RD-168 LM3423 BCS2015G1 LM556CM LM324AM LMV431IM5 CMSD6263 BAT54-7-F LM3423MH

RD-168 Applications Design Center November 2008 1.0 Design Specifications Inputs Output #1 VinMin=6V Vout1=20V VinMax=12V

National Semiconductor RD-168 RD-168 Applications Design Center November 2008 1.0 Design Specifications Inputs Output #1 VinMin=6V Vout1=20V VinMax=12V Iout1=0.1A The ambient light adaptive LED driver automatically adjusts display brightness for changing ambient light conditions, providing a more viewable display with better contrast. The design drives five series connected LEDs from an input voltage of 4.5V to 12V. The LED current is nominally 100 mA and the output voltage is 15 to 20V. amplitude of the LED current constant, but periodically turning the LED driver on and off, thus varying the LEDs' average current and light intensity. The switching is at done at a low frequency, but one which is high enough for the human eye to sense only changes in the light's average intensity. Linear dimming, where the amplitude of the current in the LEDs is modulated, is not suitable because the LEDs' light color temperature is sensitive to the current's amplitude. The heart of the design is National's PowerWise® LM3423 LM3423 LED driver which has all the features needed to implement a boost converter with output current regulation and PWM dimming of the current. The LM3423 LM3423 achieves output current regulation by sensing and appropriately adjusting the voltage across a resistor connected in series with the LED string. Ambient Light Adaptive LED Driver Ambient Light Adaptive LED Driver The dimming range of the average LED current is from 20 to nearly 100% percent of its nominal value as ambient light conditions increase from dark to bright. To reduce power consumption and prolong battery life the output current is clamped to no more than 50% of the nominal value when the input (battery) voltage falls below a predetermined level. 2.0 Design Description Dimming or brightening the backlight of a display in response to ambient light condition can greatly increase the life of batteries in portable devices, and improves the visibility of the display under different viewing conditions. In this design a photodiode circuit is used to measure ambient light and provide a signal for dimming the LEDs. PWM dimming is the preferred method of dimming because it allows the light's color temperature to remain constant irrespective of its intensity. PWM dimming is done by keeping the 3.0 Features 4.5-12V battery input voltage range 100 mA Constant current LED at 15-20V High switching frequency High efficiency (85%) PWM LED dimming up to 25kHz LED brightness limited to less than 50% of maximum at low input voltage to prolong battery life LED brightness proportional to ambient light RD-168 RD-168 © 2008 National Semiconductor Corporation www.national.com D C B A GND C26 0.01uF 1 D3 BCS2015G1 BCS2015G1 C16 0.1uF R14 10.0k R10 100k Discharge Vcc LM556CM LM556CM GND Trigger Output Reset 100k R25 5 6 C22 4.7uF V+ V- B R32 100k R31 100k VIN 7 C21 0.1uF Vref V+ V- D 2 U3D LM324AM LM324AM 14 REF1 LMV431IM5 LMV431IM5 Low battery detector 12 13 1 C11 0.1uF A K 10.0k R26 C14 0.22uF R7 100k VOUT U3A LM324AM LM324AM CV V+ A V- D4 CMSD6263 CMSD6263 3 2 1.00k 8 9 10 11 12 13 14 Light to Voltage Converter U3B LM324AM LM324AM VIN R23 Tirgger Output Reset Control Voltage Control Voltage Threshold Threshold Discharge VIN PWM Modulator for LED Dimming 7 6 5 4 3 2 1 U2 4 11 VIN=6-12V 1 3 2 1 3 1 D2 BAT54-7-F BAT54-7-F NC R22 2.00k C5 10uF 3 2 1 www.national.com 10 9 Vref 10 D5 3.3V 3 C19 CV 0.01uF C8 2.2uF 4 10 3 8 2 11 9 5 16 1 U1 CSH TIMR COMP nDIM EN L-RDY FLT RCT VCC VIN AGND DDRV RPD OVP DPOL HSN HSP PGND IS GATE LM3423MH LM3423MH 6 13 18 7 12 20 19 14 17 15 4 C17 47pF C13 47pF C12 47pF C10 100pF 1.00k R5 2.00k R13 R6 0.5 R18 4.02k R16 100k 470pF C25 Q1 Si3442BDV Si3442BDV D1 4 National Semiconductor and/or its licensors do not warrant the accuracy or completeness of this specification or any information contained therein. National and/or its licensors do not warrant that this design will meet the specifications, will be suitable for your application or fit for any particular purpose, or will operate in an implementation. National and/or its licensors do not warrant that the design is production worthy. You should completely validate and test your design implementation to confirm the system functionality for your application. 12.4k R20 0.022uF C18 C7 0.1uF C15 1000pF 100k R12 100k R9 R2 51.1k 8 21.0k R19 1200pF C9 V+ V- C R4 U3C LM324AM LM324AM R24 100k C6 0.1uF 47uH L1 3 FIGURE 1. LED Driver Board Schematic 2 1.00k R33 C2 10uF 2.00 R1 VOUT Vout=15-20V LED+ LED- I_LED=100mA 6 5 6 D C B A schematic2 Designed for: Public Release Mod. Date: 11/9/2008 Project: Ambient Light Adaptive LED Driver Sheet Title: Ambient Light Adaptive LED Driver Sheet: 1 of 1 Size: B Schematic: 870xxxxxx Rev: X1 Assembly Variant: variantName: No Variant Selected PADC: http://www.national.com File: LM3423 LM3423_RefDes_Proto.SchDoc Contact: http://www.national.com/support © Copyright, National Semiconductor, 2008 R11 2.00k 5 RD-168 RD-168 4.0 Schematic RD-168 RD-168 5.0 Bill of Materials Designator Value C2 10uF C5 10uF C6 0.1uF C7, C11, 0.1uF C21 C8 2.2uF C9 1200pF C10 100pF PackageReference 1206 1210 1206 0805 Characteristics Ceramic, X5R, 25V, 20% Ceramic, X7R, 25V, 20% Ceramic, X7R, 50V, 10% Ceramic, X7R, 25V, 10% Manufacturer Taiyo Yuden TDK Yageo America AVX PartNumber TMK316BJ106ML-T TMK316BJ106ML-T C3225X7R1E106M C3225X7R1E106M 12062R104K9B20D 12062R104K9B20D 08053C104KAT2A 08053C104KAT2A RoHS 0805 0805 0805 Ceramic, X5R, 16V, 10% Ceramic, X7R, 100V, 10% Ceramic, C0G/NP0, 100V, 1% AVX AVX AVX 0805YD225KAT2A 0805YD225KAT2A 08051C122KAT2A 08051C122KAT2A 08051A101FAT2A 08051A101FAT2A Y Y Y Y Y C12, C13 47pF 0805 Ceramic, C0G/NP0, 100V, 5% AVX 08051A470JAT2A 08051A470JAT2A Y C14 C15 C16 C17 0.22uF 1000pF 0.1uF 47pF 0805 0805 0805 0805 AVX AVX TDK AVX 08053C224MAT2A 08053C224MAT2A 08051C102KAT2A 08051C102KAT2A C2012X7R1E104K C2012X7R1E104K 08051A470KAT2A 08051A470KAT2A Y Y Y Y C18 C19, C26 C22 C25 D1 0.022uF 0.01uF 4.7uF 470pF 0.5V 0805 0805 0805 0805 SMA Ceramic, X7R, 25V, 20% Ceramic, X7R, 100V, 10% Ceramic, X7R, 25V, 10% Ceramic, C0G/NP0, 100V, 10% Ceramic, X7R, 100V, 10% Ceramic, X7R, 50V, 10% Ceramic, X7R, 10V, 20% Ceramic, X7R, 50V, 10% Vr = 25V, Io = 1.5A, Vf = 0.5V 08051C223KAT2A 08051C223KAT2A 08055C103KAT2A 08055C103KAT2A C2012X7R1A475M C2012X7R1A475M 08055C471KAT2A 08055C471KAT2A BYS10-25-E3/TR BYS10-25-E3/TR Y Y Y Y Y D2 0.65V SOT-23 Vr = 30V, Io = 0.2A, Vf = 0.65V AVX AVX TDK AVX VishaySemiconductor Diodes Inc. BAT54-7-F BAT54-7-F Y D3 D4 0.24V SOT-323 Vr = 70V, Vf = 0.41V BCS2015G1 BCS2015G1 CMSD6263 CMSD6263 Y O D5 3.3V SOT-23 0.225W TDK Central Semiconductor ON Semiconductor BZX84C3V3ET1G BZX84C3V3ET1G Y Terminal, Turret, TH, Double Keystone Electronics 1503-2 Y Shielded Drum Core, 1.6A, 0.128 Ohm Coilcraft Inc. MSS1038-473MLB MSS1038-473MLB Y GND, Double I_LED=100m A, LED-, VIN=6-12V L1 47uH MSS1038 MSS1038 Q1 R1 R2 R4 R5, R23, R33 R6 30V 2.00 51.1k 10 1.00k SOT23-6 0805 0805 0805 0805 1%, 0.125W 1%, 0.125W 5%, 0.125W 1%, 0.125W Vishay Vishay-Dale Vishay-Dale Vishay-Dale Vishay-Dale Si3442BDV Si3442BDV CRCW08052R00FNEA CRCW08052R00FNEA CRCW080551k1FKEA CRCW080510R0JNEA CRCW080510R0JNEA CRCW08051k00FKEA Y Y Y Y Y 0.5 1206 1%, 0.25W Susumu Co RL1220S-R47-F RL1220S-R47-F Y bom2 FIGURE 2. LED Driver Board BOM Page 1 of 2 3 www.national.com RD-168 RD-168 Designator R7, R9, R10, R12, R16, R24, R25, R31, R32 Value 100k PackageReference 0805 Characteristics 1%, 0.125W Manufacturer Vishay-Dale R11, R13, R22 R14, R26 R18 R19 R20 REF1 2.00k 0805 1%, 0.125W Vishay-Dale CRCW08052k00FKEA Y 10.0k 4.02k 21.0k 12.4k 0805 0805 0805 0805 MF05A MF05A 1%, 0.125W 1%, 0.125W 1%, 0.125W 1%, 0.125W Low-Voltage (1.24V) Adjustable Precision Shunt Regulator N-Channel Controllers for Constant Current LED Drivers Vishay-Dale Vishay-Dale Vishay-Dale Vishay-Dale National Semiconductor CRCW080510k0FKEA CRCW08054k02FKEA CRCW080521k0FKEA CRCW080512k4FKEA LMV431IM5 LMV431IM5 Y Y Y Y O National Semiconductor LM3423MH LM3423MH Y National Semiconductor National Semiconductor LM556CM LM556CM O LM324AM LM324AM O U1 MXA20A MXA20A U2 M14A Dual Timer U3 M14A Low Power Quad Operational Amplifier PartNumber RoHS CRCW0805100kFKEA Y b FIGURE 3. LED Driver Board BOM Page 2 of 2 6.0 Other Operating Values Operating Values Description Parameter Value Unit Modulation Frequency Frequency 300 KHz Total output power Pout 2 W Peak-to-peak ripple voltage Vout p-p n/a mV Static load regulation Static load n/a mV Dynamic load regulation Dynamic load n/a mV Steady state efficiency Efficiency 85 % 7.0 Board Photos www.national.com 4 RD-168 RD-168 boardphoto FIGURE 4. 4_LED_Driver_2482 5 www.national.com RD-168 RD-168 If there is a fault and the LED strings open, the resistors R16 and R18 limit the output voltage to a safe value that is set to be somewhat greater than the highest normal operating voltage. 8.0 Quick Start A photograph of the reference design is shown in Fig 3. To operate the board follow the steps outlined below. 1. Connect an LED string with a rated current of 100 mA at a voltage in the 15-20V range between the LED+ (anode side) and LED- posts (cathode side) 2. Apply a 9V battery to the VIN and GND pins which are adjacent to each other 3. The LED string should now be lighted. 4. To investigate the dimming function, shine a light more or less directly at the photodiode D4 on the board and notice how the Leeds brighten or dim according to whether there is more or less light falling on D4. 5. The equations 5, 6 and 7 given above can be used to fine tune the dimming function to conform to the characteristics of the application being designed for. 6. To investigate the battery life-extending function, replace the battery with an adjustable power supply set at 9V, ensure that a bright light is shining on the photo diode, and observe that the LEDs are on. 7. Now reduce the power supply voltage, and notice that when the voltage falls to about 6V, the LEDs' brightness drops to about 50% and stays there. Notice also that the brightness can fall below this value if the light falling on the photodiode decreases, but it cannot increase above 50%. 8. The equations 8 through 10 can be used to select the voltage level at which the brightness changes. The LM3423 LM3423 supports dimming of the LEDs via the nDIM pin. A PWM signal applied to the nDIM pin turns the boost converter on and off, allowing and interrupting LED current flow, thus achieving dimming. If faster dimming than is possible by this method is desired, it can be implemented by switching on and off with a signal from the DDRV pin (this signal is derived from that at the nDIM pin) a MOSFET placed in the LED current path. See application note AN1872 AN1872 for details on exactly how to do this. It is also desirable for the brightness of the LEDs to be proportional to ambient light conditions so that the display is not too bright in dark conditions and is easy to view under bright light. A photodiode circuit comprising D3, op-amp U3A and U3B and associated components is used to implement this. If diode D4 is used the output voltage of this circuit appearing at pin 7 of U3B is exponentially proportional to the light intensity sensed by D3. If D4 is omitted then U3B becomes a voltage follower and the output voltage of the circuit is simply directly proportional to the light intensity. A circuit comprising D5, R22, R24 and R26 applies an offset voltage to the dimming voltage so that even if there is no ambient light a minimum voltage representing a minimum brightness level remains at pin 10 of UC3C A comparator circuit built around U3D monitors the input battery voltage. If the battery voltage falls below about 6.6V the output of U3D goes low, and a clamping voltage of about 2V created by reference REF1 and diode D2 is imposed on pin 10 of U3C. Thus no matter what the ambient light conditions are the voltage at pin 10 can not go above 2V, though it if free to assume any value below 2V. The 2V represents a maximum dimming duty ratio of about 50%. Limiting the maximum duty ratio in this way extends battery life under bright light conditions. 9.0 Hardware Description The reference design schematic is shown in Figure 1 and the bill of materials is shown in Figures 2 and 3. A battery input voltage powers a boost converter comprising U1 and the power components C5, C6, L1, Q1, D1 and C2. The output voltage of the boost converter, which appears across C2, is given by Eq .1. Eq. 1 is derived as follows. One switching period consists of a time interval DT during which Q1 is on, and a complementary interval (1-D)T during which D1 is on. During Q1's on time the voltage across the inductor L1 is the input voltage, causing the inductor current to increase. When Q1 goes off the diode D1 turns on and the inductor current commutates into it to the output capacitor and the LED string. Under steady state conditions the output voltage is greater than the input voltage, so that during the (1-D)T interval the voltage across the inductor is equal to the difference between the output and input voltages. The net volt-seconds across the inductor are zero over one switching period, as given by Eq. 2 The voltage at pin 8 of voltage follower buffer U3C is the control voltage to which dimming duty ratio of the LEDs is proportional. The PWM dimming signal is generated by a PWM modulator built around an LM556 LM556 timer, U2. The left-hand side of U2 is configured as an astable multivibrator that generates a clock of narrow negative pulse and determines the dimming frequency. The dimming frequency is given by Eq. 4. The astable multivibrator's output voltage is used as the clock for a PWM modulator built around the right-hand side of U2. Each time the astable multivibrator's output (pin 5) produces a negative pulse the PWM modulator starts a new switching cycle. The output voltage of the modulator at pin 9 goes high and the timing capacitor C14 starts charging from zero to the output voltage through R7 according to Eq. 5. The output voltage of the dimming circuit at pin 8 of U3C is applied as a control voltage to the PWM modulator at pin11 of U2 (Note that U3C is a buffer which is required between the dimming circuit and the control pin of the PWM modulator, which has a low impedance.). When the rising voltage across capacitor C14 intersects this control voltage the output of the modulator appearing at pin 9 of U2, and applied to the nDIM pin of U1, goes low, and stays low until the multivibrator starts a new switching cycle. Consequently the dimming duty ratio Rearranging Eq. 2 gives Eq. 1. The output current of the boost converter flows to the LEDs through a high side sense resistor R1. High side sensing reduces the number of wires in a system with many parallel LED strings because the cathodes of all the LEDS share a common ground. The sense resistor's voltage is applied via the HSP and HSN pins to a current amplifier within the LM3423 LM3423 and used to regulate the LED current to a value given by Eq. 3. The LM3423 LM3423's datasheet derives Eq. 3 and explains all of the IC's features. www.national.com 6 The total voltage at pin 10 of U3C is the sum of the voltages in equations 8 and 9 and is given by Eq. 11. The quantities in Eq. 11 can be adjusted to give the desired dimming behavior. The following equations can be used to calculate component values in the dimming circuit. 10.0 Test Results - Figure 5 shows the LED current in the two strings at maximum brightness. The current is almost continuous at 100 mA. The duty ratio is slightly less than 100% because of way in which the LM556 LM556 PWM dimming works. - Figure 6 shows the LEDs dimmed to the minimum possible dimmed brightness of about 20%, obtained when there is no ambient light and the input voltage is above 6.6V. - Figure 7 shows the current in the LEDs defaulting to a maximum dimmed value of 50% when the input voltage falls below 6.6V while the photodiode is in bright light. - Figure 8 shows the current in the LEDs when the input voltage is below 6.6V and there is no ambient light. Thus under low voltage conditions the circuit adjusts correctly to light conditions that correspond to dimming duty ratios less than 50%. The output voltage of the photodiode amplifier at pin 1 of U1 is given by Eq. 6 For the linear version of the dimming circuit i.e., with D4 omitted, the output voltage at pin 7 is equal to that at pin 1. For the exponential version the voltage at pin 7 is given by Eq. 7. The exponential term in the expression for the diode current in Eq. 7 is typically much greater than unity, so that the equation reduces to Eq. 8. This equation is nearly exponential if R23 is made large so that the voltage across it is much greater than that across D4. The fraction of the voltage at pin 7 of U3B voltage that appears at pin 10 of U3C is given by Eq. 9. The offset voltage that ensures that some light is produced by the LEDs even when no light impinges on the photodiode is related to the breakdown voltage of zener diode D5 by Eq. 10. 11.0 Waveforms waveform FIGURE 5. Vin =9_ diode current @max. light 7 www.national.com RD-168 RD-168 is proportional to the voltage produced by the dimming circuit built around U3. RD-168 RD-168 waveform1 FIGURE 6. Vin = 9_diode current @min. light www.national.com 8 RD-168 RD-168 waveform2 FIGURE 7. Vin = 6_diode current @max. light 9 www.national.com RD-168 RD-168 waveform3 FIGURE 8. Vin = 6_diode current @min. light 12.0 Appendix www.national.com 10 RD-168 RD-168 Equations VOUT = VIN 1- D Eq. 1 D is the duty ratio of Q1. VIN DT = (VOUT - VIN )(1 - D ) T I LED = Eq. 2 VREF R11 R1 R20 Eq. 3 Where VREF = 1.235V is the reference voltage of the error amplifier of the LM3423 LM3423. f DIM = 1.44 ( R10 + 2 R14 ) C16 t - C14 (t ) = VOUT 1 - e C14 R7 Eq. 4 Eq. 5 VP = I P R25 Eq. 6 where I P is the photodiode current. VD 4 Eq. 7 V7 = VD 4 + iD 4 R23 = VD 4 + R23iD 4 VD 4 + R23 I o e 0.026 - 1 VD 4 where VD 4 and I D 4 = I o e 0.026 - 1 are respectively the voltage across and the current in D4, and I o is the diode's saturation current. VD 4 V7 = VD 4 + R23 I o e 0.026 V10 P = Eq. 8. V7 R24 R24 + R26 Eq. 9 image FIGURE 9. LM3423 LM3423_Equations 11 www.national.com Ambient Light Adaptive LED Driver Notes National Semiconductor's design tools attempt to recreate the performance of a substantially equivalent physical implementation of the design. Reference designs are created using National's published specifications as well as the published specifications of other device manufacturers. While National does update this information periodically, this information may not be current at the time the reference design is built. National and/or its licensors do not warrant the accuracy or completeness of the specifications or any information contained therein. National and/or its licensors do not warrant that any designs or recommended parts will meet the specifications you entered, will be suitable for your application or fit for any particular purpose, or will operate as shown in the simulation in a physical implementation. National and/or its licensors do not warrant that the designs are production worthy. You should completely validate and test your design implementation to confirm the system functionality for your application. National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, 2. (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 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