LM3410 PowerWise® 525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with Internal Compensation General Description Features The LM3410 constant current LED driver is a monolithic, high frequency, PWM DC/DC converter in 5-pin SOT23, 6-pin LLP, & 8-pin eMSOP packages. With a minimum of external components the LM3410 is easy to use. It can drive 2.8A typical peak currents with an internal 170 mΩ NMOS switch. Switching frequency is internally set to either 525 kHz or 1.60 MHz, allowing the use of extremely small surface mount inductors and chip capacitors. Even though the operating frequency is high, efficiencies up to 88% are easy to achieve. External shutdown is included, featuring an ultra-low standby current of 80 nA. The LM3410 utilizes current-mode control and internal compensation to provide high-performance over a wide range of operating conditions. Additional features include dimming, cycle-by-cycle current limit, and thermal shutdown. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Space Saving SOT23-5 & 6-LLP Package Input voltage range of 2.7V to 5.5V Output voltage range of 3V to 24V 2.8A Typical Switch Current High Switching Frequency — 525 KHz (LM3410-Y) — 1.6 MHz (LM3410-X) 170 mΩ NMOS Switch 190 mV Internal Voltage Reference Internal Soft-Start Current-Mode, PWM Operation Thermal Shutdown Applications ■ ■ ■ ■ LED Backlight Current Source LiIon Backlight OLED & HB LED Driver Handheld Devices LED Flash Driver Typical Boost Application Circuit 30038501 30038502 © 2008 National Semiconductor Corporation 300385 www.national.com LM3410 PowerWise® 525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with Internal Compensation January 23, 2008 LM3410 Connection Diagrams Top View Top View Top View 30038503 5–Pin SOT23 30038504 30038505 6-Pin LLP 8-Pin eMSOP Ordering Information Order Number Frequency LM3410YMF LM3410YMFX LM3410YSD LM3410YSDX 525 kHz LM3410YMY LM3410YMYX LM3410XMF LM3410XMFX LM3410XSD LM3410XSDX LM3410XMY LM3410XMYX www.national.com 1.6 MHz Package Type Package Drawing SOT23-5 MF05A LLP-6 SDE06A eMSOP-8 MUY08A SOT23-5 MF05A LLP-6 SDE06A eMSOP-8 MUY08A 2 Supplied As 1000 units Tape & Reel 3000 units Tape & Reel 1000 units Tape & Reel 4500 units Tape & Reel 1000 units Tape & Reel 3500 units Tape & Reel 1000 units Tape & Reel 3000 units Tape & Reel 1000 units tape & reel 4500 units Tape & Reel 1000 units Tape & Reel 3500 units Tape & Reel LM3410 Pin Descriptions - 5-Pin SOT23 Pin Name 1 SW Function 2 GND 3 FB Feedback pin. Connect FB to external resistor divider to set output voltage. 4 DIM Dimming & shutdown control input. Logic high enables operation. Duty Cycle from 0 to 100%. Do not allow this pin to float or be greater than VIN + 0.3V. 5 VIN Supply voltage pin for power stage, and input supply voltage. Output switch. Connect to the inductor, output diode. Signal and power ground pin. Place the bottom resistor of the feedback network as close as possible to this pin. Pin Descriptions - 6-Pin LLP Pin Name Function 1 PGND Power ground pin. Place PGND and output capacitor GND close together. 2 VIN Supply voltage for power stage, and input supply voltage. 3 DIM Dimming & shutdown control input. Logic high enables operation. Duty Cycle from 0 to 100%. Do not allow this pin to float or be greater than VIN + 0.3V. 4 FB Feedback pin. Connect FB to external resistor divider to set output voltage. 5 AGND 6 SW DAP GND Signal ground pin. Place the bottom resistor of the feedback network as close as possible to this pin & pin 4. Output switch. Connect to the inductor, output diode. Signal & Power ground. Connect to pin 1 & pin 5 on top layer. Place 4-6 vias from DAP to bottom layer GND plane. Pin Descriptions - 8-Pin eMSOP Pin Name 1 - Function 2 PGND 3 VIN Supply voltage for power stage, and input supply voltage. 4 DIM Dimming & shutdown control input. Logic high enables operation. Duty Cycle from 0 to 100%. Do not allow this pin to float or be greater than VIN + 0.3V. 5 FB Feedback pin. Connect FB to external resistor divider to set output voltage. 6 AGND 7 SW 8 - DAP GND No Connect Power ground pin. Place PGND and output capacitor GND close together. Signal ground pin. Place the bottom resistor of the feedback network as close as possible to this pin & pin 5 Output switch. Connect to the inductor, output diode. No Connect Signal & Power ground. Connect to pin 2 & pin 6 on top layer. Place 4-6 vias from DAP to bottom layer GND plane. 3 www.national.com LM3410 Storage Temp. Range Soldering Information Infrared/Convection Reflow (15sec) Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN SW Voltage FB Voltage DIM Voltage ESD Susceptibility (Note 4) Human Body Model Junction Temperature (Note 2) Operating Ratings -0.5V to 7V -0.5V to 26.5V -0.5V to 3.0V -0.5V to 7.0V -65°C to 150°C 220°C (Note 1) VIN VDIM (Note 5) VSW Junction Temperature Range Power Dissipation (Internal) SOT23-5 2kV 150°C 2.7V to 5.5V 0V to VIN 3V to 24V -40°C to 125°C 400 mW Electrical Characteristics Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of -40°C to 125°C. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. VIN = 5V, unless otherwise indicated under the Conditions column. Symbol VFB ΔVFB/VIN Parameter Feedback Voltage Line Regulation IFB Feedback Input Bias Current FSW Switching Frequency DMAX Maximum Duty Cycle DMIN Minimum Duty Cycle RDS(ON) Switch On Resistance ICL Switch Current Limit SU Start Up Time IQ Quiescent Current (switching) Quiescent Current (shutdown) UVLO VDIM_H Conditions Min Typ Max Units 178 190 202 mV - 0.06 - %/V - 0.1 1 µA LM3410-X 1200 1600 2000 LM3410-Y 360 525 680 LM3410-X 88 92 - LM3410-Y 90 95 - LM3410-X - 5 - LM3410-Y - 2 - SOT23-5 and eMSOP-8 - 170 330 190 350 2.80 - A µs Feedback Voltage Undervoltage Lockout VIN = 2.7V to 5.5V LLP-6 2.1 - 20 - LM3410-X VFB = 0.25 - 7.0 11 LM3410-Y VFB = 0.25 - 3.4 7 All Options VDIM = 0V - 80 - VIN Rising - 2.3 2.65 VIN Falling 1.7 1.9 - - - 0.4 1.8 - - Shutdown Threshold Voltage Enable Threshold Voltage kHz % % mΩ mA nA V V ISW Switch Leakage VSW = 24V - 1.0 - µA IDIM Dimming Pin Current Sink/Source - 100 - nA www.national.com 4 Parameter θJA Junction to Ambient 0 LFPM Air Flow (Note 3) θJC Junction to Case (Note 3) TSD Thermal Shutdown Temperature (Note 2) Conditions Min Typ Max LLP-6 and eMSOP-8 Package - 80 - SOT23-5 Package - 118 - LLP-6 and eMSOP-8 Package - 18 - SOT23-5 Package - 60 - - 165 - Units °C/W °C/W °C Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but does not guarantee specific performance limits. For guaranteed specifications and conditions, see the Electrical Characteristics. Note 2: Thermal shutdown will occur if the junction temperature exceeds the maximum junction temperature of the device. Note 3: Applies for packages soldered directly onto a 3” x 3” PC board with 2oz. copper on 4 layers in still air. Note 4: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Test method is per JESD22-A114. Note 5: Do not allow this pin to float or be greater than VIN +0.3V. 5 www.national.com LM3410 Symbol LM3410 Typical Performance Characteristics All curves taken at VIN = 5.0V with configuration in typical application circuit shown in Application Information section of this datasheet. TJ = 25C, unless otherwise specified. LM3410-X Efficiency vs VIN (RSET = 4Ω) LM3410-X Start-Up Signature 30038507 30038502 4 x 3.3V LEDs 500 Hz DIM FREQ D = 50% DIM Freq & Duty Cycle vs Avg I-LED 30038508 30038509 Current Limit vs Temperature RDSON vs Temperature 30038510 www.national.com 30038511 6 Oscillator Frequency vs Temperature - "Y" 30038512 30038513 VFB vs Temperature 30038580 7 www.national.com LM3410 Oscillator Frequency vs Temperature - "X" LM3410 Simplified Internal Block Diagram 30038514 FIGURE 1. Simplified Block Diagram begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the internal NMOS control switch. During this on-time, the SW pin voltage (VSW) decreases to approximately GND, and the inductor current (IL) increases with a linear slope. IL is measured by the current sense amplifier, which generates an output proportional to the switch current. The sensed signal is summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage and VREF. When the PWM comparator output goes high, the output switch turns off until the next switching cycle begins. During the switch off-time, inductor current discharges through diode D1, which forces the SW pin to swing to the output voltage plus the forward voltage (VD) of the diode. The regulator loop adjusts the duty cycle (D) to maintain a regulated LED current. Application Information THEORY OF OPERATION The LM3410 is a constant frequency PWM, boost regulator IC. It delivers a minimum of 2.1A peak switch current. The device operates very similar to a voltage regulated boost converter except that it regulates the output current through LEDs. The current magnitude is set with a series resistor. This series resistor multiplied by the LED current creates the feedback voltage (190 mV) which the converter regulates to. The regulator has a preset switching frequency of either 525 kHz or 1.60 MHz. This high frequency allows the LM3410 to operate with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a minimum amount of board space. The LM3410 is internally compensated, so it is simple to use, and requires few external components. The LM3410 uses current-mode control to regulate the LED current. The following operating description of the LM3410 will refer to the Simplified Block Diagram (Figure 1) the simplified schematic (Figure 2), and its associated waveforms (Figure 3). The LM3410 supplies a regulated LED current by switching the internal NMOS control switch at constant frequency and variable duty cycle. A switching cycle www.national.com 8 LM3410 Design Guide SETTING THE LED CURRENT 30038515 FIGURE 2. Simplified Boost Topology Schematic 30038517 FIGURE 4. Setting ILED The LED current is set using the following equation: where RSET is connected between the FB pin and GND. DIM PIN / SHUTDOWN MODE The average LED current can be controlled using a PWM signal on the DIM pin. The duty cycle can be varied between 0 & 100% to either increase or decrease LED brightness. PWM frequencies in the range of 1 Hz to 25 kHz can be used. For controlling LED currents down to the µA levels, it is best to use a PWM signal frequency between 200-1 kHz. The maximum LED current would be achieved using a 100% duty cycle, i.e. the DIM pin always high. LED-DRIVE CAPABILITY When using the LM3410 in the typical application configuration, with LEDs stacked in series between the VOUT and FB pin, the maximum number of LEDs that can be placed in series is dependent on the maximum LED forward voltage (VFMAX). (VFMAX x NLEDs) + 190 mV < 24V When inserting a value for maximum VFMAX the LED forward voltage variation over the operating temperature range should be considered. THERMAL SHUTDOWN Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature exceeds 165°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature drops to approximately 150°C. 30038516 FIGURE 3. Typical Waveforms INDUCTOR SELECTION The inductor value determines the input ripple current. Lower inductor values decrease the physical size of the inductor, but increase the input ripple current. An increase in the inductor value will decrease the input ripple current. CURRENT LIMIT The LM3410 uses cycle-by-cycle current limiting to protect the internal NMOS switch. It is important to note that this current limit will not protect the output from excessive current during an output short circuit. The input supply is connected to the output by the series connection of an inductor and a diode. If a short circuit is placed on the output, excessive current can damage both the inductor and diode. 9 www.national.com LM3410 From the previous equations, the inductor value is then obtained. Where 30038519 1/TS = fSW FIGURE 5. Inductor Current One must also ensure that the minimum current limit (2.1A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (Lpk I) in the inductor is calculated by: ILpk = IIN + ΔIL or ILpk = IOUT /D' + ΔiL When selecting an inductor, make sure that it is capable of supporting the peak input current without saturating. Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating correctly. Because of the speed of the internal current limit, the peak current of the inductor need only be specified for the required maximum input current. For example, if the designed maximum input current is 1.5A and the peak current is 1.75A, then the inductor should be specified with a saturation current limit of >1.75A. There is no need to specify the saturation or peak current of the inductor at the 2.8A typical switch current limit. Because of the operating frequency of the LM3410, ferrite based inductors are preferred to minimize core losses. This presents little restriction since the variety of ferrite-based inductors is huge. Lastly, inductors with lower series resistance (DCR) will provide better operating efficiency. For recommended inductors see Example Circuits. The Duty Cycle (D) for a Boost converter can be approximated by using the ratio of output voltage (VOUT) to input voltage (VIN). Therefore: Therefore: INPUT CAPACITOR An input capacitor is necessary to ensure that VIN does not drop excessively during switching transients. The primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and ESL (Equivalent Series Inductance). The recommended input capacitance is 2.2 µF to 22 µF depending on the application. The capacitor manufacturer specifically states the input voltage rating. Make sure to check any recommended deratings and also verify if there is any significant change in capacitance at the operating input voltage and the operating temperature. The ESL of an input capacitor is usually determined by the effective cross sectional area of the current path. At the operating frequencies of the LM3410, certain capacitors may have an ESL so large that the resulting impedance (2πfL) will be higher than that required to provide stable operation. As a result, surface mount capacitors are strongly recommended. Multilayer ceramic capacitors (MLCC) are good choices for both input and output capacitors and have very low ESL. For MLCCs it is recommended to use X7R or X5R dielectrics. Consult capacitor manufacturer datasheet to see how rated capacitance varies over operating conditions. Inductor ripple in a LED driver circuit can be greater than what would normally be allowed in a voltage regulator Boost & Sepic design. A good design practice is to allow the inductor to produce 20% to 50% ripple of maximum load. The increased ripple shouldn’t be a problem when illuminating LEDs. OUTPUT CAPACITOR The LM3410 operates at frequencies allowing the use of ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple. The output capacitor is selected based upon the desired output ripple and transient response. The initial current of a load transient is provided mainly by the output capacitor. The output impedance will therefore determine the maximum voltage perturbation. The output ripple of the converter is a function Power losses due to the diode (D1) forward voltage drop, the voltage drop across the internal NMOS switch, the voltage drop across the inductor resistance (RDCR) and switching losses must be included to calculate a more accurate duty cycle (See Calculating Efficiency and Junction Temperature for a detailed explanation). A more accurate formula for calculating the conversion ratio is: Where η equals the efficiency of the LM3410 application. Or: www.national.com 10 LM3410 of the capacitor’s reactance and its equivalent series resistance (ESR): When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the output ripple will be approximately sinusoidal and 90° phase shifted from the switching action. Given the availability and quality of MLCCs and the expected output voltage of designs using the LM3410, there is really no need to review any other capacitor technologies. Another benefit of ceramic capacitors is their ability to bypass high frequency noise. A certain amount of switching edge noise will couple through parasitic capacitances in the inductor to the output. A ceramic capacitor will bypass this noise while a tantalum will not. Since the output capacitor is one of the two external components that control the stability of the regulator control loop, most applications will require a minimum at 0.47 µF of output capacitance. Like the input capacitor, recommended multilayer ceramic capacitors are X7R or X5R. Again, verify actual capacitance at the desired operating voltage and temperature. 30038530 FIGURE 6. Overvoltage Protection Circuit PCB Layout Considerations When planning layout there are a few things to consider when trying to achieve a clean, regulated output. The most important consideration when completing a Boost Converter layout is the close coupling of the GND connections of the COUT capacitor and the LM3410 PGND pin. The GND ends should be close to one another and be connected to the GND plane with at least two through-holes. There should be a continuous ground plane on the bottom layer of a two-layer board except under the switching node island. The FB pin is a high impedance node and care should be taken to make the FB trace short to avoid noise pickup and inaccurate regulation. The RSET feedback resistor should be placed as close as possible to the IC, with the AGND of RSET (R1) placed as close as possible to the AGND (pin 5 for the LLP) of the IC. Radiated noise can be decreased by choosing a shielded inductor. The remaining components should also be placed as close as possible to the IC. Please see Application Note AN-1229 for further considerations and the LM3410 demo board as an example of a four-layer layout. Below is an example of a good thermal & electrical PCB design. DIODE The diode (D1) conducts during the switch off time. A Schottky diode is recommended for its fast switching times and low forward voltage drop. The diode should be chosen so that its current rating is greater than: ID1 ≥ IOUT The reverse breakdown rating of the diode must be at least the maximum output voltage plus appropriate margin. OUTPUT OVER-VOLTAGE PROTECTION A simple circuit consisting of an external zener diode can be implemented to protect the output and the LM3410 device from an over-voltage fault condition. If an LED fails open, or is connected backwards, an output open circuit condition will occur. No current is conducted through the LED’s, and the feedback node will equal zero volts. The LM3410 will react to this fault by increasing the duty-cycle, thinking the LED current has dropped. A simple circuit that protects the LM3410 is shown in figure 6. Zener diode D2 and resistor R3 is placed from VOUT in parallel with the string of LEDs. If the output voltage exceeds the breakdown voltage of the zener diode, current is drawn through the zener diode, R3 and sense resistor R1. Once the voltage across R1 and R3 equals the feedback voltage of 190mV, the LM3410 will limit its duty-cycle. No damage will occur to the LM3410, the LED’s, or the zener diode. Once the fault is corrected, the application will work as intended. 30038532 FIGURE 7. Boost PCB Layout Guidelines This is very similar to our LM3410 demonstration boards that are obtainable via the National Semiconductor website. The demonstration board consists of a two layer PCB with a common input and output voltage application. Most of the routing is on the top layer, with the bottom layer consisting of a large ground plane. The placement of the external components satisfies the electrical considerations, and the thermal perfor- 11 www.national.com LM3410 mance has been improved by adding thermal vias and a top layer “Dog-Bone”. For certain high power applications, the PCB land may be modified to a "dog bone" shape (see Figure 8). Increasing the size of ground plane and adding thermal vias can reduce the RθJA for the application. consideration. This contradiction is the placement of external components that dissipate heat. The greatest external heat contributor is the external Schottky diode. It would be nice if you were able to separate by distance the LM3410 from the Schottky diode, and thereby reducing the mutual heating effect. This will however create electrical performance issues. It is important to keep the LM3410, the output capacitor, and Schottky diode physically close to each other (see PCB layout guidelines). The electrical design considerations outweigh the thermal considerations. Other factors that influence thermal performance are thermal vias, copper weight, and number of board layers. Thermal Definitions Heat energy is transferred from regions of high temperature to regions of low temperature via three basic mechanisms: radiation, conduction and convection. Radiation: Electromagnetic transfer of heat between masses at different temperatures. Conduction: Transfer of heat through a solid medium. Convection: Transfer of heat through the medium of a fluid; typically air. Conduction & Convection will be the dominant heat transfer mechanism in most applications. RθJA: Thermal impedance from silicon junction to ambient air temperature. RθJC: Thermal impedance from silicon junction to device case temperature. CθJC: Thermal Delay from silicon junction to device case temperature. CθCA: Thermal Delay from device case to ambient air temperature. RθJA & RθJC: These two symbols represent thermal impedances, and most data sheets contain associated values for these two symbols. The units of measurement are °C/ Watt. RθJA is the sum of smaller thermal impedances (see simplified thermal model Figures 9 and 10). Capacitors within the model represent delays that are present from the time that power and its associated heat is increased or decreased from steady state in one medium until the time that the heat increase or decrease reaches steady state in the another medium. The datasheet values for these symbols are given so that one might compare the thermal performance of one package against another. To achieve a comparison between packages, all other variables must be held constant in the comparison (PCB size, copper weight, thermal vias, power dissipation, VIN, VOUT, load current etc). This does shed light on the package performance, but it would be a mistake to use these values to calculate the actual junction temperature in your application. 30038533 FIGURE 8. PCB Dog Bone Layout Thermal Design When designing for thermal performance, one must consider many variables: Ambient Temperature: The surrounding maximum air temperature is fairly explanatory. As the temperature increases, the junction temperature will increase. This may not be linear though. As the surrounding air temperature increases, resistances of semiconductors, wires and traces increase. This will decrease the efficiency of the application, and more power will be converted into heat, and will increase the silicon junction temperatures further. Forced Airflow: Forced air can drastically reduce the device junction temperature. Air flow reduces the hot spots within a design. Warm airflow is often much better than a lower ambient temperature with no airflow. External Components: Choose components that are efficient, and you can reduce the mutual heating between devices. PCB design with thermal performance in mind: The PCB design is a very important step in the thermal design procedure. The LM3410 is available in three package options (5 pin SOT23, 8 pin eMSOP & 6 pin LLP). The options are electrically the same, but difference between the packages is size and thermal performance. The LLP and eMSOP have thermal Die Attach Pads (DAP) attached to the bottom of the packages, and are therefore capable of dissipating more heat than the SOT23 package. It is important that the customer choose the correct package for the application. A detailed thermal design procedure has been included in this data sheet. This procedure will help determine which package is correct, and common applications will be analyzed. There is one significant thermal PCB layout design consideration that contradicts a proper electrical PCB layout design www.national.com LM3410 Thermal Models Heat is dissipated from the LM3410 and other devices. The external loss elements include the Schottky diode, inductor, and loads. All loss elements will mutually increase the heat on the PCB, and therefore increase each other’s temperatures. 12 LM3410 30038534 FIGURE 9. Thermal Schematic 30038535 FIGURE 10. Associated Thermal Model 13 www.national.com LM3410 Calculating Efficiency, and Junction Temperature We will talk more about calculating proper junction temperature with relative certainty in a moment. For now we need to describe how to calculate the junction temperature and clarify some common misconceptions. One can see that if the loss elements are reduced to zero, the conversion ratio simplifies to: And we know: A common error when calculating R θJA is to assume that the package is the only variable to consider. RθJA [variables]: • Input Voltage, Output Voltage, Output Current, RDS(ON) • Ambient temperature & air flow • Internal & External components power dissipation • Package thermal limitations • PCB variables (copper weight, thermal via’s, layers component placement) Another common error when calculating junction temperature is to assume that the top case temperature is the proper temperature when calculating RθJC. RθJC represents the thermal impedance of all six sides of a package, not just the top side. This document will refer to a thermal impedance called . represents a thermal impedance associated with just the top case temperature. This will allow one to calculate the junction temperature with a thermal sensor connected to the top case. The complete LM3410 Boost converter efficiency can be calculated in the following manner. Therefore: Calculations for determining the most significant power losses are discussed below. Other losses totaling less than 2% are not discussed. A simple efficiency calculation that takes into account the conduction losses is shown below: The diode, NMOS switch, and inductor DCR losses are included in this calculation. Setting any loss element to zero will simplify the equation. VD is the forward voltage drop across the Schottky diode. It can be obtained from the manufacturer’s Electrical Characteristics section of the data sheet. The conduction losses in the diode are calculated as follows: Power loss (PLOSS) is the sum of two types of losses in the converter, switching and conduction. Conduction losses usually dominate at higher output loads, where as switching losses remain relatively fixed and dominate at lower output loads. Losses in the LM3410 Device: PLOSS = PCOND + PSW + PQ Where PQ = quiescent operating power loss Conversion ratio of the Boost Converter with conduction loss elements inserted: PDIODE = VD x ILED Depending on the duty cycle, this can be the single most significant power loss in the circuit. Care should be taken to choose a diode that has a low forward voltage drop. Another concern with diode selection is reverse leakage current. Depending on the ambient temperature and the reverse voltage across the diode, the current being drawn from the output to the NMOS switch during time D could be significant, this may increase losses internal to the LM3410 and reduce the overall efficiency of the application. Refer to Schottky diode manufacturer’s data sheets for reverse leakage specifications, and typical applications within this data sheet for diode selections. Another significant external power loss is the conduction loss in the input inductor. The power loss within the inductor can be simplified to: Where RDCR = Inductor series resistance www.national.com 14 Quiescent Power Losses IQ is the quiescent operating current, and is typically around 1.5 mA. or PQ = IQ x VIN RSET Power Loss The LM3410 conduction loss is mainly associated with the internal power switch: PCOND-NFET = I2SW-rms x RDSON x D Example Efficiency Calculation: Operating Conditions: 5 x 3.3V LEDs + 190mVREF ≊ 16.7V TABLE 1. Operating Conditions VIN 3.3V VOUT 16.7V ILED 50mA 30038542 FIGURE 11. LM3410 Switch Current VD 0.45V fSW 1.60MHz IQ 3mA tRISE 10nS tFALL 10nS (small ripple approximation) RDSON 225mΩ PCOND-NFET = IIN2 x RDSON x D LDCR 75mΩ or D 0.82 IIN 0.31A ΣPCOND + PSW + PDIODE + PIND + PQ = PLOSS Quiescent Power Loss: PQ = IQ x VIN = 10 mW The value for RDSON should be equal to the resistance at the junction temperature you wish to analyze. As an example, at 125°C and RDSON = 250 mΩ (See typical graphs for value). Switching losses are also associated with the internal power switch. They occur during the switch on and off transition periods, where voltages and currents overlap resulting in power loss. The simplest means to determine this loss is to empirically measuring the rise and fall times (10% to 90%) of the switch at the switch node: Switching Power Loss: PSWR = 1/2(VOUT x IIN x fSW x tRISE) ≊ 40 mW PSWF = 1/2(VOUT x IIN x fSW x tFALL) ≊ 40 mW PSW = PSWR + PSWF = 80 mW Internal NFET Power Loss: RDSON = 225 mΩ PCONDUCTION = IIN2 x D x RDSON = 17 mW PSWR = 1/2(VOUT x IIN x fSW x tRISE) IIN = 310 mA PSWF = 1/2(VOUT x IIN x fSW x tFALL) Diode Loss: VD = 0.45V PSW = PSWR + PSWF PDIODE = VD x ILED = 23 mW Typical Switch-Node Rise and Fall Times VIN VOUT tRISE tFALL 3V 5V 6nS 4nS 5V 12V 6nS 5nS 3V 12V 8nS 7nS 5V 18V 10nS 8nS Inductor Power Loss: RDCR = 75 mΩ PIND = IIN2 x RDCR = 7 mW 15 www.national.com LM3410 PIND = IIN2RDCR LM3410 Total Power Losses are: TABLE 2. Power Loss Tabulation VIN 3.3V VOUT 16.7V ILED 50mA POUT 825W VD 0.45V PDIODE 23mW fSW 1.6MHz IQ 10nS PSWR 40mW tRISE 10nS PSWF 40mW IQ 3mA PQ 10mW RDSON 225mΩ PCOND 17mW LDCR 75mΩ PIND 7mW PLOSS 137mW D 0.82 η 85% SOT23-5 = 93°C/W SOT23-5 = 56°C/W Typical LLP & eMSOP typical applications will produce numbers in the range of 50°C/W to 65°C/W, and will vary between 18°C/W and 28°C/W. These values are for PCB’s with two and four layer boards with 0.5 oz copper, and four to six thermal vias to bottom side ground plane under the DAP. The thermal impedances calculated above are higher due to the small amount of power being dissipated within the device. Note: To use these procedures it is important to dissipate an amount of power within the device that will indicate a true thermal impedance value. If one uses a very small internal dissipated value, one can see that the thermal impedance calculated is abnormally high, and subject to error. Figure 12 shows the nonlinear relationship of internal power dissipation vs . . PINTERNAL = PCOND + PSW = 107 mW Calculating and We now know the internal power dissipation, and we are trying to keep the junction temperature at or below 125°C. The next step is to calculate the value for and/or . This is actually very simple to accomplish, and necessary if you think you may be marginal with regards to thermals or determining what package option is correct. The LM3410 has a thermal shutdown comparator. When the silicon reaches a temperature of 165°C, the device shuts down until the temperature drops to 150°C. Knowing this, one or the of a specific application. Becan calculate the cause the junction to top case thermal impedance is much lower than the thermal impedance of junction to ambient air, the error in calculating is lower than for . However, you will need to attach a small thermocouple onto the top case value. of the LM3410 to obtain the Knowing the temperature of the silicon when the device shuts down allows us to know three of the four variables. Once we calculate the thermal impedance, we then can work backwards with the junction temperature set to 125°C to see what maximum ambient air temperature keeps the silicon below the 125°C temperature. Procedure: Place your application into a thermal chamber. You will need to dissipate enough power in the device so you can obtain a good thermal impedance value. Raise the ambient air temperature until the device goes into thermal shutdown. Record the temperatures of the ambient air and/or the top case temperature of the LM3410. Calculate the thermal impedances. Example from previous calculations (SOT23-5 Package): PINTERNAL = 107 mW TA @ Shutdown = 155°C TC @ Shutdown = 159°C www.national.com 30038551 FIGURE 12. RθJA vs Internal Dissipation For 5-pin SOT23 package typical applications, RθJA numbers will vary between will range from 80°C/W to 110°C/W, and 50°C/W and 65°C/W. These values are for PCB’s with two & four layer boards with 0.5 oz copper, with two to four thermal vias from GND pin to bottom layer. Here is a good rule of thumb for typical thermal impedances, and an ambient temperature maximum of 75°C: If your design requires that you dissipate more than 400mW internal to the LM3410, or there is 750mW of total power loss in the application, it is recommended that you use the 6 pin LLP or the 8 pin eMSOP package with the exposed DAP. SEPIC Converter The LM3410 can easily be converted into a SEPIC converter. A SEPIC converter has the ability to regulate an output voltage that is either larger or smaller in magnitude than the input voltage. Other converters have this ability as well (CUK and Buck-Boost), but usually create an output voltage that is opposite in polarity to the input voltage. This topology is a perfect fit for Lithium Ion battery applications where the input voltage for a single cell Li-Ion battery will vary between 2.7V & 4.5V and the output voltage is somewhere in between. Most of the 16 Therefore: 30038556 FIGURE 13. Inductor Volt-Sec Balance Waveform Small ripple approximation: In a well-designed SEPIC converter, the output voltage, and input voltage ripple, the inductor ripple IL1 and IL2 is small in comparison to the DC magnitude. Therefore it is a safe approximation to assume a DC value for these components. The main objective of the Steady State Analysis is to determine the steady state duty-cycle, voltage and current stresses on all components, and proper values for all components. In a steady-state converter, the net volt-seconds across an inductor after one cycle will equal zero. Also, the charge into a capacitor will equal the charge out of a capacitor in one cycle. Therefore: Applying Charge balance on C1: Since there are no DC voltages across either inductor, and capacitor C3 is connected to Vin through L1 at one end, or to ground through L2 on the other end, we can say that VC3 = VIN Therefore: This verifies the original conversion ratio equation. It is important to remember that the internal switch current is equal to IL1 and IL2 during the D interval. Design the converter so that the minimum guaranteed peak switch current limit (2.1A) is not exceeded. Substituting IL1 into IL2 IL2 = ILED 30038552 FIGURE 14. HB/OLED SEPIC CONVERTER Schematic 17 www.national.com LM3410 The average inductor current of L2 is the average output load. analysis of the LM3410 Boost Converter is applicable to the LM3410 SEPIC Converter. SEPIC Design Guide: SEPIC Conversion ratio without loss elements: LM3410 Steady State Analysis with Loss Elements 30038559 FIGURE 15. SEPIC Simplified Schematic Using inductor volt-second balance & capacitor charge balance, the following equations are derived: TABLE 3. Efficiencies for Typical SEPIC Applications IL2 = (ILED) and IL1 = (ILED) x (D/D') VIN 2.7V VIN 3.3V VIN 5V VOUT 3.1V VOUT 3.1V VOUT 3.1V IIN 770mA IIN 600mA IIN 375mA ILED 500mA ILED 500mA ILED 500mA η 75% η 80% η 83% SEPIC Converter PCB Layout The layout guidelines described for the LM3410 Boost-Converter are applicable to the SEPIC OLED Converter. Figure 16 is a proper PCB layout for a SEPIC Converter. Therefore: One can see that all variables are known except for the duty cycle (D). A quadratic equation is needed to solve for D. A less accurate method of determining the duty cycle is to assume efficiency, and calculate the duty cycle. 30038565 FIGURE 16. HB/OLED SEPIC PCB Layout www.national.com 18 LM3410 LM3410X SOT23-5 Design Example 1: 5 x 1206 Series LED String Application LM3410X (1.6MHz): VIN = 2.7V to 5.5V, 5 x 3.3V LEDs, (VOUT ≊ 16.5V) ILED ≊ 50mA 30038581 Part ID Part Value Manufacturer Part Number U1 2.8A ISW LED Driver NSC LM3410XMF C1, Input Cap 10µF, 6.3V, X5R TDK C2012X5R0J106M C2 Output Cap 2.2µF, 25V, X5R TDK C2012X5R1E225M D1, Catch Diode 0.4Vf Schottky 500mA, 30VR Diodes Inc MBR0530 L1 10µH 1.2A Coilcraft DO1608C-103 R1 4.02Ω, 1% Vishay CRCW08054R02F R2 100kΩ, 1% Vishay CRCW08051003F LED's SMD-1206, 50mA, Vf ≊ 3 .6V Lite-On LTW-150k 19 www.national.com LM3410 LM3410Y SOT23-5 Design Example 2: 5 x 1206 Series LED String Application LM3410Y (550kHz): VIN = 2.7V to 5.5V, 5 x 3.3V LEDs, (VOUT ≊ 16.5V) ILED ≊ 50mA 30038581 Part ID Part Value Manufacturer Part Number U1 2.8A ISW LED Driver NSC LM3410YMF C1, Input Cap 10µF, 6.3V, X5R TDK C2012X5R0J106M C2 Output Cap 2.2µF, 25V, X5R TDK C2012X5R1E225M D1, Catch Diode 0.4Vf Schottky 500mA, 30VR Diodes Inc MBR0530 L1 15µH 1.2A Coilcraft DO1608C-153 R1 4.02Ω, 1% Vishay CRCW08054R02F R2 100kΩ, 1% Vishay CRCW08051003F LED's SMD-1206, 50mA, Vf ≊ 3 .6V Lite-On LTW-150k www.national.com 20 LM3410 LM3410X LLP-6 Design Example 3: 7 LEDs x 5 LED String Backlighting Application LM3410X (1.6MHz): VIN = 2.7V to 5.5V, 7 x 5 x 3.3V LEDs, (VOUT ≊ 16.7V), ILED ≊ 25mA 300385a2 Part ID Part Value Manufacturer Part Number U1 2.8A ISW LED Driver NSC LM3410XSD C1, Input Cap 10µF, 6.3V, X5R TDK C2012X5R0J106M C2 Output Cap 4.7µF, 25V, X5R TDK C2012X5R1E475M D1, Catch Diode 0.4Vf Schottky 500mA, 30VR Diodes Inc MBR0530 L1 8.2µH, 2A Coilcraft MSS6132-822ML R1 1.15Ω, 1% Vishay CRCW08051R15F R2 100kΩ, 1% Vishay CRCW08051003F LED's SMD-1206, 50mA, Vf ≊ 3 .6V Lite-On LTW-150k 21 www.national.com LM3410 LM3410X LLP-6 Design Example 4: 3 x HB LED String Application LM3410X (1.6MHz): VIN = 2.7V to 5.5V, 3 x 3.4V LEDs, (VOUT ≊ 11V) ILED ≊ 340mA 30038567 Part ID Part Value Manufacturer Part Number U1 2.8A ISW LED Driver NSC LM3410XSD C1, Input Cap 10µF, 6.3V, X5R TDK C2012X5R0J106M C2 Output Cap 2.2µF, 25V, X5R TDK C2012X5R1E225M D1, Catch Diode 0.4Vf Schottky 500mA, 30VR Diodes Inc MBR0530 L1 10µH 1.2A Coilcraft DO1608C-103 R1 1.00Ω, 1% Vishay CRCW08051R00F R2 100kΩ, 1% Vishay CRCW08051003F R3 1.50Ω, 1% Vishay CRCW08051R50F HB - LED's 340mA, Vf ≊ 3 .6V CREE XREWHT-L1-0000-0901 www.national.com 22 LM3410 LM3410Y SOT23-5 Design Example 5: 5 x 1206 Series LED String Application with OVP LM3410Y (525kHz): VIN = 2.7V to 5.5V, 5 x 3.3V LEDs, (VOUT ≊ 16.5V) ILED ≊ 50mA 30038568 Part ID Part Value Manufacturer Part Number U1 2.8A ISW LED Driver NSC LM3410YMF C1 Input Cap 10µF, 6.3V, X5R TDK C2012X5R0J106M C2 Output Cap 2.2µF, 25V, X5R TDK C2012X5R1E225M D1, Catch Diode 0.4Vf Schottky 500mA, Diodes Inc MBR0530 D2 18V Zener diode Diodes Inc 1N4746A L1 15µH, 0.70A TDK VLS4012T-150MR65 R1 4.02Ω, 1% Vishay CRCW08054R02F R2 100kΩ, 1% Vishay CRCW08051003F R3 100kΩ, 1% Vishay CRCW06031000F LED’s SMD-1206, 50mA, Vf ≊ 3 .6V Lite-On LTW-150k 23 www.national.com LM3410 LM3410X SEPIC LLP-6 Design Example 6: HB/OLED Illumination Application LM3410X (1.6MHz): VIN = 2.7V to 5.5V, (VOUT ≊ 3.8V) ILED ≊ 300mA 30038552 Part ID Part Value Manufacturer U1 2.8A ISW LED Driver NSC LM3410XSD C1 Input Cap 10µF, 6.3V, X5R TDK C2012X5R0J106K C2 Output Cap 10µF, 6.3V, X5R TDK C2012X5R0J106K C3 Cap 2.2µF, 25V, X5R TDK C2012X5R1E225M D1, Catch Diode 0.4Vf, Schottky 1A, 20VR Diodes Inc DFLS120L L1 & L2 4.7µH 3A Coilcraft MSS6132-472 R1 665 mΩ, 1% Vishay CRCW0805R665F R2 100kΩ, 1% Vishay CRCW08051003F HB - LED’s 350mA, Vf ≊ 3 .6V CREE XREWHT-L1-0000-0901 www.national.com 24 Part Number LM3410 LM3410X LLP-6 Design Example 7: Boost Flash Application LM3410X (1.6MHz): VIN = 2.7V to 5.5V, (VOUT ≊ 8V) ILED ≊ 1.0A Pulsed 30038570 Part ID Part Value Manufacturer Part Number U1 2.8A ISW LED Driver NSC LM3410XSD C1 Input Cap 10µF, 6.3V, X5R TDK C2012X5R0J106M C2 Output Cap 10µF,16V, X5R TDK C2012X5R1A106M D1, Catch Diode 0.4Vf Schottky 500mA, 30VR Diodes Inc MBR0530 L1 4.7µH, 3A Coilcraft MSS6132-472 R1 200mΩ, 1% Vishay CRCW0805R200F LED’s 500mA, Vf ≊ 3 .6V, IPULSE = 1.0A CREE XREWHT-L1-0000-0901 25 www.national.com LM3410 LM3410X SOT23-5 Design Example 8: 5 x 1206 Series LED String Application with VIN > 5.5V LM3410X (1.6MHz): VPWR = 9V to 14V, (VOUT ≊ 16.5V) ILED ≊ 50mA Part ID Part Value Mfg 30038571 Part Number U1 2.8A ISW LED Driver NSC LM3410XMF C1 Input VPWR Cap 10µF, 6.3V, X5R TDK C2012X5R0J106M C2 Output Cap 2.2µF, 25V, X5R TDK C2012X5R1E225M C2 Input VIN Cap 0.1µF, 6.3V, X5R TDK C1005X5R1C104K D1, Catch Diode 0.43Vf, Schotky, 0.5A, 30VR Diodes Inc MBR0530 www.national.com L1 10µH 1.2A Coilcraft DO1608C-103 R1 4.02Ω, 1% Vishay CRCW08054R02F R2 100kΩ, 1% Vishay CRCW08051003F R3 576Ω, 1% 3.3V Zener, SOT23 Vishay CRCW08055760F D2 Diodes Inc BZX84C3V3 LED’s SMD-1206, 50mA, Vf ≊ 3 .6V Lite-On LTW-150k 26 LM3410 LM3410X LLP-6 Design Example 9: Camera Flash or Strobe Circuit Application LM3410X (1.6MHz): VIN = 2.7V to 5.5, (VOUT ≊ 7.5V), ILED ≊ 1.5A Flash 30038572 Part ID Part Value Mfg Part Number U1 2.8A ISW LED Driver NSC LM3410XSD C1608X5R0J106K C1 Input VPWR Cap 10µF, 6.3V, X5R TDK C2 Output Cap 220µF, 10V, Tanatalum KEMET T491V2271010A2 C3 Cap 10µF, 16V, X5R TDK C3216X5R0J106K D1, Catch Diode 0.43Vf, Schotky, 1.0A, 20VR Diodes Inc DFLS120L L1 3.3µH 2.7A Coilcraft MOS6020-332 R1 1.0kΩ, 1% Vishay CRCW08051001F R2 37.4kΩ, 1% Vishay CRCW08053742F R3 100kΩ, 1% Vishay CRCW08051003F R4 Vishay CRCW0805R150F Q1, Q2 0.15Ω, 1% 30V, ID = 3.9A ZETEX ZXMN3A14F LED’s 500mA, Vf ≊ 3 .6V, IPULSE = 1.5A CREE XREWHT-L1-0000-00901 27 www.national.com LM3410 LM3410X SOT23-5 Design Example 10: 5 x 1206 Series LED String Application with VIN & VPWR Rail > 5.5V LM3410X (1.6MHz): VPWR = 9V to 14V, VIN = 2.7V to 5.5V, (VOUT ≊ 14V) ILED ≊ 50mA 30038573 Part ID Part Value Mfg Part Number U1 2.8A ISW LED Driver NSC LM3410XMF C1 Input VPWR Cap 10µF, 6.3V, X5R TDK C2012X5R0J106M C2 VOUT Cap 2.2µF, 25V, X5R TDK C2012X5R1E225M C3 Input VIN Cap 0.1µF, 6.3V, X5R TDK C1005X5R1C104K D1, Catch Diode 0.43Vf, Schotky, 0.5A, 30VR Diodes Inc MBR0530 www.national.com L1 10µH 1.5A Coilcraft DO1608C-103 R1 4.02Ω, 1% Vishay CRCW08054R02F R2 100kΩ, 1% Vishay CRCW08051003F LED’s SMD-1206, 50mA, Vf ≊ 3 .6V Lite-On LTW-150k 28 LM3410 LM3410X LLP-6 Design Example 11: Boot-Strap Circuit to Extended Battery Life 30038574 LM3410X (1.6MHz): VIN = 1.9V to 5.5V, VIN > 2.3V (TYP) for Start Up Part ID Part Value Mfg U1 2.8A ISW LED Driver NSC Part Number LM3410XSD C1 Input VPWR Cap 10µF, 6.3V, X5R TDK C1608X5R0J106K C2 VOUT Cap 10µF, 6.3V, X5R TDK C1608X5R0J106K C3 Input VIN Cap 0.1µF, 6.3V, X5R TDK C1005X5R1C104K D1, Catch Diode 0.43Vf, Schotky, 1.0A, 20VR Diodes Inc DFLS120L D2, D3 Dual Small Signal Schotky Diodes Inc BAT54CT L1, L2 3.3µH 3A Coilcraft MOS6020-332 R1 665 mΩ, 1% Vishay CRCW0805R665F R3 100kΩ, 1% Vishay CRCW08051003F HB/OLED 3.4Vf, 350mA TT Electronics/Optek OVSPWBCR44 29 www.national.com LM3410 Physical Dimensions inches (millimeters) unless otherwise noted 6-Lead LLP Package NS Package Number SDE06A 5-Lead SOT23-5 Package NS Package Number MF05A www.national.com 30 LM3410 8-Lead eMSOP Package NS Package Number MUY08A 31 www.national.com LM3410 PowerWise® 525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with Internal Compensation Notes 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 www.national.com/webench Audio www.national.com/audio Analog University www.national.com/AU Clock Conditioners www.national.com/timing App Notes www.national.com/appnotes Data Converters www.national.com/adc Distributors www.national.com/contacts Displays www.national.com/displays Green Compliance www.national.com/quality/green Ethernet www.national.com/ethernet Packaging www.national.com/packaging Interface www.national.com/interface Quality and Reliability www.national.com/quality LVDS www.national.com/lvds Reference Designs www.national.com/refdesigns Power Management www.national.com/power Feedback www.national.com/feedback Switching Regulators www.national.com/switchers LDOs www.national.com/ldo LED Lighting www.national.com/led PowerWise www.national.com/powerwise Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors Wireless (PLL/VCO) www.national.com/wireless THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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