APPLICATION NOTE Multi-Channel WLED Driver Solution in Single-Cell, Li-Ion Battery Applications by AAT1451/1405/1407 and AAT3110 Introduction The white LED backlight is very popular in notebook, portable DVD/TV, iPad and similar applications. Larger display screens need greater number of WLEDs to meet their requirement of brightness. General industry consensus is to adopt a step-up converter to drive WLEDs in series with several strings from Li-ion battery. Panel sizes of 7 inch to 10 inch usually use 24~36 WLEDs in a 4~8 WLEDs in series, 4~6 strings in parallel, configuration for the backlight with 20mA to 30mA maximum WLED current. How to use a multi-channel WLED step-up power management IC to drive so many WLEDs with only a single Li-ion battery as input power is a key design consideration for such applications. This document gives a solution for the single cell Li-ion battery backlighting application using AAT1451/1405/1407 and AAT3110 to drive the WLEDs. AAT1451/1405 plus AAT3110 is used to drive 6series-4parallel (6S4P) to 8series-4parallel (8S4P) and AAT1407 plus AAT3110 is employed to drive 4series-6parallel (4S6P) to 6series-6parallel (6S6P) configurations. AAT3110 is not required when the system is powered by a 2 cell Li-ion battery. AAT1451/05/07 can be powered directly by the 2 cell Li-ion battery in such applications. Application Solution Figure 1 shows the solution structure of the WLED driver by multi-channel WLED boost in Li-ion battery application using AAT1451/05/07 and AAT3110. The battery provides the power to the inductor and AAT3110. The AAT3110-5.0 outputs 5V for AAT1451/05/07. L Battery 2.7-4.2V D VOUT AAT1451/05/07 AAT3110-5.0 SW WLEDs VIN VOUT ON/OFF signal SHDN 5V VIN FB1 FB2 FBn Figure 1: Solution Structure for Multi-channel WLED Backlight in Li-ion Battery Application Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202383A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 27, 2012 1 APPLICATION NOTE Multi-Channel WLED Driver Solution in Single-Cell, Li-Ion Battery Applications by AAT1451/1405/1407 and AAT3110 AAT3110, a charge pump device, delivers up to 100mA with three small capacitors, to the voltage regulated output. AAT1451 is a step-up converter white LED driver for netbooks, notebook computers, monitors and portable TVs. It provides a high voltage output to drive multiple strings of series WLEDs. Four precision current sinks can provide constant current drive for up to 44 white LEDs depending upon WLED forward voltage. The WLED current, up to 30mA per string, is set by a single external resistor for a total output current capability of 120mA. External PWM signal is used to dim the WLED. For more detailed information on AAT3110 and AAT1451, please refer to their datasheets. Figure 2 shows the multi-channel WLED backlight solution in Li-ion battery application with AAT1451 and AAT3110 DS1 SS16L L1 4.7µH Battery 2.7-4.2V U2 AAT3110IJS-5.0-T1 1 COUT2 10µF ON/OFF signal 2 VOUT GND 3 SHDN VIN C+ C- 8 4 10 9 CFLY 1µF 11 CIN2 10µF 13 WLEDs 6S4P~8S4P VDD 3 2 FB1 COMP FB2 ISET FB3 FSLCT FB4 4 5 6 7 12 0 RFS 20k PWM EP CCOMP 15nF RSET 5.23k SHDN GND RCOMP 20k OVP CVDD 2.2µF 5 6 VIN SW 1 CIN1 2.2µF U1 AAT1451 VOUT COUT1 2.2µF/50V 19 CIN 4.7µF Figure 2: Application Solution with AAT1451 and AAT3110 AAT1405 and AAT1407 have similar function, structure and driving capability compared to AAT1451. AAT1405 integrates a boost (step-up) converter and provides 45V output for driving series WLEDs. Four precision current sinks are programmable up to 30mA per string through one external RSET resistor, supporting up to 44 white WLEDs at 120mA total output current. AAT1407 integrates a boost (step-up) converter and provides up to 45V output for driving series WLEDs. Six precision current sinks are programmable up to 30mA per string through one external RSET resistor, supporting up to 66 white WLEDs at 180mA total output current. 2 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202383A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 27, 2012 APPLICATION NOTE Multi-Channel WLED Driver Solution in Single-Cell, Li-Ion Battery Applications by AAT1451/1405/1407 and AAT3110 Figure 3 and Figure 4 show the application solutions using AAT1405 with AAT3110 and AAT1407 with AAT3110. L1 4.7µH Battery 2.7-4.2V CIN 1 COUT2 10µF 2 VOUT VIN GND C+ 3 ON/OFF signal C- SHDN 23 EN 10µF 5 22 CVDD J4 2.2µF 6 CCOMP CFLY 1µF RCOMP 4.99k 15nF CIN2 4 10µF OVP 17 24 IN CIN1 U2 AAT3110 MSS1P5 U1 AAT1405 J1 19 LX 4.7µF DS1 16 RSET J3 GND 6 2 FSET 1 GND 7 GND 13 GND 14 3 10 GND 21 GND 18 4 GND GND EP 0 PGND 20 SYNC SGND J2 WLEDs 6S4P~8S4P CS2 9 CS3 11 CS4 12 COMP 15 RSET 4.32k CS1 8 2.2µF 50V R2 12.1k VCC 5 PWM COUT1 R1 442k Figure 3: Application Solution with AAT1405 and AAT3110 L1 4.7µH Battery 2.7-4.2V CIN COUT2 10µF ON/OFF signal 2 VOUT GND 3 SHDN VIN C+ C- 23 EN 5 10µF 22 CVDD J4 2.2µF 6 CCOMP CFLY 1µF 4 15nF CIN2 10µF OVP 17 24 IN CIN1 U2 AAT3110 1 MSS1P5 U1 AAT1407 J1 19 LX 4.7µF DS1 J3 RCOMP 4.99k RSET 4.32k CS1 CS2 COMP CS3 CS4 15 RSET 1 6 COUT1 2.2µF 50V WLEDs 4S6P~6S6P 7 8 9 CS5 11 CS6 12 2 FSET GND GND 21 GND 18 4 EP 0 PGND 20 GND 13 GND 14 SYNC GND 3 SGND 10 J2 R2 12.1k VCC 5 PWM 16 R1 442k Figure 4: Application Solution with AAT1407 and AAT3110 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202383A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 27, 2012 3 APPLICATION NOTE Multi-Channel WLED Driver Solution in Single-Cell, Li-Ion Battery Applications by AAT1451/1405/1407 and AAT3110 Design Considerations The number of WLEDs in the application is determined by the WLED forward voltage, the required maximum LED current, the Schottky diode forward voltage and the output voltage capability of the boost. We will discuss them one by one in this section. WLED I-V Curve Whether a WLED backlighting solution is good or not is not only determined by the WLED driver, but also by the selected WLEDs. AAT1451/1405/1407 are specifically intended for driving WLEDs. These devices are suitable to drive most types of WLEDs with forward voltage specifications typically ranging from 2.2V to 4.7V, depending upon supply voltage. WLED applications may include mixed arrangements for display backlighting, keypad display, and any other application that needs a constant current sink generated from a varying input voltage. Since the FB1 to FB4 constant current sinks are matched within 2% with negligible supply voltage dependence, the constant current channels will be matched regardless of the specific WLED forward voltage (VF) levels. The low dropout current sinks in the AAT1451/1405/1407 maximize performance and make them capable of driving WLEDs with high forward voltages. In the past, the forward voltage of WLED was usually specified around 4V. With the WLED manufacturing technology continuing to improve, the forward voltage has decreased. Currently, there are many WLEDs available with typical forward voltages (VF) in the 2.8 to 3.6V range. The typical I-V curve is shown in Figure 5, the typical forward voltage in the figure is specified to be 3.2V at 20mA. Figure 5: WLED I-V curve of RS-0805UW Schottky Diode I-V Curve Figure 6 shows the Schottky diode (SS16L) I-V curve. When WLED is in 4 strings and its current is 30mA, the Schottky diode current is 120mA. Under these condition, the forward voltage is 350mV. 4 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202383A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 27, 2012 APPLICATION NOTE Multi-Channel WLED Driver Solution in Single-Cell, Li-Ion Battery Applications by AAT1451/1405/1407 and AAT3110 SS16L Schottky Diode If vs Vf 400 Forward Current (mA) 360 SS16L Schottky If vs Vf 320 280 240 200 160 120 80 40 0 0.28 0.29 0.3 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4 Forward Voltage (V) Figure 6: I-V curve of Schottky Diode VOUT Calculation As mentioned above, WLED has a 3.26V typical forward voltage VF when ILED is 30mA. The Schottky diode (DS1) has a typical 0.35V forward voltage VD at 120mA forward current. With the sink voltage of feedback 0.5 to 0.9V VFB, we may calculate the required VOUT which the LED driver needs to output using the following formula: VOUT = n · VF + VFB Where n is the quantity of WLEDs. So, for 8 WLEDs in series (8S), the boost output needs to be 27V to obtain 30mA for each WLED as shown by the calculation below. VOUT = 8 · 3.26V + 0.5~0.9V = 26.58~26.98V The actual VOUT is limited by the switching duty cycle D, VIN, Schottky diode forward VD and the boost internal NMOSFET VDS. (VBAT - VDS) · D = (VOUT + VD - VBAT) · (1-D) So, VOUT = (VBAT - VDS) · D (1-D) + VBAT - VD The maximum switching duty cycle of the three products is 90%. If VBAT = 2.7V, VOUT can be calculated according to the above equation: VOUT = (2.7 – 0.5) · 0.9 = 22.15V (1-0.9) + 2.7 - 0.35 With battery voltage decreasing, the VDS will increase. Here, we assume that VDS is 0.5V. So, under 2.7V VBAT, the solution cannot drive 8S WLED with max 30mA each. If VBAT = 3.3V, VOUT is 28.15V. Then under 3.3V VBAT, the solution can drive 8S WLED with max 30mA each. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202383A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 27, 2012 5 APPLICATION NOTE Multi-Channel WLED Driver Solution in Single-Cell, Li-Ion Battery Applications by AAT1451/1405/1407 and AAT3110 Measurement Result WLED 6S4P (6Series-4Parallel) For 6S4P WLEDs, there is a total of 24WLEDs as the load. After changing PWM dimming duty and varying the battery voltage, we obtain the measurement results shown in Figure 7. The WLED current changes linearly with PWM dimming duty and can get up to 30mA LED current at full PWM dimming duty. ILED vs PWM dimming duty for 8S4P 50 45 Vbat=2.7V LED Current (mA) 40 Vbat=3.0V Vbat=3.6V 40 60 Vbat=4.2V 35 30 25 20 15 10 5 0 0 10 20 30 50 70 80 90 100 PWM Dimming Duty (%) Figure 7: ILED vs PWM Dimming Duty for 6S4P Configuration Figures 8 shows that for 6S4P, the WLED maximum current is 30mA in the VBAT range of 2.7V to 4.2V. ILED vs Vbat 35 33 6S4P LED Current (mA) 31 29 27 25 23 21 19 17 15 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 Battery Voltage (V) 4.3 4.5 4.7 4.9 Figure 8: ILED vs VBAT for 6S4P Configuration The curves show that the WLED current can get to 30mA under any battery voltage from 2.7V to 4.2V. So these three products are suitable for the application when the WLED arrays are 6S4P 6 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202383A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 27, 2012 APPLICATION NOTE Multi-Channel WLED Driver Solution in Single-Cell, Li-Ion Battery Applications by AAT1451/1405/1407 and AAT3110 WLED 8S4P (8Series-4Parallel) For 8S4P, 32WLEDs, when the battery voltage is 2.7V, with the increase of PWM dimming duty, WLED current increases until the PWM dimming duty equal to about 80%. When the switching duty gets to its maximum value (90%), WLED current will not increase. See Figure 9. ILED vs PWM dimming duty for 8S4P 50 45 Vbat=2.7V LED Current (mA) 40 Vbat=3.0V Vbat=3.6V 40 60 Vbat=4.2V 35 30 25 20 15 10 5 0 0 10 20 30 50 70 80 90 100 PWM Dimming Duty (%) Figure 9: ILED vs PWM Dimming Duty for 8S4P Configuration Figures 10 shows that when the attery voltage is less than 3.2V, the WLED current cannot get to 30mA as derived earlier. ILED vs Vbat 35 8S4P 33 LED Current (mA) 31 29 27 25 23 21 19 17 15 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 Battery Voltage (V) Figure 10: ILED vs VBAT for 8S4P Configuration Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202383A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 27, 2012 7 APPLICATION NOTE Multi-Channel WLED Driver Solution in Single-Cell, Li-Ion Battery Applications by AAT1451/1405/1407 and AAT3110 WLED 6S6P (6Series-6Parallel) In a 6S6P configuration, there are 36 WLEDs as the load. Therefore, AAT1407 is used because it has 6 channels for current sink. When the battery voltage drops from 4.2 to 2.9V, WLED current will not change anymore, and when the battery voltage is less than 2.9V, the IC will get into over-temperature protection (OTP) mode because of the increased internal power dissipation. Figure 11 shows the maximum WLED current is 30mA during the whole battery voltage range from 2.9V to 4.2V. ILED vs Vbat 35 33 31 LED Current (mA) 29 27 25 6S6P 23 21 19 17 15 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 Battery Voltage (V) Figure 11: ILED vs VBAT for 6S6P Configuration Conclusion The solutions using AAT1451, AAT1405 and AAT1407 with AAT3110 meet the application requirements for single-cell, Li-Ion battery to drive 24~36 WLEDs with 30mA maximum LED current. The maximum WLED current is limited by the maximum switching duty cycle for some combinations of array at low battery voltage. The table below summarizes the details about the solutions using these products. AAT1451/AAT1405 + AAT3110 Configuration (Number of WLEDs) 6S4P (24 LEDs) 7S4P (28 LEDs) 8S4P (32 LEDs) Maximum WLED current up to 30mA VBAT = 2.7~4.2V VBAT = OK ILED=28mA at VIN=2.7V ILED=23mA at VIN=2.7V 3.3~4.2V OK OK OK Maximum WLED current up to 30mA VBAT = 2.7~4.2V VBAT = OK OK OTP at VIN<2.8V 3.3~4.2V OK OK OK AAT1407 + AAT3110 Configuration (Number of WLEDs) 4S6P (24 LEDs) 5S6P (30 LEDs) 6S6P (36 LEDs) 8 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202383A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 27, 2012 APPLICATION NOTE Multi-Channel WLED Driver Solution in Single-Cell, Li-Ion Battery Applications by AAT1451/1405/1407 and AAT3110 Copyright © 2012 Skyworks Solutions, Inc. All Rights Reserved. Information in this document is provided in connection with Skyworks Solutions, Inc. (“Skyworks”) products or services. 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Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202383A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 27, 2012 9