APPLICATION NOTE AAT1239-1 Enhanced Efficiency Application Solution Introduction The AAT1239-1 is a varied frequency, constant current boost converter capable of driving up to 20 white LEDs. This paper discusses how to select external components for the AAT1239-1 and enhance its efficiency based on power loss analysis in Li-ion battery applications. AAT1239-1 Typical Application Connection Figure 1 shows a typical application connection for the AAT1239-1. The AAT1239-1 can drive up to 20 WLEDs (10 in series, and 2 branches). S2C High or low level 1 VIN 3.0V~5.5V U1 AAT1239-1 PVIN EN/SET 3 SEL 2 C1 4 5 6 L1 12 LIN 11 OVP 10 FB 9 VIN AGND N/C SW PGND 7 SW 8 DS1 SS16L IOUT R2 C2 R3 12k VOUT D11 D21 D12 D22 D13 D23 D1x D2x R1 Figure 1: AAT1239-1 Typical Application Connection. Power Loss Analysis Several factors influence the AAT1239-1's efficiency, including switching loss, PMOSFET disconnect power loss, inductor copper loss, control circuit power consumption, and external resistor power loss. Each factor will be analyzed and calculated in the following sections. PMOSFET Power Loss The AAT1239-1 has a P-channel MOSFET connected between PVIN and LIN. This P-channel MOSFET senses input current. Power loss on the P-channel MOSFET is determined by RDS(ON) current level. Figure 2 shows the power loss of the P-channel MOSFET vs. current level. The curve shows that the power loss is less than 25mW when the current level is less than 30mA. Table 1 shows the AAT1239-1’s input current and PMOSFET power loss with a 2.2µH inductor driving 10 WLEDs in series (31V VOUT) with 20mA LED current. The table shows that the PMOSFET power loss is only 1.5%. This has little effect on the efficiency of the application. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202371A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 1 APPLICATION NOTE AAT1239-1 Enhanced Efficiency Application Solution 100 Power Loss (mW) 90 80 70 60 50 40 30 VIN = 3.0V VIN = 3.6V VIN = 4.2V 20 10 0 0 50 100 150 200 250 300 350 400 450 500 550 600 Current (mA) Figure 2: AAT1239-1 PMOSFET Power Loss vs. Current. Measurement Data Calculation Result L (µH) VIN (V) IIN (mA) VOUT (V) IOUT (mA) RDS(ON) (mΩ) PIN (mW) PLOSS (mW) PLOSS/PIN (%) 2.2 3.61 251 31.2 20.8 211 906 13.3 1.5 Table 1: AAT1239-1 PMOSFET Power Loss with 2.2µH Inductor. Inductor Copper Loss Inductor copper power loss is determined by the RMS current flowing through the inductor and the inductor DCR value. Inductor RMS current is a root-mean-square value of inductor current which includes DC and AC factors. Inductance directly affects inductor AC current, also called ripple current. The ripple current in Continuous Conduction Mode from a boost converter can be derived from the following formula: ∆IINDUCTOR_PP = VIN · D f·L Higher inductance creates a lower ripple current; lower inductance creates a higher ripple current. Figure 3 shows a AAT1239-1 operating waveform at 3.6V VIN, 31V VOUT (10 white LEDs in series), 20mA LED current with 2.2µH and 10µH inductors with similar DCR (48mΩ). Differences in inductor RMS current can be observed at varying inductance, leading to differences in inductor copper loss. For example, with a 2.2µH (CDRH2D18/HP-2R2) inductor: PIN = VIN · IIN = 3.61 · 251 = 906mW PLOSS_L = I2L_RMS · DCR = (336mA)2 · 48mΩ = 5.4mW PLOSS_L 5.4 = · 100% = 0.6% PIN 906 Table 2 illustrates inductor copper loss with 2.2µH and 10µH inductors, 10 WLEDs in series (31V VOUT), and 20mA LED current. In the AAT1239-1 application, inductor copper loss leads to approximately 2% power loss. 2 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202371A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 APPLICATION NOTE AAT1239-1 Enhanced Efficiency Application Solution (a) 2.2µH inductor: CDRH2D18/HP-2R2 (b) 10µH inductor: CDRH5D28-100 Figure 3: AAT1239-1 Operating Waveform with Different Inductors at 3.6V VIN, 10 WLEDs in series (VOUT = 31V), 20mA LED Current; Ch1: SW, Ch3: IINDUCTOR. Inductor Measurement Data Calculation Result Inductance (µH) Part Number DCR (mΩ) VIN (V) IIN (mA) IL_RMS (mA) PIN (mW) PLOSS (mW) PLOSS/PIN (%) 2.2 10 10 CDRH2D18/HP-2R2 CDRH5D28-100 CDRH3D18-100 48 48 164 3.61 3.61 3.61 251 227 223 336 287 282 906 819 805 5.4 4.0 13.0 0.6 0.5 1.6 Table 2: AAT1239-1 Inductor Copper Loss at Different Inductor Values. Switching Loss Switching transitions of the power MOSFET can lead to a very large instantaneous power loss. Even though transition times are short, the resulting average power loss is significant. The average switching loss is positively proportional to the switching frequency, switching transition time, etc. The switching frequency of the AAT1239-1 varies with different conditions (VIN, VOUT, IOUT and L) according to its constant current control structure. This leads to different switching losses with varying inductor values. A large switching frequency difference is illustrated in Figure 3. As a result, there is an 87mW difference in switching loss, which is 9.6% of the total power at 3V VIN as shown in Table 3. Measurement Data Calculation Result L (µH) VIN (V) IIN (mA) VOUT (V) IOUT (mA) Freq. (MHz) PIN (mW) POUT (mW) 2.2 10 2.2 10 3.0 3.0 4.2 4.2 300 271 216 196 31.4 31.4 31.4 31.4 20.8 20.8 20.9 20.8 1.49 0.58 1.87 0.94 900 813 907 823 653 653 654 654 P∆ (mW) P∆/PIN (%) 87 9.6 84 9.3 Table 3: Power Loss Difference Mainly Caused by Switching Loss. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202371A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 3 APPLICATION NOTE AAT1239-1 Enhanced Efficiency Application Solution The switching frequency of the AAT1239-1 increases as input voltage increases and decreases as inductance increases. Figure 4 shows at VIN = 3.0V, different inductance, frequency vs. IOUT curve. The curve shows that the AAT1239-1's switching frequency is 1.49MHz for 2.2µH, 0.86MHz for 4.7µH, and 0.58MHz for 10µH at 3.0V VIN. The frequency change is almost 2.6 times greater when the inductor is changed from 2.2µH to 10µH. Paired with Table 3, this tells us that a 10µH inductor can decrease the switching loss by 9% compared to a 2.2µH inductor. 1.8 L = 2.2µH L = 4.7µH L = 10µH Frequency (MHz) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 10 20 30 40 50 60 70 80 90 IOUT (mA) Figure 4: AAT1239-1 Frequency vs. IOUT at VIN = 3.0V. Control Circuit and External Resistor Power Loss The quiescent current of AAT1239-1 is only 5µA at 3.6V VIN when SEL and EN = logic high, VFB = 0.6V and no LED load. At this instant, the AAT1239-1 internal control circuit only consumes 0.02mW. External power consumption is from the OVP divider (R2 and R3) and the ballast resistor (R1). Even with a load of 10 WLEDs in series (VOUT voltage is assumed to be 31V), the OVP resistor divider power consumption is 2.5mW which is only 0.3% of the total 906mW at 3.6V VIN. Table 4 gives a summary of the above analysis and illustrates that the switching loss is the most significant power loss. L (µH) PMOSFET Power Loss (%) Inductor Copper Loss (%) Switching Loss (%) Control Circuit and External Resistor Power Loss (%) 2.2 10 1.5 1.4 2 2 24 12 0.3 0.3 Table 4: AAT1239-1 Power Loss Summary at 3.6V VIN, 10 WLEDs in Series, 20mA LED Current. Application Solution with Enhanced Efficiency From the efficiency analysis above, decreasing the switching loss can remarkably improve efficiency by increasing the inductance of the inductor. Inductance of the inductor can not be selected randomly because the designer has to consider the loop stability and application circuit size. Proper inductor selection is important in the actual application. Under sizing or over sizing inductance may cause instability to the AAT1239-1’s control loop which might generate a greater output ripple (above 500mV) and cause the inductor to vibrate. A bigger inductor value with higher saturation current might lead to the increase of the total application solution (circuitry size). 4 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202371A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 APPLICATION NOTE AAT1239-1 Enhanced Efficiency Application Solution Inductor and Capacitor Selection In inductor selection, inductance and saturation current are the most important two factors. The principle of inductor selection for AAT1239-1 is that the IC should produce stable LED current under whole application operating range. Table 5 gives a selection guide of inductance, output capacitance and loop stable area when AAT12391-1 drives up to 10 WLEDs in Li+ battery application. For example, in the application of 10S2P (10 WLEDs in series and 2 in parallel) with 20mA LED current in each string, both a 2.2µH and a 4.7µH inductance can be used. However, the combination of a 4.7µH inductor and a 2.2µF output capacitor is more efficient. Loop Stable Area L (µH) COUT IOUT_MAX (mA) Operating Range (V) 2.2 4.7 10 10 2.2µF/50V 2.2µF/50V 2.2µF/50V 4.7µF/50V 60 40 25 30 3.0 ~ 5.5 Table 5: AAT1239-1 Stable Area with Inductor and Output Capacitor Combination. Inductor saturation current is another important factor. The saturation current specified in the inductor datasheet represents the current when inductance become 35% lower than its nominal value. Under a basic boost structure, in CCM (Continuous Current Mode), the inductor peak current value is determined by the following formula: IINDUCTOR_PEAK = ILOAD V 1 + IN · D · 1·D 2·L f D=1- VIN VOUT This formula is also suitable for AAT1239-1. The device's frequency varies by up to 2MHz when VIN, IOUT and L change. Though we can calculate inductor peak current value based on the formula, it is difficult to derive the value because there is no formula to estimate the frequency. Figure 5 illustrates inductor peak current value vs. output current at 3.0V VIN. 3.0V VIN is the least suitable input voltage condition in most applications. The customer can select the minimum inductor saturation current based on Figure 5. For example, when IOUT is 40mA, saturation current for a 4.7µH inductor should be higher than 1.1A. IINDUCTOR_MAX (mA) 2.00 1.75 1.50 1.25 1.00 0.75 L = 2.2µH L = 4.7µH L = 10µH 0.50 0.25 0.00 10 20 30 40 50 60 IOUT (mA) Figure 5: AAT1239-1 Inductor Peak Current Value Under Different Inductance at VIN = 3.0V. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202371A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 5 APPLICATION NOTE AAT1239-1 Enhanced Efficiency Application Solution Resistor Selection The OVP resistor divider (R2 and R3) threshold is used to detect the trip voltage if one of the LEDs in the string is disconnected. The output voltage is limited and will recover once the open connection condition is removed. Recommended OVP resistor selection when R1 is equal to 12kΩ is shown in Table 6. Number of LED (in series) R2 (kΩ) OVP Trip Point (V) 4 5 6 7 8 9 10 158 182 215 255 287 324 374 15.8 18.2 21.5 25.5 28.7 32.4 37.4 Table 6: AAT1239-1 OVP Resistor Selection. Resistor R1 is used to program the LED current. The value of R1 can be calculated using the following equation: R1 = 0.6V ILED Table 7 shows several resistor values corresponding to the most commonly used LED current levels. R1 (Ω) Maximum ILED (mA) SEL = High SEL = Low 40 35 30 25 20 15 10 5 15.0 16.9 20.0 24.3 30.1 40.2 60.4 121.0 10.0 11.3 13.3 16.2 20.0 26.7 40.2 80.6 Table 7: Maximum LED Current and R1 Resistor Values (1% Resistor Tolerance). Application Solution Example Table 8 illustrates two application examples of external component selection. The key factor is the inductor. As shown in Table 5, a 10µH inductor can be used for a 10S/20mA application (10 WLEDs in series and 20mA LED current). According to Figure 5, the 10µH inductor should have greater than 0.6A saturation current for the full scale LED current (20mA). The Sumida CDRH3D18-100NC SMD inductor is suitable for the application and has a 0.9A saturation current and an inductor size of 4.0x4.0x2.0mm. 6 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202371A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 APPLICATION NOTE AAT1239-1 Enhanced Efficiency Application Solution Application: WLED/IOUT(MAX) Component C1 L1 C2 R1 (Ω) R2 (kΩ) 10S/20mA 10S2P/40mA 4.7µF/10V/0805 GRM219R61A475KE19 10µH CDRH3D18-100NC 2.2µF/50V/1206 GRM31CR71H225KA88 30.1 374 4.7µF/10V/0805 GRM219R61A475KE19 4.7µH CDRH4D22/HP-4R7 2.2µF/50V/1206 GRM31CR71H225KA88 15 374 90.0 85.0 87.5 82.5 85.0 80.0 82.5 80.0 77.5 VIN = 3.0V VIN = 3.6V VIN = 4.2V 75.0 72.5 70.0 2 4 Efficiency (%) Efficiency (%) Table 8: Application Solution Example. 6 8 10 12 14 16 18 Output Current (mA) (a) 10S/20mA Solution Efficiency 77.5 75.0 72.5 VIN = 3.0V VIN = 3.6V VIN = 4.2V 70.0 67.5 65.0 20 5 10 15 20 25 30 35 40 Output Current (mA) (b) 10S2P/40mA Solution Efficiency Figure 6: Application Solution Efficiency. Summary Power loss sources in the AAT1239-1 include switching loss, PMOSFET disconnect power loss, inductor copper loss, control circuit power consumption, and external resistor power loss. Among these, switching loss is the highest power loss source. Increasing inductance can significantly improve the efficiency of the AAT1239-1 by decreasing the switching frequency, which reduces switching loss. However, large inductance can make the AAT1239-1 unstable. Designers should select the correct inductance for their specific application according to Table 5. It is also important to choose an inductor with sufficient inductor saturation current as shown in Figure 5. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202371A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 7 APPLICATION NOTE AAT1239-1 Enhanced Efficiency Application Solution Copyright © 2012 Skyworks Solutions, Inc. All Rights Reserved. Information in this document is provided in connection with Skyworks Solutions, Inc. (“Skyworks”) products or services. These materials, including the information contained herein, are provided by Skyworks as a service to its customers and may be used for informational purposes only by the customer. Skyworks assumes no responsibility for errors or omissions in these materials or the information contained herein. Skyworks may change its documentation, products, services, specifications or product descriptions at any time, without notice. Skyworks makes no commitment to update the materials or information and shall have no responsibility whatsoever for conflicts, incompatibilities, or other difficulties arising from any future changes. No license, whether express, implied, by estoppel or otherwise, is granted to any intellectual property rights by this document. Skyworks assumes no liability for any materials, products or information provided hereunder, including the sale, distribution, reproduction or use of Skyworks products, information or materials, except as may be provided in Skyworks Terms and Conditions of Sale. THE MATERIALS, PRODUCTS AND INFORMATION ARE PROVIDED “AS IS” WITHOUT WARRANTY OF ANY KIND, WHETHER EXPRESS, IMPLIED, STATUTORY, OR OTHERWISE, INCLUDING FITNESS FOR A PARTICULAR PURPOSE OR USE, MERCHANTABILITY, PERFORMANCE, QUALITY OR NON-INFRINGEMENT OF ANY INTELLECTUAL PROPERTY RIGHT; ALL SUCH WARRANTIES ARE HEREBY EXPRESSLY DISCLAIMED. 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Customers are responsible for their products and applications using Skyworks products, which may deviate from published specifications as a result of design defects, errors, or operation of products outside of published parameters or design specifications. Customers should include design and operating safeguards to minimize these and other risks. Skyworks assumes no liability for applications assistance, customer product design, or damage to any equipment resulting from the use of Skyworks products outside of stated published specifications or parameters. Skyworks, the Skyworks symbol, and “Breakthrough Simplicity” are trademarks or registered trademarks of Skyworks Solutions, Inc., in the United States and other countries. Third-party brands and names are for identification purposes only, and are the property of their respective owners. Additional information, including relevant terms and conditions, posted at www.skyworksinc.com, are incorporated by reference. 8 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202371A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012

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