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Driving HB LEDs, Issues and Resolutions Chris Richardson (Santa Clara) Clinton Jensen (Longmont) 1 LED Applications • Old days – Signal indicator – Numeric and Alpha-numeric display • Nowadays – – – – – – – – – Automotive Backlight Flashlight for portable device General illumination Projector Light Source Signage Torchlight Traffic Light LCD and DLP backlighting 2 © 2006 National Semiconductor Corporation LED Market Segmentation Automotive • • • • • • • • • • • Headlamps Forward Lighting High Beam Lights Interior Lighting Dashboard Lighting CHMStopL Rear Lights Turn Signals Puddle lights Emergency Vehicle Lighting Aftermarket Accent Lighting General Illumination • Bulb replacement – Home – Office – Flashlights • Traffic lights and signs • Signage – Billboards – Community Information – Security, Exits • Indicator / Fun lights – Gaming Machines – Clavilux (Disco) • Medical / Dental – Endoscopes – MRI / CAT scan Other Markets • Backlighting – – – – – – Monitors TVs Portable devices Camera/Camcorders Handsets Instruments • Camera Flash • User experience – – – – Handset fun lights Handset indicators Toys Instruments 3 © 2006 National Semiconductor Corporation Topics Covered • Critical Design Considerations • Topology Selection • High Current Challenges and Solutions 4 © 2006 National Semiconductor Corporation Critical Design Considerations and Topology Selection 5 Device and Topology Selection Criteria Outline 1. 2. – – 3. – – 4. – – – 5. – Input Voltage Output Voltage # of LEDs LED Type LED Current How much? How accurate? Light Output Control. LED ripple current allowed. Dimming? Analog (average current) or PWM? Two wire? How fast do the transitions have to be? Current Sense High side or low side? 6 © 2006 National Semiconductor Corporation Input Voltage • Not as easy as “what is it?”, there are many concerns: – Is the design of the source still flexible? – How much will it vary and under what conditions? Automotive cold crank and load dump are good examples. – What functionality is required / desired over the range? • Regulation / Safety Shutdown / Survival – What does the source schematic look like, a wall wart, etc. – Can the source handle low frequency, unfiltered PWM dimming? – Can the source handle 100% load steps? If not, sequencing or softstart may be required similar to voltage regulator sequencing. 7 © 2006 National Semiconductor Corporation Output Voltage • Always determined by the LEDs, but dependent on many factors: – Number of LEDs. – Manufacturer and color which determines forward voltage at operating current. – Temperature. – Age dependent. – Dynamics of dimming method. – Dynamic resistance (see next slide). 8 © 2006 National Semiconductor Corporation Dynamic Resistance as a Load • Dynamic Resistance, rD, is the inverse of the IF vs. VF curve • rD is typically 5x to 10x lower than the result of simply dividing VF-TYP by IF-TYP • The control loop sees rD, so the load impedance • RL = rD + RSNS 9 © 2006 National Semiconductor Corporation LED Current • How much current can they run effectively? – Different applications will change the order of importance of the following: • • • • Heat sinking of the LED in the application. Desired dominant wavelength. LED lifetime. Peak efficiency requirements. • Any transient steps? How fast? What is the dynamic range? • Accuracy – can be important because: – LEDs are not tightly manufactured / matched – Binned LEDs are expensive. 2D binning (light per current, dominant wavelength) is not uncommon and hugely expensive. – Color (dominant wavelength) variation is very perceptible • Ripple current allowed. – Higher ripple tends to broaden the spectrum, which can be pleasing to the eye. – High ripple usually means smaller components (inductor), especially because there is often no output capacitor (buck regulators). 10 © 2006 National Semiconductor Corporation Light Output vs. Current • Luminus flux output can vary greatly depending on operating region. Yes, those are Amps on the x-axis (we’ll get to that). • Observe the difference between continuous wave (CW) operation and pulsed. * Information courtesy of Luminus Devices, Inc. 11 © 2006 National Semiconductor Corporation Dominant Wavelength vs. Current • Again, notice the difference between continuous wave (CW) operation and pulsed. * Information courtesy of Luminus Devices, Inc. 12 © 2006 National Semiconductor Corporation CCT vs. Current • CCT is the Correlated Color Temperature, which is effectively the “white” that the LED is providing. • Cx and Cy are coordinates that specify that white point. Observe how they shift with changes in current. 13 © 2006 National Semiconductor Corporation Advanced Topology Selection Input Current Output Current Buck Boost Buck-Boost Discontinuous Continuous Discontinuous L J L Discontinuous Discontinuous L L Continuous J Perfect for LEDs! • Discontinuous input current can lead to EMI issues. • Discontinuous output current requires an output capacitor which makes PWM dimming more difficult, and usually requires multiple sense points. • Buck-boost is best for wide input ranges but is most difficult for an LED driver. 14 © 2006 National Semiconductor Corporation Why Bucks Are So Great • Continuous output current allows us to only sense the current in one place. Such a simple control algorithm requires no compensation! • Hysteretic or simple constant on-time control. • Single sense point. • Constant on-time. 15 © 2006 National Semiconductor Corporation Multiple Sense Points • For the boost and buck-boost, output current is a function of duty cycle, so both inductor current and LED current must be sensed. VIN LM3423 Vin Vcc GATE IS RCT PGND FLT HSP LED Ready Enable LRDY EN nDIM COMP CSH LED current sense adds an element to the control function to ensure the average current in the LEDs in correct. HSN TIMR DDRV OVP AGND BUT! Compensation is required for the more complex control loop. L 16 © 2006 National Semiconductor Corporation COT’s So-so Tolerance of ΔiF and IF • LM3402 and LM3404 fix valley of inductor current – Peak changes with VIN – Peak changes with VO: • LED VF shifts with temp • Large shifts if one circuit drives different # of LEDs • COT Control was designed for constant VO – Basic circuit controls fSW as VIN changes – On-time, tON, varies with VIN only – How to make tON proportional to VO as well? 17 © 2006 National Semiconductor Corporation The Constant On-Time Buck Architecture (1) • Control architecture for LM3402 / LM3404 / LM3406. TON is proportional to current in the RON pin. • Valley-mode control – the valley is a fixed reference and the on-time is adjustable. • How does this provide for constant frequency? (1) (2) tON + tOFF = C (for constant frequency) (V IN - V OUT) tON = V OUT tOFF (volt - sec product) combining (1) and (2) yields... tON = C V OUT V IN But, VOUT is constant (first order) because it is an LED driver, so if tON is proportional to 1/VIN, frequency will be constant. 18 © 2006 National Semiconductor Corporation The Constant On-Time Buck Architecture (2) • Some customers have complained that frequency shifts when they change the number of LEDs in production, so we fixed that on the LM3406. LM3406 VOUT VIN Terminate Cycle Instead of having a fixed reference, this timer references VOUT so that frequency is truly fixed. RON 19 © 2006 National Semiconductor Corporation The Constant On-Time Buck Architecture (3) • Why constant frequency? EMC issues are supposedly lessened. • Why not constant frequency? Because ripple is important in LEDs and we want ripple to be constant Hysteretic. • Can we do constant ripple / hysteretic with the constant on-time architecture? Sure! ΔI = (V IN - V OUT) tON L Because the inductor is a constant (first order), if we make tON proportional to 1/(VIN-VOUT), ripple will be constant with variation in V VIN and VOUT. IN RON VOUT This simple circuit transforms the LM3402/04/06 from constant frequency to constant ripple. to RON pin 20 © 2006 National Semiconductor Corporation Adding a PNP to Get Constant Ripple CB VIN VIN BOOT L1 VO SW RON CIN V0 IF D1 LM3402/04 Q1 CS tON α IRON RON RSNS DIM GND VCC CF t ON = 1 .34 10 -10 RON V IN - VO + 0 .6 Fixing ΔiL means that fSW will vary! 21 © 2006 National Semiconductor Corporation Test Circuit Results • VIN = 24V, IF-NOM = 350 mA • Drives one to five white LEDs 22 © 2006 National Semiconductor Corporation Test Circuit Results Sw IF Valley is controlled by comparator Peak is constant because ΔiF is constant 23 © 2006 National Semiconductor Corporation Main Drawback: Parallel FET Dimming Sw DIM IF VRON • VRON drops to 0.2V + VSAT of PNP • VSAT can drop to ~0.2V at hot • VRON is used for low power shutdown Potential for unwanted shutdown 24 © 2006 National Semiconductor Corporation The Constant On-Time Buck Architecture (4) • Will this trick still work with the LM3406 now that our on-timer references VOUT instead of a fixed potential? • Yes, but we need to adjust the circuit a bit. VCC LM3406 VOUT VIN Terminate Cycle Tie the “VOUT” pin to the VCC pin, the internal reference is once again a fixed point and the circuit works the same as before. RON 25 © 2006 National Semiconductor Corporation Boost VIN LM3423 Vin Vcc IS High side sense resistor GATE RCT PGND FLT HSP LED Ready Enable LRDY EN nDIM COMP CSH HSN TIMR DDRV OVP DIM FET can be Removed for Industrial and Automotive apps. AGND • High side current sense and what it allows. 26 © 2006 National Semiconductor Corporation Flyback / Buck-Boost VIN LM3423 Vin TIMR RCT IS FLT LED Ready LRDY GATE PGND HSN Enable EN HSP • LM3423 in a buck-boost configuration for a wide input voltage range. Vcc nDIM OVLO DDRV COMP DPOL CSH AGND 27 © 2006 National Semiconductor Corporation PWM vs. Analog Dimming • Analog dimming consists of changing the constant current through the LED by adjusting the sense voltage. – Quiet, does not generate additional noise in the system. – The dominant wavelength varies with LED current however, so the color will change using this method. • PWM dimming consists of setting a desired LED current and turning the LED on and off at speeds faster than the human eye can detect. – Noisier. The input supply must be filtered properly to accommodate the high input current transients. – The dominant wavelength does not change so color can be well controlled. This is usually the preferred method of dimming high current LEDs. 28 © 2006 National Semiconductor Corporation High Speed Dimming • High speed PWM dimming can be desirable in order to avoid certain frequency bands, such as audio. • Some big questions… – How do we do this when LED current is very high? – How do we do this with various topologies? 29 © 2006 National Semiconductor Corporation Buck – Short Out LEDs CB VIN = 24V VIN CIN BOOT L1 Inductor current is continuous in buck converters when dimming SW RON PWM D1 IF = 1A RON LM3404/04HV CS RSNS DIM GND VCC CF 30 © 2006 National Semiconductor Corporation Parallel FET Results IF FET 31 © 2006 National Semiconductor Corporation Boost – Series Dimming FET VIN LM3423 Vin Vcc IS Inductor current is not continuous in boost converters when dimming GATE RCT PGND FLT HSP LED Ready Enable LRDY EN nDIM COMP CSH HSN TIMR Series DIM FET DDRV OVP AGND • High frequency dimming in boost converters requires a FET in series with the LEDs. 32 © 2006 National Semiconductor Corporation Boost – Series Shutdown Switch • In the event of an output short circuit, a boost converter cannot provide short circuit protection for the source. The LM3423 includes a series shutdown switch which will disconnect the circuit from the source in the event of an output short circuit. Shutdown Switch VIN LM3423 Vin Vcc IS GATE RCT PGND FLT HSP LED Ready Enable LRDY EN nDIM COMP CSH HSN TIMR DDRV OVP AGND 33 © 2006 National Semiconductor Corporation 2-Wire Dimming CB VIN L1 D1 VIN BOOT SW RON CIN D2 RON LM3406B or LM3406BHV VINS VOUT CS RSNS COMP CC GND VCC CF • 2 wire dimming is achieved by sensing when an input voltage is present. When the input is present the LED is powered normally. When IF the input voltage is not present the regulator does not supply the LED any current but is kept alive using a diode and a capacitor so that it can quickly supply current to the LED when the input voltage returns. 34 © 2006 National Semiconductor Corporation High Current Challenges and Solutions 35 The Challenge • Drive a 6-36A LED that has it’s Anode tied to chassis ground for thermal performance. • Pulse Width Modulate the LED with duty cycles from 0100% at 30 kHz. • Sense as low a voltage as possible with 5% or better accuracy. • 90%+ Efficiency required. 36 © 2006 National Semiconductor Corporation The Standard Buck Regulator NMOS Switch 12V Control and Gate Drive Main FET Sync FET Using a series sense resistor for average current mode control turns the inductor into a true current source ideal for LED driving. 12V Main FET Control and Gate Drive VCC Charge Discharge Sync FET The NMOS solution uses a charge pump or bootstrap type of drive. 37 © 2006 National Semiconductor Corporation LEDs Go Megalythic • It turns out that the Magical Longhairs at Luminus Devices Inc. determined that the light prefers to exit the Cathode side of the LED and the heat prefers to exit the Anode side. The LEDs are upside down! • This requires entry into the underground… a mirrored Buck converter. Under the normal light of day it looks a little confusing with the top and bottom FETs swapped. Boost pump driving the synchronous FET producing negative output voltages and currents! 38 © 2006 National Semiconductor Corporation Negative Buck Regulator Chassis Ground Sync FET Control and Gate Drive VCC 12V Discharge Charge Main FET Current is flowing through the LED is both charge and discharge phases of the cycle. 39 © 2006 National Semiconductor Corporation The LM3433 Negative Buck Regulator with Dimming Chassis Ground 12V Sync FET LM3433 VCC Main FET The shunt NMOS determines where the current is steered. The regulator runs 100% of the time. Thus, the inductor is used as a current source and is indifferent as to where the current is going. 40 © 2006 National Semiconductor Corporation Fast Pulse Width Modulating of Massive LED Currents PHYSICISTS INSIST THAT E = L * dI/dT Inductance is the problem. 41 © 2006 National Semiconductor Corporation The LED • LEDs have inherent inductance that cannot be ignored at these current levels. • In PWM Dimming, the ideal is to go from zero current to full current in zero time, then return back to zero current just as fast. This is practice would take infinite voltage, both positive and negative! • What we end up doing is compromise… we trade rise and fall time for output voltage compliance of our driver. • The two sides of this compromise are: – (1) the inductance in the DIM FET to LED circuit – (2) the allowable output compliance of the driver (our switcher) 42 © 2006 National Semiconductor Corporation All Components Have Inductance • Even the best LEDs. • If we have a current source driving an LED closely coupled with a DIM MOSFET such that the MOSFET shunts the current away from the LED, our current fall time in the LED will not be instantaneous… 36A • The decay loop contains the inductance of the LED, the DIM FET, and all interconnections. • The current fall time is determined by the voltage and the inductance. For this case, the voltage is VF of the LED, the inductance is that as described above. • For a 36A LED with a VF of 6V, 250nH of total inductance, tdecay=1.5ms. A similar overvoltage (VF + 6V) would be require to achieve this rise time. 43 © 2006 National Semiconductor Corporation Fast PWM LED Dimming Scope Photo of Light Out of Green LED at 6A Light Output (LED Current) DIM Pin 44 © 2006 National Semiconductor Corporation The FETs (1) • The synchronous FET body diode is the single biggest problem in a synchronous buck converter. • A 6A converter can have a 50A commutation current (looks like shoot through but isn’t). • They’ll say, “Put a Schottky diode in parallel with the FET, that’ll fix it!” – but sadly it won’t for the same ‘ol reason – inductance! The current flowing in the FET would take 2ms to transfer from the FET to the diode if total L=100nH (V=0.3V, the difference between the VF’s of the body diode and Schottky)!!!! 45 © 2006 National Semiconductor Corporation The FETs (2) • What’s the real problem? 50A reverse recovery current drops efficiency and bounces the whole system (120MHz ringing at 30V). • So, what’s the solution? Add some ferrite beads to the drain of the low side (main switcher) FET to slow the transition, decrease the reverse recovery current, and create a softer recovery which effectively snubs the ringing! Chassis Ground People are tempted to put a resistor in the gate of the main FET to slow the transition down, but the SYNC FET body diode is a short during this time, so no Miller effect can be used! Sync FET 12V 6A Ferrite Bead * All switching losses are in the main FET. Main FET 46 © 2006 National Semiconductor Corporation The Board – “Star” Grounds The quasar, the ultimate magnetic field generator! 47 © 2006 National Semiconductor Corporation Magnetic Field Lines • Remember the right hand rule? Current passing through a conductor will always generate a magnetic field. Current in a loop of wire will also generate a magnetic field. 48 © 2006 National Semiconductor Corporation First Pass, Non-ideal Layout • Our first pass LM3433 layout. As you can see, the current loops are parallel to the board plane, which means magnetic flux lines will be coming out of, and going into the board planes, inducing high frequency voltages. Board top view D GND S SW C D L S -12V Red: Current path with main FET on. Blue: Current path with synchronous FET on. 49 © 2006 National Semiconductor Corporation Solution? Ideal Layout! • Placing the two high current FETs on opposite sides of the board allowed us to have a current path orthogonal to the board planes. The result? Magnetic flux lines parallel to the board planes, meaning they don’t cross the planes and induce voltage! Board side view S SW via D D C S GND via -12V Red: Current path with main FET on. Blue: Current path with synchronous FET on. 50 © 2006 National Semiconductor Corporation Almost Ideal Current Source • Error amplifier loop provides better than 80dB of common mode rejection. • Output impedance of current source is 1KΩ 1000Ω 6000V Equivalent Circuit 51 © 2006 National Semiconductor Corporation 52 Driving HB LEDs, Issues and Resolutions What advantage does fast PWM dimming provide over analog dimming? A. Lower input current ripple B. Low color variation over dim range. C. Dim time allows components to cool. D. Allows color shift to account for LED aging. 25% 25% 25% 25% 10 A. B. C. 53 D. © 2006 National Semiconductor Corporation Driving HB LEDs, Issues and Resolutions Which of the following factors is not important concerning the input supply? A. Range and required functionality over range. B. Startup / UVLO / sequencing requirements. C. Load regulation. D. Ability to handle the unfiltered dimming frequency ripple. 25% 25% 25% 25% 10 A. B. C. 54 D. © 2006 National Semiconductor Corporation Driving HB LEDs, Issues and Resolutions What is the steady state output voltage not dependent on? A. B. C. D. E. Dimming method. Type of LED. Temperature. DC LED current. LED dynamic resistance. 20% 20% 20% 20% 20% 10 A. B. C. D. 55 E. © 2006 National Semiconductor Corporation Driving HB LEDs, Issues and Resolutions Which of the following fact is true regarding the importance of LED current accuracy? A. LEDs are tightly manufactured. B. As an LED Heats up its apparent brightness decreases, thus compensating for the current value. (Self Ballasts) The high bulk resistively in LEDs is designed to compensate for minor current variations. C. D. 25% 25% 25% 25% Binned LEDs are very expensive. 10 A. B. C. 56 D. © 2006 National Semiconductor Corporation Driving HB LEDs, Issues and Resolutions What statement is true of an adaptive COT (constant on time) architecture. A. It is difficult to stabilize. B. The frequency is always unpredictable. C. You can achieve constant ripple if adapted to Vout. D. You can achieve constant frequency if adapted to Vin. E. C & D. 20% 20% 20% 20% 20% 10 A. B. C. D. 57 © 2006 National Semiconductor Corporation E. Driving HB LEDs, Issues and Resolutions What is the most important factor determining LED operating current? A. Heat sinking of the LED. B. Dominant wavelength. C. LED life. D. LED current to light efficiency. E. All of the above. 20% 20% 20% 20% 20% 10 A. B. C. D. 58 E. © 2006 National Semiconductor Corporation Driving HB LEDs, Issues and Resolutions Why is ripple current sometimes desirable in LEDs? A. It isn’t. B. Broadens the wavelength which is nicer to look at. C. Smaller, lighter, cheaper drive electronics. D. B and C. 25% 25% 25% 25% 10 A. B. C. 59 D. © 2006 National Semiconductor Corporation Driving HB LEDs, Issues and Resolutions Why do customers want the anode of the LED tied to the grounded chassis? A. Better mechanical shock resistance. B. Safety concerns (shock hazard). C. Safety concerns (fire hazard). D. Efficient thermal transfer. E. C and D. 20% 20% 20% 20% 20% 10 A. B. C. D. 60 E. © 2006 National Semiconductor Corporation Driving HB LEDs, Issues and Resolutions What is the best way to slow down a rapid body diode commutation in the synchronous FET of a buck switcher? A. Slow down the turn on speed of the main FET with a gate resistor. B. Put a schottky diode in parallel with the synchronous FET. C. Place ferrite beads in series with the drain of the main FET. D. Put a snubber across the synchronous FET. 25% 25% 25% 25% 10 A. B. C. 61 D. © 2006 National Semiconductor Corporation Driving HB LEDs, Issues and Resolutions According to the LM3402 datasheet, tON=1.34x1010 / I RON. What RON should be used if we have a 220mH inductor and we want 100mA of constant ripple? Neglect the Vbe drop of the pnp. 25% 25% 25% 25% VIN A. 62 kW Clues B. 247 kW C. 110 kW D. 164 kW ΔI = (VIN - VOUT ) tON RON L VOUT 10 What is the equation for IRON is this circuit? to RON pin 62 © 2006 National Semiconductor Corporation 63 Team Scores 0 Team 1 0 Team 2 0 Team 3 0 Team 4 0 Team 5 64 © 2006 National Semiconductor Corporation 65