National Semiconductor Application Note 1839 Matthew Reynolds December 10, 2008 Introduction Thermal Performance The LM3402/02HV and LM3404/04HV are buck regulator derived controlled current sources designed to drive a series string of high power, high brightness LEDs (HBLEDs) at forward currents of up to 0.5A (LM3402/02HV) or 1.0A (LM3404/04HV). This evaluation board demonstrates the enhanced thermal performance, fast dimming, and true constant LED current capabilities of the LM3402 and LM3404 devices. The PSOP-8 package is pin-for-pin compatible with the SO-8 package with the exception of the thermal pad, or exposed die attach pad (DAP). The DAP is electrically connected to system ground. When the DAP is properly soldered to an area of copper on the top layer, bottom layer, internal planes, or combinations of various layers, the θJA of the LM3404/04HV can be significantly lower than that of the SO-8 package. The PSOP-8 evaluation board is two layers of 1oz copper each, and measures 1.25" x 1.95". The DAP is soldered to approximately 1/2 square inch of top and two square inches of bottom layer copper. Three thermal vias connect the DAP to the bottom layer of the PCB. A recommended DAP/via layout is shown in figure 2. Circuit Performance with LM3404 This evaluation board (figure 1) uses the LM3404 to provide a constant forward current of 750 mA ±10% to a string of up to five series-connected HBLEDs with a forward voltage of approximately 3.4V each from an input of 18V to 36V. LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board 30061601 FIGURE 1. LM3402 / 04 Schematic AN-1839 © 2008 National Semiconductor Corporation 300616 www.national.com AN-1839 30061604 FIGURE 3. Buck Converter Inductor Current Waveform A voltage signal, VSNS, is created as the LED current flows through the current setting resistor, RSNS, to ground. VSNS is fed back to the CS pin, where it is compared against a 200 mV reference (VREF). A comparator turns on the power MOSFET when VSNS falls below VREF. The power MOSFET conducts for a controlled on-time, tON, set by an external resistor, RON. 30061602 FIGURE 2. LM3402/04 PSOP Thermal PAD and Via Layout Connecting to LED Array The LM3402 / 04 evaluation board includes two standard 94 mil turret connectors for the cathode and anode connections to a LED array. Low Power Shutdown The LM3402/04 can be placed into a low power shutdown state (IQ typically 90 µA) by grounding the DIM terminal. During normal operation this terminal should be left open-circuit. Constant On Time Overview The LM3402 and LM3404 are buck regulators with a wide input voltage range and a low voltage reference. The controlled on-time (COT) architecture is a combination of hysteretic mode control and a one-shot on-timer that varies inversely with input voltage. With the addition of a PNP transistor, the on-timer can be made to be inversely proportional to the input voltage minus the output voltage. This is one of the application improvements made to this demonstration board that will be discussed later (improved average LED current circuit). The LM3402 / 04 were designed with a focus of controlling the current through the load, not the voltage across it. A constant current regulator is free of load current transients, and has no need for output capacitance to supply the load and maintain output voltage. Therefore, in this demonstration board in order to demonstrate the fast transient capabilities, I have chosen to omit the output capacitor. With any Buck regulator, duty cycle (D) can be calculated with the following equations. 30061605 FIGURE 4. VSNS Circuit SETTING THE AVERAGE LED CURRENT Knowing the average LED current desired and the input and output voltages, the slopes of the currents within the inductor can be calculated. The first step is to calculate the minimum inductor current (LED current) point. This minimum level needs to be determined so that the average LED current can be determined. The average inductor current equals the average LED current whether an output capacitor is used or not. www.national.com 2 iTARGET x RSNS = 0.20V Therefore: Finally RSNS can be calculated. 30061606 FIGURE 5. ISENSE Current Waveform Standard On-Time Set Calculation The control MOSFET on-time is variable, and is set with an external resistor RON (R2 from Figure1). On-time is governed by the following equation: Using figures 3 and 5 and the equations of a line, calculate ILED-MIN. Where Where IF = ILED-Average k = 1.34 x 10-10 The delta of the inductor current is given by: At the conclusion of tON the control MOSFET turns off for a minimum OFF time (tOFF-MIN) of 300 ns, and once tOFF-MIN is complete the CS comparator compares VSNS and VREF again, waiting to begin the next cycle. The LM3402 / 04 have minimum ON and OFF time limitations. The minimum on time (tON) is 300 ns, and the minimum allowed off time (tOFF) is 300 ns. Designing for the highest switching frequency possible means that you will need to know when minimum ON and OFF times are observed. Minimum OFF time will be seen when the input voltage is at its lowest allowed voltage, and the output voltage is at its maximum voltage (greatest number of series LEDs). The opposite condition needs to be considered when designing for minimum ON time. Minimum ON time is the point at which the input voltage is at its maximum allowed voltage, and the output voltage is at its lowest value. There is a 220 ns delay (tD) from the time that the current sense comparator trips to the time at which the control MOSFET actually turns on. We can solve for iTARGET knowing there is a delay. ΔiD is the magnitude of current beyond the target current and equal to: Therefore: 3 www.national.com AN-1839 The point at which you want the current sense comparator to give the signal to turn on the FET equals: AN-1839 Application Circuit Calculations To better explain the improvements made to the COT LM3402 / 04 demonstration board, a comparison is shown between the unmodified average output LED current circuit to the improved circuit. Design examples 1 and 2 use two original LM3402 / 04 circuits. The switching frequencies will be maximized to provide a small solution size. Design example 3 is an improved average current application. Example 3 will be compared against example 2 to illustrate the improvements. Example 4 will use the same conditions and circuit as example 3, but the switching frequency will be reduced to improve efficiency. The reduced switching frequency can further reduce any variations in average LED current with a wide operating range of series LEDs and input voltages. Design Example 1 • VIN = 48V (±20%) • Driving three HB LEDs with VF = 3.4V • VOUT = (3 x 3.4V +200 mV) = 10.4V • IF = 500 mA (typical application) • Estimated efficiency = 82% • fSW = fast as possible • Design for typical application within tON and tOFF limitations LED (inductor) ripple current of 10% to 60% is acceptable when driving LEDs. With this much allowed ripple current, you can see that there is no need for an output capacitor. Eliminating the output capacitor is actually desirable. An LED connected to an inductor without a capacitor creates a near perfect current source, and this is what we are trying to create. In this design we will choose 50% ripple current. 30061615 FIGURE 6. VOUT-MAX vs fSW ΔiL = 500 mA x 0.50 = 250 mA IPEAK = 500 mA + 125 mA = 625 mA Calculate tON, tOFF & RON From the datasheet there are minimum control MOSFET ON and OFF times that need to be met. tOFF minimum = 300 ns tON minimum = 300 ns The minimum ON time will occur when VIN is at its maximum value. Therefore calculate RON at VIN = 60V, and set tON = 300 ns. A quick guideline for maximum switching frequency allowed versus input and output voltages are shown below in the two graphs (figures 6 & 7). www.national.com 30061616 FIGURE 7. VOUT-MIN vs fSW RON = 135 kΩ (use standard value of 137 kΩ) tON = 306 ns Check to see if tOFF minimum is satisfied. This occurs when VIN is at its minimum value. At VIN = 36V, and RON = 137 kΩ calculate tON from previous equation. tON = 510 ns We know that: 4 tOFF = 938 ns (satisfied) Example 1 ON & OFF Times VIN (V) VOUT (V) tON tOFF 36 10.4 5.10E-07 9.38E-07 48 10.4 3.82E-07 1.06E-06 60 10.4 3.06E-07 1.14E-06 Calculate Switching Frequency VIN = 36V, 48 and 60V. Substituting equations: fSW = 691kHz (VIN = 36V, 48V, & 60V) Calculate Inductor Value With 50% ripple at VIN = 48V • IF = 500 mA Therefore: RSNS = 467 mΩ Calculate Average LED current (IF) Calculate average current through the LEDs for VIN = 36V and 60V. • ΔiL = 250 mA (target) • L = 57 µH (68 µH standard value) Calculate Δi for VIN = 36V, 48V, and 60V with L = 68 µH Example 1 Ripple Current VIN (V) VOUT (V) ΔiL (A) 36 10.4 0.192 48 10.4 0.211 VIN (V) VOUT (V) IF (A) 60 10.4 0.223 36 10.4 0.490 48 10.4 0.500 60 10.4 0.506 Example 1 Average LED Current Calculate RSNS Calculate RSNS at VIN typical (48V), and average LED current (IF) set to 500 mA. 30061623 FIGURE 8. Inductor Current Waveform 5 www.national.com AN-1839 • IF = 500 mA • VIN = 48V • VOUT = 10.4V • L = 68 µH • tD = 220 ns • tON = 382 ns Using equations from the COT Overview section, calculate RSNS. Rearranging the above equation and solving for tOFF with tON set to 510 ns AN-1839 Example 2 On & Off Time Design Example 2 Design example 2 demonstrates a design if a single Bill of Materials (Bom) is desired over many different applications (number of series LEDs, VIN, VOUT etc). • VIN = 48V (±20%) • Driving 3, 4, or 5 HB LEDs with VF = 3.4V • IF = 500 mA (typical application) • Estimated efficiency = 82% • fSW = fast as possible • Design for typical application within tON and tOFF limitations The inductor, RON resistor, and the RSNS resistor is calculated for a typical or average design. • VOUT = 3 x 3.4V + 200 mV = 10.4V • VOUT = 4 x 3.4V + 200 mV = 13.8V • VOUT = 5 x 3.4V + 200 mV = 17.2V Calculate tON, tOFF & RON In this design we will maximize the switching frequency so that we can reduce the overall size of the design. In a later design, a slower switching frequency is utilized to maximize efficiency. If the design is to use the highest possible switching frequency, you must ensure that the minimum on and off times are adhered to. Minimum on time occurs when VIN is at its maximum value, and VOUT is at its lowest value. Calculate RON at VIN = 60V, VOUT = 10.4V, and set tON = 300 ns: Three Series LEDs VIN (V) VOUT (V) RON tON tOFF 36 10.4 137 kΩ 5.10E-07 9.38E-07 48 10.4 137 kΩ 3.82E-07 1.06E-06 60 10.4 137 kΩ 3.06E-07 1.14E-06 Four Series LEDs 36 13.8 137 kΩ 5.10E-07 5.81E-07 48 13.8 137 kΩ 3.82E-07 7.08E-07 60 13.8 137 kΩ 3.06E-07 7.85E-07 Five Series LEDs 36 17.2 137 kΩ 5.10E-07 3.65E-07 48 17.2 137 kΩ 3.82E-07 4.93E-07 60 17.2 137 kΩ 3.06E-07 5.69E-07 Calculate Switching Frequency The switching frequency will only change with output voltage. Substituting equations: Or: RON = 137 kΩ, tON = 306 ns Check to see if tOFF minimum is satisfied: tOFF minimum occurs when VIN is at its lowest value, and VOUT is at its maximum value. At VIN = 36V, VOUT = 17.2V, and R ON = 137 kΩ calculate tON from the above equation: tON = 510 ns • fSW = 691 kHz (VOUT = 10.4V) • fSW = 916 kHz (VOUT = 13.8V) • fSW = 1.14 MHz (VOUT = 17.2V) Calculate Inductor Value Rearrange the above equation and solve for tOFF with tON set to 510 ns With 50% ripple at VIN = 48V, and VOUT = 10.4V • IAVG = 500 mA • ΔiL = 250 mA (target) • L = 53 µH (68 uH standard value) Calculate Δi for VIN = 36V, 48V, & 60V with L = 68 µH. tOFF = 365 ns (satisfied) www.national.com 6 Example 2 Average LED Current VOUT (V) ΔiL (A) 10.4 0.192 VIN (V) IF (A) 36 10.4 0.511 48 10.4 0.521 60 10.4 0.526 Three Series LEDs Three Series LEDs 36 VOUT (V) 48 10.4 0.211 60 10.4 0.223 Four Series LEDs Four Series LEDs 36 13.8 0.166 36 13.8 0.487 48 13.8 0.192 48 13.8 0.500 60 13.8 0.208 60 13.8 0.508 Five Series LEDs Four Series LEDs 36 17.2 0.141 36 17.2 0.463 48 17.2 0.173 48 17.2 0.479 0.193 60 17.2 0.489 60 17.2 In this application you can see that there is a difference of 63 mA between the low and high of the average LED current. Calculate RSNS Calculate RSNS at VIN typical (48V), with four series LEDs (13.8V = VOUT), and average LED current (IF) set to 500 mA. • IF = 500 mA • VIN = 48V • VOUT = 13.8V • L = 68 µH • tD = 220 ns • tON = 382 ns Modified COT Application Circuit With the addition of one pnp transistor and one resistor (Q1 and R3) the average current through the LEDs can be made to be more constant over input and output voltage variations. Refer to page one figure 1. Resistor RON (R2) and Q1 turn the tON equation into: Ignore the PNP transistor’s VBE voltage drop. Design to the same criteria as the previous example with the improved application and compare results. RSNS = 446 mΩ Calculate Average Current through LED All combinations of VIN, VOUT with RSNS = 446 mΩ 7 www.national.com AN-1839 Example 2 Ripple Current VIN (V) AN-1839 Calculate tON, tOFF & RON Minimum ON time occurs when VIN is at its maximum value, and VOUT is at its lowest value. Calculate RON at VIN = 60V, VOUT = 10.4V, and set tON = 300 ns: Modified Application Circuit Design Example 3 Design Example 1 • VIN = 48V (±20%) • Driving 3, 4, or 5 HB LEDs with VF = 3.4V • IF = 500 mA (typical application) • Estimated efficiency = 82% • fSW = fast as possible • Design for typical application within tON and tOFF limitations RON = 111 kΩ (113 kΩ) tON = 306 ns Check to see if tOFF minimum is satisfied. At VIN = 36V, VOUT = 17.2V, and RON = 113 kΩ calculate tON:. tON = 806 ns The inductor, RON resistor, and the RSNS resistor are calculated for a typical or average design. • VOUT = 3 x 3.4V + 200 mV = 10.4V • VOUT = 4 x 3.4V + 200 mV = 13.8V • VOUT = 5 x 3.4V + 200 mV = 17.2V tOFF = 577 ns (satisfied) 30061637 FIGURE 9. Improved Average LED Current Application Circuit www.national.com 8 VIN (V) VOUT (V) RON tON tOFF 36 10.4 113 kΩ 5.92E-07 1.09E-07 48 10.4 113 kΩ 4.03E-07 1.12E-06 60 10.4 113 kΩ 3.06E-07 1.14E-06 Four Series LEDs 36 13.8 113 kΩ 6.83E-07 7.78E-07 48 13.8 113 kΩ 4.43E-07 8.21E-07 60 13.8 113 kΩ 3.28E-07 8.41E-07 Five Series LEDs 36 17.2 113 kΩ 8.06E-07 5.77E-07 48 17.2 113 kΩ 4.92E-07 6.34E-07 60 17.2 113 kΩ 3.54E-07 6.59E-07 Substitute improved circuit tON calculation: Calculate Switching Frequency Simplified: Example 3 Switching Frequency VIN (V) VOUT (V) fSW (kHz) 36 10.4 595 48 10.4 656 60 10.4 692 36 13.8 685 48 13.8 791 60 13.8 855 36 17.2 723 48 17.2 888 60 17.2 987 Three Series LEDs Typical Application: • VOUT = 13.8V • IF = 500 mA • RON= 113 kΩ • L = 68 µH • tD = 220 ns RSNS = 462 mΩ This equation shows that only variations in VOUT will affect the average current over the entire application range. These variations should be very minor even with large variations in output voltage. Calculate Average Current through LED Modified application circuit average forward current equation. Four Series LEDs Five Series LEDs Calculate Inductor Value Simplified: Therefore: 9 www.national.com AN-1839 You can quickly see one benefit of the modified circuit. The improved circuit eliminates the input and output voltage variation on RMS current. • IF = 500 mA (typical application) • ΔiL = 250 mA (target) • RON= 113 kΩ • L = 59 µH (68 µH standard value) • ΔiL = 223 mA (L = 68 µH all combinations) Calculate RSNS Original RSNS equation: Example 3 On & Off Times Three Series LEDs AN-1839 Example 3 Average LED Current VIN (V) VIN (V) VOUT (V) IF (A) 13.8 0.500 36 17.2 0.489 48 17.2 0.489 60 17.2 0.489 VOUT (V) IF (A) Three Series LEDs 36 10.4 0.511 Five Series LEDs 48 10.4 0.511 60 10.4 0.511 Three Series LEDs 60 Four Series LEDs 36 13.8 0.500 48 13.8 0.500 www.national.com In this application you can see that there is a difference of 22 mA between the low and high of the average LED current. 10 VIN (V) VOUT (V) fSW (kHz) 10.4 435 36 13.8 430 48 13.8 497 60 13.8 537 36 17.2 454 48 17.2 558 60 17.2 620 Three Series LEDs 60 • VIN = 48V (±20%) • Driving 3, 4, or 5 HB LEDs with VF = 3.4V • IF = 500 mA (typical application) • Estimated efficiency = 82% • fSW = 500 kHz (typ app) The inductor, RON resistor, and the RSNS resistor are calculated for a typical or average design. • VOUT = 3 x 3.4V + 200 mV = 10.4V • VOUT = 4 x 3.4V + 200 mV = 13.8V • VOUT = 5 x 3.4V + 200 mV = 17.2V Reduce switching frequency for the typical application to about 500 kHz to increase efficiency. Calculate tON, tOFF & RON Four Series LEDs Five Series LEDs Calculate RSNS • VOUT = 13.8V • VIN = 48V • IF = 500 mA • tD = 220 ns • η = 0.85 • L = 100 µH RSNS = 488 mΩ Calculate Average Current through LED • VOUT = 13.8V • VIN = 48V • IF = 500 mA • tD = 220 ns • η = 0.85 • fSW = 500 kHz tON ≊ 705 ns RON ≊ 179 kΩ (use standard value of 182 kΩ) Calculate Inductor Value Example 4 Average LED Current VIN (V) VOUT (V) IF (A) 36 10.4 0.507 48 10.4 0.507 60 10.4 0.507 36 13.8 0.500 48 13.8 0.500 60 13.8 0.500 36 17.2 0.493 48 17.2 0.493 60 17.2 0.493 Three Series LEDs • IF = 500 mA • ΔiL = 250 mA (target) • RON = 182 kΩ • L = 100 µH Calculate ΔiL with L = 100 µH (VIN = 48V, VOUT = 13.8V) Four Series LEDs ΔiL = 241 mA (all combinations) Calculate Switching Frequency Five Series LEDs In the reduced frequency application you can see that there is a difference of 14 mA between the low and high of the average current. If the original tON circuit was used (no PNP transistor) with the switching frequency centered around 500 kHz the difference between the high and low values would be about 67 mA. Example 4 Switching Frequency VIN (V) VOUT (V) fSW (kHz) 36 10.4 374 48 10.4 412 Three Series LEDs 11 www.national.com AN-1839 Modified Application Circuit Design Example 4 AN-1839 should be at least one order of magnitude lower than the steady state switching frequency in order to prevent aliasing. Refer to figure 10 for illustrations. The interval tD represents the delay from a logic high at the DIM pin to the onset of the output current. The quantities tSU and tSD represent the time needed for the LED current to slew up to steady state and slew down to zero, respectively. As an example, assume a DIM duty cycle DDIM equal to 100% (always on) and the circuit delivers 500mA of current through the LED string. At DDIM equal to 50% you would like exactly ½ of 500 mA of current through your LED string (250 mA). This could only be possible if there were no delays (tD) between the on/off DIM signal and the on/off of the LED current. The rise and fall times (tSU and tSD) of the LED current would also need to be eliminated. If we can reduce these times, the linearity between the PWM signal and the average current will be realized. Dimming The DIM pin of the LM3402/04 is a TTL compatible input for low frequency pulse width modulation (PWM) dimming of the LED current. Depending on the application, a contrast ratio greater than what the LM3402/04 internal DIM circuitry can provide might be needed. This demonstration board comes with external circuitry that allows for dimming contrast ratios greater than 50k:1 LM3402 / 04 DIM Pin Operation To fully enable and disable the LM3402 / 04, the PWM signal should have a maximum logic low level of 0.8V and a minimum logic high level of 2.2V. Dimming frequency, fDIM, and duty cycle, DDIM, are limited by the LED current rise time and fall time and the delay from activation of the DIM pin to the response of the internal power MOSFET. In general, fDIM 30061652 FIGURE 10. Contrast Ratio Definitions Contrast Ratio Definition Contrast Ratio (CR) = 1/DMIN DMIN = (tD + tSU) x fDIM 30061653 FIGURE 11. tD & tSU (DIM Pin) www.national.com 12 Refer to figure 12. MOSFET Q4 and its drive circuitry are provided on the demonstration PCB. When MOSFET Q4 is turned on, it shorts LED+ to LED-, therefore redirecting the 30061654 FIGURE 12. 30061656 30061655 FIGURE 14. tD + tSU Graph FIGURE 13. VIN = 24V, 3 series LEDs @ 400mA 13 www.national.com AN-1839 inductor current from the LED string to the shunt MOSFET. The LM3402 / 04 is never turned off, and therefore become a perfect current source by providing continuous current to the output through the inductor (L1). A buck converter with an external shunt MOSFET is the ideal circuit for delivering the highest possible contrast ratio. Refer to figures 13-15 for typical delays and rise time for external MOSFET dimming. External MOSFET Dimming and Contrast Ratio AN-1839 when Q2 shunt MOSFET is OFF during fast dimming. This is an added benefit due to the fact that tOFF is greatly increased, and therefore the switching frequency is decreased, which leads to improved efficiency (see figure 16). Inductor L1 still remains charged, and as soon as Q4 turns off current flows through the LED string. 30061657 FIGURE 15. tD + tSD Graph Fast Dimming + Improved Average Current Circuit Using both the Improved Average LED current circuit and the external MOSFET fast dimming circuit together has additional benefits. If RON and the converter's switching frequency (fSW) is determined and set with the improved average LED current circuit, the switching frequency will decrease once VOUT is shorted during fast dimming. With MOSFET Q4 on, VOUT is equal to VFB (200 mV). The tON equation then becomes almost identical to the original unmodified circuit equation. Setting tON and RON: 30061662 FIGURE 16. Improved Avg ILED Circuit + Fast Dimming Linearity with Fast Dimming Once the delays and rise/fall times have been greatly reduced, linear average current vs, duty cycle (DDIM) can be achieved at very high dimming frequencies (fDIM) (see figure 17). tON equation becomes: when Q4 shunt MOSFET is on during fast dimming. tOFF equation during normal operation is: 30061663 tOFF equation then becomes: FIGURE 17. Linearity with Fast Dimming www.national.com 14 VIN = 9V to 18V, ILED = 750 mA, 3 x 3.4V White LED Strings (fSW ≊ 500 kHz) 15 30061601 www.national.com AN-1839 LM3404 Improved ILED Average & Fast Dimming Demonstration Board AN-1839 Bill of Materials Part ID Part Value Mfg Part Number U1 1A Buck LED Driver PSOP pkg NSC LM3404 C1, Input Cap 10 µF, 25V, X5R TDK C3225X5R1E106M C2, C6 Cap 1 µF, 16V, X5R TDK C1608X5R1C105M C3, VBOOST Cap 0.1 µF, X5R TDK C1608X5R1H104M C4 Output Cap 10 µF, 25V, X5R (Optional) TDK C3225X5R1E106M C5, VRON Cap 0.01 µF, X5R TDK C1608X5R1H103M D1, Catch Diode 0.5Vf Schottky 2A, 30VR Diodes INC B230 D2 Dual SMT small signal Diodes INC BAV199 L1 33 µH CoilCraft D01813H-333 R1A, R1B 0.62Ω 1% 0.25W 1206 ROHM MCR18EZHFLR620 R2 47.5 kΩ 1% Vishay CRCW08054752F R3 1.0 kΩ, 1% Vishay CRCW08051001F R4, R5 1Ω, 1% Vishay CRCW08051R00F R6 10 kΩ, 1% Vishay CRCW08051002F Q1 SOT23 PNP Diodes INC MMBT3906 Q4 SOT23-6 N-CH 2.4A, 20V ZETEX ZXMN2A01E6 Q3 SC70-6, P + N Channel Vishay Si1539DL Test Points Connector Keystone 1502-2 VIN, GND, LED+, LED- Connector Keystone 575-8 JMP-1 Jumper Molex 22-28-4023 J15 50Ω BNC Amphenol 112538 www.national.com 16 AN-1839 Layout 30061665 17 www.national.com LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage Reference www.national.com/vref Design Made Easy www.national.com/easy PowerWise® Solutions www.national.com/powerwise Solutions www.national.com/solutions Serial Digital Interface (SDI) www.national.com/sdi Mil/Aero www.national.com/milaero Temperature Sensors www.national.com/tempsensors Solar Magic® www.national.com/solarmagic Wireless (PLL/VCO) www.national.com/wireless Analog University® www.national.com/AU THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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