS3676(*3) AS3676 4Sx5 AS3676(*3) AS3676 Up to 7Sx6 AS3676(*4) AS3676 9 LEDs 10 LEDs 12 LEDs 15 LEDs 18 LEDs 20 LEDs More LEDs Notes: *1 = external cascode transistor may be needed / depending on Vf of the LED string *2 = recommendation criteria: optimized for size and efficiency. *3 = possible if the display module has connected top anodes and bottom cathodes together *4 = maximum current = 114mA *5 = depending on Vf and VBatmin Green Box = best efficiency ams Application Note [v1-02] 2014-Jul-18 Page 11 Document Feedback Mobile Backlight Selection Guide 7.1 DCDC Regulation 7.1.1 AS3687xm/75/76 DCDC Figure 10: AS3687xm/75/76 DCDC Battery C8 1µF L1 10µH D1 Q1 DCDC_GATE C9 10µF Vout DCDC_SNS Step Up DC/DC Converter R1 100mΩ R2 1MΩ DCDC_FB DCDC_GATE VLED HV Current Sinks each 0.15–38.25mA Curr1 CURR2 CURR6 VCURR This DCDC boost has basically no limitation in output voltage. The maximum output voltage is defined by the limits of the external NMOS, the maximum output voltage of the output capacitor and the diode. The high voltage current-sinks of the AS3687xm/75 are capable of allowing up to 15V. The high voltage current sinks of the AS3676 are able to allow up to 25V. If the voltage limits of the high voltage current-sinks are exceeded, an external cascode transistor has to be used. Otherwise an irreversible damage could happen to the IC. Please refer to selection for “Single string solution’s with higher forward voltages” for detailed explanation. The driving current of this DCDC is limited by the saturation current of the inductor, resistance of inductor / transistors, current limit of the sense resistor and the current driving ability of the high voltage current-sinks. High-voltage DCDC regulated strings need to be connected to the HV current sinks CURR1,CURR2, and CURR6. ams Application Note [v1-02] 2014-Jul-18 Page 12 Document Feedback Mobile Backlight Selection Guide 7.1.2 AS3677 DCDC Figure 11: AS3677 DCDC The AS3677 has an integrated NMOS, with internal programmable overvoltage protection and additionally internal overcurrent limitation. The voltage limit can be set to max. 25V. The maximum NMOS current is 1200mA ( typ ), for worst case calculation the minimum value has to be chosen. The maximum output current for 25V close applications is 50mA. High-voltage DCDC regulated strings need to be connected to the HV current sinks CURR1, CURR 2, and CURR 6. The AS3687xm/75/76/77 DCDC Regulates with the principle of : Vout = VLED + Vcurr for AS3687xm/75/76/77 Vcurr is regulated to 500mV ams Application Note [v1-02] 2014-Jul-18 Page 13 Document Feedback Mobile Backlight Selection Guide 7.1.3 AS3490/AS3492 DCDC Figure 12: AS3490/AS3492 DCDC The AS3490/AS3492 has an Integrated NMOS and PMOS, also unlike the other backlight drivers this device has high side current sources. High sided current sources offer the advantage that the return line of LED current can be used as GND line. This offers additional PCB routing benefits. The voltage limit of the DCDC is 9.3V ( typ ). This allows the use of 2 LEDs in series configurations, from 2Sx2 to 2Sx5. The output voltage of the AS3490/ AS3492 is regulated under the condition: VOUT = VLED + VCURR for the AS3490/ AS3492 VCURR can be assumed with 100mV ams Application Note [v1-02] 2014-Jul-18 Page 14 Document Feedback Mobile Backlight Selection Guide 7.1.4 High Voltage Current Sink Overview Figure 13: High Voltage Current Sink Overview High Voltage current sinks Max. voltage for HV-current sinks Current of HV current sinks Compliance voltage AS3687xm 3x 15V 38.25mA 500mV AS3675 3x 15V 38.25mA 500mV AS3676 3x 25V 38.25mA 500mV AS3677 3x 25V 25.5mA 500mV AS3490 3x 10V 25.5mA 100mV AS3492 5x 10V 25.5mA 100mV 7.2 General DCDC Application Component Selection The performance of the application depends on: • Inductor o too small inductors or wrong selected inductors do not improve the efficiency o for most applications Wire-wound inductors show better performance than multi layer inductors multilayer type inductors have the disadvantage that with higher output voltage the parasitic capacitance and core losses are more dominant compared to wire-wound inductors • o higher Inductances help to increase for light load conditions ( 12~15~18uH ) o much too big inductors in terms of size do not improve the efficiency LED Configuration o • • Switching Transistor o low gate charge helps to increase the efficiency in light load conditions o integrated diode in the transistor module is beneficial for size & efficiency Driver IC o • multiple strings reduce the parasitic capacitance at the switching node and core losses low quicence current at DCDC & currentsink increase the application efficiency Schottky Diode o too small discrete diodes are decreasing efficiency ams Application Note [v1-02] 2014-Jul-18 Page 15 Document Feedback Mobile Backlight Selection Guide o 7.3 too big diode capacity also decreases the DCDC efficiency Single String Solutions Single string configurations are the most used solution for displays. Single String solutions are possible with the AS3687xm/75/76/77. Figure 14: Design Example with the AS3677 for a 6Sx1 LED configuration Figure 15: AS3677 6Sx1 Application Efficiency ams Application Note [v1-02] 2014-Jul-18 Page 16 Document Feedback Mobile Backlight Selection Guide If the DCDC voltage is higher than the maximum voltage of the current-sinks it is mandatory to use a cascode transistor. Please refer to the chapter “Single string solutions with higher forward voltages” for detailed explanation. Figure 16: AS3676 DCDC Efficiency vs VOUT Figure 17: AS3676 DCDC Efficiency vs VOUT Measurement Setup IC AS3676 Load 0.5W constant power load / VOUT Sweep Transistor FDFMA3N109 Diode FDFMA3N109 Inductor different Shunt 0R1 Frequency 1MHz Used LED Resistors used Due to the parasitic capacitance at the switching node and core losses the efficiency of the DCDC converter reduces the higher the output voltage becomes. This graph has been measured with the AS3676 and a constant power of 0.5W to the LEDs, which comes close to the power requirements of a 4” Display. ams Application Note [v1-02] 2014-Jul-18 Page 17 Document Feedback Mobile Backlight Selection Guide Figure 18: AS3676 LED Configuration Comparison This graph shows the AS3676 with different single string LED configurations: 5Sx1, 6Sx1, 7Sx1,8Sx1, 9Sx1, 10Sx1. Figure 19: AS3676 LED Configuration Comparison Measurement Setup IC AS3676 Voltage Sweep Load 20mA Transistor FDFME3N311ZT Fairchild Diode FDFME3N311ZT Fairchild Inductor LPS3010-123 Coilcraft Frequency 1MHz Shunt 0R1 Config different Used LED CL-120A per LED Citizen 7.3.1 Single String Solutions with higher Forward Voltages For driving a Display with single string solutions with higher forward voltage, it needs to be considered that the current-sinks can only drive a certain amount of voltage. For the AS3687 the maximum voltage for the high voltage current-sinks is limited to 15V. The AS3676 maximum voltage limit for the high voltage current-sinks is 25V. If voltages are required which go above the maximum voltage limits of the high voltage current-sinks a cascode transistor can be used to protect the current-sink. ams Application Note [v1-02] 2014-Jul-18 Page 18 Document Feedback Mobile Backlight Selection Guide Figure 20: AS3687 with external cascode transistor, 12 LEDs in series, DCDC output Voltage = 39,6V, LED Current = 100mA The external cascode transistor has to have a maximum drain source voltage which is higher than the application voltage. Efficiency improvement possiblities for higher String voltages: • increasing the inductance • reduction of switching frequency • inductors with lower parasitic capacitance • transistors with lower capacity between drain-source and drain-gate • schottky diodes with less forward voltage • schottky diodes with less capacity ams Application Note [v1-02] 2014-Jul-18 Page 19 Document Feedback Mobile Backlight Selection Guide 7.4 Multiple String Solutions Multiple string solutions can be used to reduce the maximum output voltage of the DCDC. This helps to reduce the component sizes of capacitors and inductors. For example the output capacitors get bigger the more voltage they need to sustain. Figure 21: AS3677 design example for a 4Sx2 configuration - 4” Smartphone This example shows the AS3677 with it’s integrated NMOS and an external schottky diode for a 4Sx2 configuration. Additionally the chargepump is driving a 50mA torch LED. ams Application Note [v1-02] 2014-Jul-18 Page 20 Document Feedback Mobile Backlight Selection Guide Figure 22: AS3676 design example for a 6Sx3 configuration – 7” tablet This example shows the AS3676 with an external NMOS with integrated schottky diode for a 6Sx3 configuration which can drive up to 2 Watts of LED power. Additionally there are 2 RGB LEDs and 4 indicator LEDs for the status indication of capacitive buttons. 7.4.1 Multiple Strings for Tablets Figure 23: Design example for the AS3676 as 10.1” tablet backlight driver for a 6Sx6 LED configuration Balance Resistors R3,R4,R5,R6,R7 and R8 are Optional ams Application Note [v1-02] 2014-Jul-18 Page 21 Document Feedback Mobile Backlight Selection Guide Design example for the AS3676 as 10.1” tablet backlight driver for a 6Sx6 LED configuration. Additionally there are 2x RGB indication LEDs and 4 indication LEDs. The AS3676 is connected with the TSL253CL for ambient light control. The RGB LEDs, indication LEDs and display backlight can be controled with independent ambient light calculation tables. Light Value [Lux] Figure 24: Light Value vs Current Current[mA] Red = indication LED control curve Blue = rgb LEDs control curve Green = display backlight control curve 7.4.2 What is the best Configuration? The best configuration for LEDs depends on the amount of LEDs needed for the application, and the typical use-case of current, DCDC efficiency and the compliance voltage of the current sinks. Figure 26: AS3676 LED Configuration Comparison Measurement Setup IC AS3676 Voltage 3.7V Load 20mA Transistor FDFME3N311ZT Fairchild Diode FDFME3N311ZT Fairchild Inductor LPS3010-123 Shunt 0R1 Frequency 1MHz ams Application Note [v1-02] 2014-Jul-18 per LED - Page 22 Document Feedback Mobile Backlight Selection Guide Config different Used LED CL-120A Citizen Figure 27: AS3674/AS3492 LED Configuration Comparison Measurement Setup IC AS3674(AS3492) Voltage 3.7V Load 20mA Transistor Internal NMOS Diode Internal PMOS Inductor CIG21K4R7SCD Frequency 2MHz Config 2Sx3/4/5 Used LED Firefly ams Application Note [v1-02] 2014-Jul-18 per LED - Osram Page 23 Document Feedback Mobile Backlight Selection Guide 7.4.3 Increasing efficiency in light load conditions As light sensors are common in mobile handheld devices the dynamic range of the backlight can vary depending on the light condition. To increase the application efficiency in light load conditions it can be useful to increase the inductance. This can have, depending on the resistance of the inductor wire and the core losses, negative effects on the efficiency under high load conditions. Figure 28: AS3677 4Sx3 Applicatin Efficiency Figure 29: AS3677 4Sx3 Applicatin Efficiency Measurement Setup Pout @ 60mA 0.75W Pout @ 20mA 0.25W Used LED Config 4Sx3 10uH Inductor LQH3NPN100MJ0 15uH Inductor LQH3NPN150NJ0 Used Diode CMDSH05-4 Inductor Size 3x3x1mm ams Application Note [v1-02] 2014-Jul-18 Page 24 Document Feedback Mobile Backlight Selection Guide 7.5 Driving the DCDC with Serial Batteries Figure 30: Driving the DCDC with Serial Batteries It is possible to connect the AS3687xm/75/76/77 to supply systems which have 2 batteries in series. The way how to do this is supplying IC VBAT pin with a general purpose system supply and connecting the Battery directly to the DCDC for the backlight application. This connection will have a beneficial effect to the system application efficiency. This example shows how the AS3676 is connected in a system with two serial batteries. The 7” tablet display module is configured in a 4Sx5 configuration where the LEDs are connected together at the bottom cathodes and the top anodes. The high voltage currentsinks of the AS3676 can drive, connected together, up to 114mA of current. Additionally this example is using the ams-TAOS TSL253CL as ambient light sensor for backlight control, and the 400mA chargepump as flash LED driver. 7.6 Dimension of external Components Output Capacitor: It is mandatory that the capacity of the output capacitor for AS3687xm/75/76/77 has under all conditions more than 0.7uF. For this please check the DC bias characteristics of the capacitor with the maximum voltage which would happen in the application with max. LED forward voltage and include the tolerances of the capacitor. For AS3492 or AS3490 the minimum capacity is 4.8uF. ams Application Note [v1-02] 2014-Jul-18 Page 25 Document Feedback Mobile Backlight Selection Guide Figure 1: Example for the DC Bias characteristics of a 10uF capacitor Inductor: The inductor is an important component to select for the DCDC. Too small inductors will result in bad efficiency. Also too big inductors will also result in bad efficiency values. Also it can be that 10uH is not always the best suitable inductor as shown below. It is necessary to check if the inductor for AS3687xm/75/76/77 has all conditions more than 7uH. For AS3490 and AS3492 the minimum inductance under all conditions is 2,45uH. Figure 32: AS3677 40mA Application Efficiency This measurement graph shows that the 12uH - 3x3x1 Inductor was best in class. Also that the 2,6x2x1mm Inductor has a low application efficiency of 79%. ams Application Note [v1-02] 2014-Jul-18 Page 26 Document Feedback Mobile Backlight Selection Guide Figure 33: AS3677 40mA Application Efficiency Measurement Setup Output Current 20mA Frequency 1MHz Inductor Sweep Load 4Sx2 IC AS3677 8 per LED Conclusion Figure 34: Recommended Configurations LED Amount Recommended Configuration IC Display 4 2Sx2 AS3490 Up to 3.2” 5 5Sx1 AS3677 Up to 3.5” 6 2Sx3 AS3490 Up to 3.7” 7 7Sx1 AS3676 Up to 4.0” 8 2Sx4 AS3492 Up to 4.3” 9 3Sx3 AS3677 4.0~4.5” 10 2Sx5 AS3492 4.0~4.7” 12 4Sx3 AS3677 4.5”~5.0” 15 5Sx3 AS3676 5.0”~ 18 6Sx3 AS3676 7.0”~ 20 4Sx5 AS3676 7.0”~ 36 6Sx6 AS3676 10.1”~ 42 7Sx6 AS3676 10.1” Conclusion of single string solutions: Pro: • less connections to the display ams Application Note [v1-02] 2014-Jul-18 Page 27 Document Feedback Mobile Backlight Selection Guide Con: • efficiency suffering due to parasitic capacitance on the switching node and core losses ( higher ripple current ) • bigger inductor and lower frequency needed to improve efficiency • output capacitors get bigger due to high voltage Conclusion of multiple string solutions Pro: • higher efficiency due to lower parasitic capacitance and core losses at the DCDC • smaller external components like inductor and capacitors possible Con: • 9 9.1 more connections needed Design Examples Smartphone with 4.3” Display Figure 2: AS3492 backlight driver for smartphones This example shows the AS3492 as backlight driver for smartphones with display sizes up to 4.3” or depending on specifications even bigger. The components are tiny, as the DCDC has a switching with a frequency of 2MHz. The CIG21E4R7MNE Inductor from SEMCO, or the LQM2HPN4R7_G Inductor from Murata have a size of 2x1,2x1mm, the input capacitor is a 2.2uF ams Application Note [v1-02] 2014-Jul-18 Page 28 Document Feedback Mobile Backlight Selection Guide 0402 capacitor, while the output capacitor is a 10uF, 25V 0603 capacitor. The solution enables the AS3492 to reduce it’s solution size down to 15mm² ( with ams design rules ). Figure 3: PCB 3D Drawing of the AS3492 Figure 4: PCB Drawing of the AS3492 Additionally keeping the GND away from the Inductor helps to reduce eddy current losses. This efficiency improving PCB design technique is ams internally known as “hovering Inductor layout”, it is an additional efficiency improvement technique. The GND plane can be placed already on MID Layer #1 or Layer#2. ams Application Note [v1-02] 2014-Jul-18 Page 29 Document Feedback Mobile Backlight Selection Guide 9.2 Smartphone with 4.5” Display Figure 38: AS3676 smartphone LED driver The example shows the AS3676 as smartphone LED driver. It has an 8Sx1 LED configuration for the display backlight which is powered by the inductive DCDC. The display can have a size from 4” to 4.5”. Also there are LEDs which are powered by the capacitive charge pump for the keypad backlight, camera indication and the RGB notification LED. Figure 39: Application efficiency of the 8Sx1 configuration for display backlight ams Application Note [v1-02] 2014-Jul-18 Page 30 Document Feedback Mobile Backlight Selection Guide 9.3 Mid – range Smartphone, Backlight, Keypad, Torch and ALS Figure 40: AS3677 display backlight driver AS3677 as display backlight driver with a 6Sx1 configuration. Additionally the DCDC also drives a keypad backlight with low current. The chargepump is used for a 50mA torch LED. Generally it is recommended to have balanced LED strings – if this is not possible the efficiency will be reduced, but as the energy in the keypad LED string is usually much smaller compared to the backlight LED string it will only slightly affect efficiency. Additionally it should be checked if the power dissipation inside the driver due to unbalanced LED strings does not exceed the datasheet specifications (see absolute max. ratings) ams Application Note [v1-02] 2014-Jul-18 Page 31 Document Feedback Mobile Backlight Selection Guide 9.4 10.1” Tablet with 7Sx6 configuration (42 LEDs) + ALS Figure 5: AS3676 as display backlight driver for 42 LEDs in a 7Sx6 configuration 9.5 Smartphone backlight: 4.3 ~ 5” with higher currents Figure 6: AS3687xm as 5” tablet backlight driver; max. current = 38mA per LED ams Application Note [v1-02] 2014-Jul-18 Page 32 Document Feedback Mobile Backlight Selection Guide Figure 7: Application efficiency of the AS3687xm as 5” tablet backlight driver 9.6 Smartphone backlight: 4.3 ~ 5” + Button Backlight & ALS Figure 8: AS3677 as 5” tablet backlight driver with ambient light sensing and keypad LED control. This solution is optimized for best efficiency and lowest size, max. current per LED = 25mA. ams Application Note [v1-02] 2014-Jul-18 Page 33 Document Feedback Mobile Backlight Selection Guide Figure 9: Application efficiency of the AS3677 setup above. Inductor: Murata LQH3NPN100MJ0, Diode CMDSH05-4 9.7 7” Tablet Backlight Figure 10: This design example shows the AS3676 as 7” Tablet backlight driver with a 4Sx5 configuration, a 70mA torch LED, ambient light control, and an RGB notification LED. ams Application Note [v1-02] 2014-Jul-18 Page 34 Document Feedback Mobile Backlight Selection Guide 9.8 Headlamp for ebooks or e-ink devices Figure 11: AS3677 headlamp for ebooks and E-Ink devices This design example shows how the AS3677 is implemented as headlamp for ebooks or E-Ink devices. As these devices do not have any backlight it is not possible to read in the dark. It is possible to read by turning on side LEDs as headlamp. To optimize power consumption a light sensor can be used. As ebook or e-ink devices tend to turn off their main processor during the reading activity of the user, the automatic ambient light control will determine if actual light amount requires the need of the headlamp or not. This solution is optimized for higher efficiency for lower load currents as the application requires typically low light output. ams Application Note [v1-02] 2014-Jul-18 Page 35 Document Feedback Mobile Backlight Selection Guide 9.9 Feature phone – low size Backlight + 2x RGB LEDs Figure 12: AS3687xm with 4 LEDs as display backlight and 2 RGB LEDs as low size solution for feature phones. Balance resistors are required depending on the current accuarcy for the RGB LEDs. 9.10 Feature phone – high efficiency Backlight + 2x RGB LEDs Figure 13: AS3687xm with 4 LEDs as display backlight with high efficiency and 2 RGB LEDs which can be controlled independently. Typical application: feature phones. ams Application Note [v1-02] 2014-Jul-18 Page 36 Document Feedback Mobile Backlight Selection Guide 10 Contact Information Buy our products or get free samples online at: www.ams.com/ICdirect Technical Support is available at: www.ams.com/Technical-Support Provide feedback about this document at: www.ams.com/Document-Feedback For further information and requests, e-mail us at: ams_sales@ams.com For sales offices, distributors and representatives, please visit: www.ams.com/contact Headquarters ams AG Tobelbaderstrasse 30 8141 Unterpremstaetten Austria, Europe Tel: +43 (0) 3136 500 0 Website: www.ams.com ams Application Note [v1-02] 2014-Jul-18 Page 37 Document Feedback Mobile Backlight Selection Guide 11 Copyrights & Disclaimer Copyright ams AG, Tobelbader Strasse 30, 8141 Unterpremstaetten, Austria-Europe. Trademarks Registered. All rights reserved. The material herein may not be reproduced, adapted, merged, translated, stored, or used without the prior written consent of the copyright owner. Information in this document is believed to be accurate and reliable. However, ams AG does not give any representations or warranties, expressed or implied, as to the accuracy or completeness of such information and shall have no liability for the consequences of use of such information. Applications that are described herein are for illustrative purposes only. ams AG makes no representation or warranty that such applications will be appropriate for the specified use without further testing or modification. ams AG takes no responsibility for the design, operation and testing of the applications and end-products as well as assistance with the applications or end-product designs when using ams AG products. ams AG is not liable for the suitability and fit of ams AG products in applications and end-products planned. ams AG shall not be liable to recipient or any third party for any damages, including but not limited to personal injury, property damage, loss of profits, loss of use, interruption of business or indirect, special, incidental or consequential damages, of any kind, in connection with or arising out of the furnishing, performance or use of the technical data or applications described herein. No obligation or liability to recipient or any third party shall arise or flow out of ams AG rendering of technical or other services. ams AG reserves the right to change information in this document at any time and without notice. ams Application Note [v1-02] 2014-Jul-18 Page 38 Document Feedback Mobile Backlight Selection Guide 12 Revision Information Changes from 1-01 (2012-Aug-25) to current revision 1-02 (2014-Jul-18) Update to corporate format Page 1-45 Note: Page numbers for the previous version may differ from page numbers in the current revision. ams Application Note [v1-02] 2014-Jul-18 Page 39 Document Feedback