AND8192/D Charge Pump Based Multiple LED Driver Prepared by: Michael Bairanzade ON Semiconductor http://onsemi.com APPLICATION NOTE Abstract On the other hand, combining three functions in the same system creates a special case since the converter must be capable of driving the wide current load needed for the different functions. The typical currents used to drive the LED, summarized in Table 1, range from a low 1 mA to 350 mA when the flash is activated. Moreover, unlike the xenon photo flash, the LED system must have a relatively long pulse of light to properly illuminate the scene. Typically, a xenon pulse has a 1 ms flash duration, the LED system being in the 100 ms to 200 ms range. Consequently, the converter must be designed to support such a large demand. High powered LED capable to sustaining up to 800 mA are under development and drivers for these devices should be available within a few months. This application note describes a multi−functional system, capable of generating and controlling the power needed to utilized three features available in modern cellular phones. In addition to larger displays, with full color capability, flash and torch features have now been added to support the embedded camera and the night path finder. These features are made possible by using an ultra bright LED powered by standard battery cells. BASIC CIRCUIT DESCRIPTION Since the LED have a forward drop voltage ranging from 3 V to 4.5 V, depending upon the forward current, a straightforward connection to a standard battery is not feasible as depicted Figure 1. A boost structure must be used to make the power supply voltage compatible with the LED. Battery Voltage = F(Capacity) @ TA = +20°C OSRAM − LWY87S @10 mA−3.8 V 100 CITIZEN− CL590S @20 mA−3.9 V 90 NICHIA−NECWB205 @20 mA−4.0 V Absorbed Capacity (%) 80 OSRAM − LWT67C @20 mA−4.1 V 70 60 Vout−3 Cell Alkaline Vout−Li−ion 50 Vout−2 cell Alkaline 40 30 20 10 0 2 MB−JUNE 2004 2.5 3 2.0 X 3.5 1.5 X 4 1.33 X 4.5 5 Vout (V) 1.0 X Figure 1. Typical Lithium−Ion Battery Voltage and White LED Semiconductor Components Industries, LLC, 2004 December, 2004 − Rev. 0 1 Publication Order Number: AND8192/D AND8192/D Table 1. White LED Typical Applications LED Backlight OSRAM LWY85S 1 mA – 10 mA OSRAM – LWT67C 1 mA – 20 mA Torch OSRAM Flash 100 mA OSRAM – LWW5SG 350 mA CITIZEN − CL590S 1 mA – 20 mA NICHIA−NECWB205 1 mA – 20 mA LUMILED 800 mA LED, a good thermal contact to a dedicated layer on the printed board is essential. The LWW5SG specifications give a maximum 9°C/W junction−to−case thermal resistance, capable of limiting the temperature of the silicon to the 100°C maximum specified in the OSRAM data sheet. After dissipating 1.6 W, the maximum thermal to air resistance acceptable by the chip can be calculated as: Along with the amount of current the converter provides, it is worthwhile to note the thermal behavior of both the silicon and the power LED. According to the OSRAM’s data sheet, the Dragon LED (LWW5SG ) should have a maximum 4.5 V forward drop with 350 mA current. The power absorbed by the load will be 1.57 W and, assuming a 75% efficiency of the DC/DC converter, will translate to almost 2 W of input power. Consequently, some 400 mW will be dissipated as heat into the silicon and, according to the NCP5603 data sheet, the chip temperature will increase by RJA x Pin = 85 x 0.4 = 34°C. Such a temperature increase is acceptable since, even under the worst case +85°C ambient temperature, the junction will be below the maximum rating defined for this chip. However, we must take into account the low battery situation: in this case, the efficiency of the converter can decrease and we end up with 60% efficiency, yielding almost 54°C temperature increases. At this point, the silicon can rise above 125°C, under extreme high ambient temperature, and the global long−term reliability of the chip will be impaired. This can be avoided by either reducing the thermal resistance (using a heatsink by means of the PCB layer) or by ensuring the duty cycle is short enough to properly cool off the chip between pulses. Generally speaking, the High Intensity LED are power limited and care must be observed to avoid any thermal run out during normal operation. This is particularly true for the flash mode in which, as depicted above, nearly 1.6 W are dissipated into the LED junctions. Because the junction to ambient thermal resistance is limited by the packaging of the RJA RJA Tjmax Tamb Pchip 100 85 9.37°CW 1.6 Since the RJC is 9°C/W, it is practically impossible to achieve a 0.38°C/W case to ambient thermal resistance and the only alternative is to limit the operating ambient temperature. Assuming Tamb = 60°C, then RJA = (100−60) / 1.6 = 25°C/W. In this case, the case−to−ambient thermal resistance is 25 − 9 = 16°C/W, a value more realistic, although not so easy to achieve with a room limited PCB. NCP5603 operates without special treatment in terms of thermal sinking and a simple copper flag is built underneath the QFN package as depicted Figure 3. The schematic of the multiple application, Figure 2, illustrates the three functions: • Backlight four LED in parallel, dimming capability. • Torch one LED, no output adjustment. • Flash one power LED, pulse width adjustable. http://onsemi.com 2 AND8192/D GND J3 10 F/10 V 10R D5 R8 Q2 MGSF1N03L GND LW67C D3 82R R5 LW67C D2 82R R4 LW67C D1 82R R3 LW67C 82R R7 200R Q1 U4B GND 1 F 74HC08 U4A EN Vout U1 NCP5603 EN/PWM Fsel Vsel GND 1.5 k C6 R11 VCC 100 k 10 k TRA TRB R Q CNT/PWM FLASH 10 k S1 GND Figure 2. Multiple LED Driver Application 3 GND 10 k VCC VCC http://onsemi.com S6 S5 NL37WZ04 R1A U2A P1 100 kA 100 k GND R10 10 k NL37WZ04 10 k R13 R12 Adjust PWM 10 k R1C 10 k R1B + VCC 33 nF GND U2B 2 x 1.5 V − + + PK1 S3 Adjust FLASH S2 VSEL GND FSEL 100 nF/6.3 V VCC 200 kA R2 GND P2 C8 2.2 F TORCH GND S7 PWR CX C7 100 nF R1D GND R14 CLR Q D7 PWM A B C9 C 4.7 F/10 V Q GND RC C1 VCC U5A MC14538B EN FSEL VSEL GND RC 1 F/6.3 V Q GND C2 U5B MC74HC4538 C1P C1N C2N C1P C3 74HC08 C4 1 F/6.3 V GND MMBF0201N NL37WZ04 LWG6SC GOLDEN DRAGON 5.1R R6 D4 Vout Vbat Q3 MGSF1N03L GND 74HC08 TP1 GND R9 U2C D6 U4C C5 GND VCC AND8192/D TOP Layer BOTTOM Layer Figure 3. Printer Circuit Board GERBER Files (scale 1:1) http://onsemi.com 4 AND8192/D switch S1 is flipped to the Vcc position, the RESET of U5A is released and the EN pin is clocked High / Low by the clock generated by U2A / U2B. Simultaneously, diode LED D7 turns ON to identify the PWM mode of operation. The duty cycle of the U5A / Q output is manually adjusted by potentiometer P1 to set the brightness of the four associated LED. The efficiency of the system has been evaluated at room temperature (see Table 2), the results being fully within the NCP5603 data sheet specifications. The system is powered by two AA cells in series, assembled in a standard battery holder, the operating mode being selected by the S1, S5 and S6 switches. Since the total current is limited by the DC/DC converter, the backlights LEDs are automatically deactivated when either the Torch or the Flash are selected. Moreover, the Flash is not available while the Torch is running. An extra feature, backlight dimming, is provided by switch S1 is associated with potentiometer P1. When the switch is connected to ground, the NCP5603 enabling pin EN is high and the brightness is maximized. When the Table 2. Demo Board Efficiency Vbat Ibat Vout Iout/LED Iout Total Yield Comments 3.50 V 2.3 mA 0V 0 mA 0 mA − No Load 3.50 V 132 mA 4.42 V 16.5 mA 66 mA 63.14% 3.50 V 170 mA 4.92 V 21.4 mA 85.6 mA 70.78% 3.10 V 131 mA 4.42 V 16.5 mA 66 mA 71.83% 3.10 V 169 mA 4.92 V 21.4 mA 85.6 mA 80.38% 3.10 V 300 mA 4.92 V 142 mA 142 mA 75.12% Torch operation The inrush current is internally limited by the chip, as depicted Figure 4, and no uncontrolled current takes place when the system starts up from scratch. Figure 5. Typical Digital Dimming Although there is no dedicated pin, the LED brightness can be dimmed by means of the EN digital control. The waveform captured in Figure 5 illustrate this behavior, the PWM being intentionally arranged out of the audio band for a portable system. Figure 4. Typical Startup Timing With a startup time well below 1 ms (from zero to full Vout, see Figure 4), the NCP5603 is fast enough to accommodate a flash application as shown in the demo board. http://onsemi.com 5 AND8192/D NCP5603 – MULTIPLE DRIVE CIRCUIT − Bill Of Material QTY Designator Description Footprint Manufacturer Part Number 1 R1 10 k, pack four independent elements SIP−5 BOURNS 4605X series 3 R2, R10, R13 10 k 0805 Vishay Draloric 4 R3, R4, R5, R6 82 0805 Vishay Draloric 1 R7 200 0805 Vishay Draloric 1 R8 1.2 0805 Vishay Draloric 1 R9 10 0805 Vishay Draloric 2 R11, R12 100 k 0805 Vishay Draloric 1 R14 1.5 k 0805 Vishay Draloric 1 P1 100 k A Potentiometer VR4 BOURNS 3386F−TW 1 P2 200 k A Potentiometer VR4 BOURNS 3386F−TW 1 C1 4.7 F/10 V 1206 TDK 3 C2, C3, C4 1 F/6.3 V 1206 TDK 1 C5 10 F/10 V 1206 TDK 1 C6 33 nF 0805 KEMET 1 C7 2.2 F 0805 TDK 2 C8, C9 100 nF 0805 KEMET 4 D1, D2, D3, D4 LW67C OSRAM_LED OSRAM 1 D5 GOLDEN DRAGON OSRAM_DRAGON OSRAM 1 D6 LWG6SC OSRAM_LWG OSRAM 1 D7 LED OSRAM_LED OSRAM 1 Q1 MMBF0201N SOT_23A ON Semiconductor 2 Q2, Q3 MGSF1N03L SOT_23A ON Semiconductor 1 U1 NCP5603 QFN10_COB ON Semiconductor 1 U2 NL37WZ04 US8 ON Semiconductor 1 U4 74HC08 SO−14 ON Semiconductor 1 U5 MC14538B SO−16 ON Semiconductor 4 S1, S2, S3, S6 Toggle Switch APEM_CMS APEM TL36WS84000 1 S5 Push Button PUSH_BUT_B CANNON ITT KSA 0M210 1 S7 Toggle Switch CKSWITCH_V C&K ET01MD1 CBE 1 TP1 TEST POINT TEST_POINT KEYSTONE 5005 (THM) 1 J3 GND GND_TEST HARWIN D3082−01 (tin) D3082−05 (gold) 1 PK1 2 x 1.5V Battery holder, 2 x AA BPACK2 KEYSTONE 2223 LWW5SG ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). 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