LM3501 Synchronous Step-up DC/DC Converter for White LED Applications General Description Features The LM3501 is a fixed-frequency step-up DC/DC converter that is ideal for driving white LEDs for display backlighting and other lighting functions. With fully intergrated synchronous switching (no external schottky diode required) and a low feedback voltage (515 mV), power efficiency of the LM3501 circuit has been optimized for lighting applications in wireless phones and other portable products (single cell Li-Ion or 3-cell NiMH battery supplies). The LM3501 operates with a fixed 1 MHz switching frequency. When used with ceramic input and output capacitors, the LM3501 provides a small, low-noise, low-cost solution. n Synchronous rectification, high efficiency and no external schottky diode required n Uses small surface mount components n Can drive 2-5 white LEDs in series (may function with more low VF LEDs) n 2.7V to 7V input range n True shutdown isolation, no LED leakage current n DC voltage LED current control n Input undervoltage lockout n Internal output over-voltage protection (OVP) circuitry, with no external zener diode required LM3501-16: 15.5V OVP; LM3501-21: 20.5V OVP. n Requires only a small 16V (LM3501-16) or 25V (LM3501-21) ceramic capacitor at the input and output n Thermal Shutdown n 0.1µA shutdown current n Small 8-bump thin micro SMD package Two LM3501 options are available with different output voltage capabilities. The LM3501-21 has a maximum output voltage of 21V and is typically suited for driving 4 or 5 white LEDs in series. The LM3501-16 has a maximum output voltage of 16V and is typically suited for driving 3 or 4 white LEDs in series (maximum number of series LEDs dependent on LED forward voltage). If the primary white LED network should be disconnected, the LM3501 uses internal protection circuitry on the output to prevent a destructive overvoltage event. A single external resistor is used to set the maximum LED current in LED-drive applications. The LED current can easily be adjusted by varying the analog control voltage on the control pin or by using a pulse width modulated (PWM) signal on the shutdown pin. In shutdown, the LM3501 completely disconnects the input from output, creating total isolation and preventing any leakage currents from trickling into the LEDs. Applications n n n n n LCD Bias Supplies White LED Back-Lighting Handheld Devices Digital Cameras Portable Applications Typical Application Circuit 20065301 FIGURE 1. Typical 3 LED Application © 2005 National Semiconductor Corporation DS200653 www.national.com Synchronous Step-up DC/DC Converter for White LED Applications May 2005 LM3501 Connection Diagram Top View 20065302 8-bump micro SMD Ordering Information Order Number Package Type NSC Package Drawing Top Mark Supplied As LM3501TL-16 micro SMD TL08SSA 19 250 Units, Tape and Reel LM3501TLX-16 micro SMD TL08SSA 19 3000 Units, Tape and Reel LM3501TL-21 micro SMD TL08SSA 30 250 Units, Tape and Reel LM3501TLX-21 micro SMD TL08SSA 30 3000 Units, Tape and Reel Pin Description/Functions Pin Name Function A1 AGND B1 VIN C1 VOUT PMOS source connection for synchronous rectification. C2 VSW Switch pin. Drain connections of both NMOS and PMOS power devices. C3 GND B3 FB A3 CNTRL Analog LED current control. A2 SHDN Shutdown control pin. Analog ground. Analog and Power supply input. Power Ground. Output voltage feedback connection. AGND (pin A1): Analog ground pin. The analog ground pin should tie directly to the GND pin. VIN (pin B1):Analog and Power supply pin. Bypass this pin with a capacitor, as close to the device as possible, connected between the VIN and GND pins. VOUT (pin C1):Source connection of internal PMOS power device. Connect the output capacitor between the VOUT and GND pins as close as possible to the device. VSW (pin C2):Drain connection of internal NMOS and PMOS switch devices. Keep the inductor connection close to this pin to minimize EMI radiation. GND (pin C3):Power ground pin. Tie directly to ground plane. www.national.com FB (pin B3):Output voltage feedback connection. Set the primary White LED network current with a resistor from the FB pin to GND. Keep the current setting resistor close to the device and connected between the FB and GND pins. CNTRL (pin A3): Analog control of LED current. A voltage above 125 mV will begin to regulate the LED current. Decreasing the voltage below 75 mV will turn off the LEDs. SHDN (pin A2):Shutdown control pin. Disable the device with a voltage less than 0.3V and enable the device with a voltage greater than 1.1V. The white LED current can be controlled using a PWM signal at this pin. There is an internal pull down on the SHDN pin, the device is in a normally off state. 2 ESD Ratings (Note 3) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Human Body Model VIN −0.3V to 7.5V VOUT (LM3501-16)(Note 2) −0.3V to 16V VOUT (LM3501-21)(Note 2) −0.3V to 21V VSW (Note 2) Junction Temperature (Note 4) −0.3V to 7.5V Maximum Junction Temperature 150˚C Lead Temperature (Soldering 10 sec.) 300˚C Vapor Phase (60 sec.) 215˚C Infrared (15 sec.) 220˚C 2.7V to 7V CNTRL Max. −0.3V to VIN+0.3V CNTRL −40˚C to +125˚C Supply Voltage −0.3V to 7.5V SHDN Voltage 200V Operating Conditions −0.3V to VOUT+0.3V FB Voltage 2kV Machine Model 2.7V Thermal Properties 75˚C/W Junction to Ambient Thermal Resistance (θJA) (Note 5) Electrical Characteristics Specifications in standard type face are for TA = 25˚C and those in boldface type apply over the Operating Temperature Range of TA = −10˚C to +85˚C. Unless otherwise specified VIN = 2.7V and specifications apply to both LM3501-16 and LM3501-21. Symbol IQ VFB Parameter Conditions Min (Note 6) Typ (Note 7) Max (Note 6) 0.95 1.2 2 2.5 0.1 2 Quiescent Current, Device Not Switching FB > 0.54V Quiescent Current, Device Switching FB = 0V Shutdown SHDN = 0V Feedback Voltage CNTRL = 2.7V, VIN = 2.7V to 7V 0.485 0.515 0.545 CNTRL = 1V, VIN = 2.7V to 7V 0.14 0.19 0.24 0.1 0.5 Units mA µA V ∆VFB Feedback Voltage Line Regulation VIN = 2.7V to 7V ICL Switch Current Limit (LM3501-16) VIN = 2.7V, Duty Cycle = 80% 275 400 480 VIN = 3.0V, Duty Cycle = 70% 255 400 530 VIN = 2.7V, Duty Cycle = 70% 420 640 770 VIN = 3.0V, Duty Cycle = 63% 450 670 800 45 200 nA 7.0 V Switch Current Limit (LM3501-21) IB FB Pin Bias Current VIN Input Voltage Range RDSON DLimit FSW mA FB = 0.5V (Note 8) 2.7 NMOS Switch RDSON VIN = 2.7V, ISW = 300 mA PMOS Switch RDSON VOUT = 6V, ISW = 300 mA Duty Cycle Limit (LM3501-16) FB = 0V Duty Cycle Limit (LM3501-21) FB = 0V %/V 0.43 1.3 80 87 85 94 0.85 1.0 2.3 Ω % Switching Frequency 3 1.15 MHz www.national.com LM3501 Absolute Maximum Ratings (Note 1) LM3501 Electrical Characteristics (Continued) Specifications in standard type face are for TA = 25˚C and those in boldface type apply over the Operating Temperature Range of TA = −10˚C to +85˚C. Unless otherwise specified VIN = 2.7V and specifications apply to both LM3501-16 and LM3501-21. Symbol ISD ICNTRL IL UVP OVP IVout IVL Typ (Note 7) Max (Note 6) 1.8 4 SHDN = 2.7V 1 2.5 SHDN = GND 0.1 VCNTRL = 2.7V 10 20 VCNTRL = 1V 4 15 0.01 0.5 Parameter SHDN Pin Current (Note 9) CNTRL Pin Current (Note 9) Conditions Min (Note 6) SHDN = 5.5V Switch Leakage Current (LM3501-16) VSW = 15V Switch Leakage Current (LM3501-21) VSW = 20V Input Undervoltage Lockout ON Threshold OFF Threshold µA µA µA 0.01 2.0 2.4 2.5 2.6 2.3 2.4 2.5 Output Overvoltage Protection (LM3501-16) ON Threshold 15 15.5 16 OFF Threshold 14 14.6 15 Output Overvoltage Protection (LM3501-21) ON Threshold 20 20.5 21 OFF Threshold 19 19.5 20 260 400 300 460 0.01 3 0.01 3 VOUT Bias Current (LM3501-16) VOUT = 15V, SHDN = 1.5V VOUT Bias Current (LM3501-21) VOUT = 20V, SHDN = 1.5V PMOS Switch Leakage Current (LM3501-16) VOUT = 15V, VSW = 0V PMOS Switch Leakage Current (LM3501-21) VOUT = 20V, VSW = 0V CNTRL Threshold Units V V V µA µA LED power off 75 LED power on 125 mV SHDN Threshold SHDN low 0.65 0.3 V SHDN High 1.1 0.65 Specifications in standard type face are for TJ = 25˚C and those in boldface type apply over the full Operating Temperature Range (TJ = −40˚C to +125˚C). Unless otherwise specified VIN =2.7V and specifications apply to both LM3501-16 and LM3501-21. Symbol IQ VFB Parameter Conditions Min (Note 6) Typ (Note 7) Max (Note 6) 0.95 1.2 2 2.5 Quiescent Current, Device Not Switching FB > 0.54V Quiescent Current, Device Switching FB = 0V Shutdown SHDN = 0V 0.1 2 Feedback Voltage CNTRL = 2.7V, VIN = 2.7V to 7V 0.485 0.515 0.545 CNTRL = 1V, VIN = 2.7V to 7V 0.14 0.19 0.24 0.1 0.5 mA ∆VFB Feedback Voltage Line Regulation VIN = 2.7V to 7V ICL Switch Current Limit (LM3501-16) VIN = 3.0V, Duty Cycle = 70% 400 Switch Current Limit (LM3501-21) VIN = 3.0V, Duty Cycle = 63% 670 IB FB Pin Bias Current FB = 0.5V (Note 8) VIN Input Voltage Range www.national.com Units V %/V mA 45 2.7 4 µA 200 nA 7.0 V (Continued) Specifications in standard type face are for TJ = 25˚C and those in boldface type apply over the full Operating Temperature Range (TJ = −40˚C to +125˚C). Unless otherwise specified VIN =2.7V and specifications apply to both LM3501-16 and LM3501-21. Symbol RDSON DLimit Parameter NMOS Switch RDSON VIN = 2.7V, ISW = 300 mA PMOS Switch RDSON VOUT = 6V, ISW = 300 mA Duty Cycle Limit (LM3501-16) FB = 0V Duty Cycle Limit (LM3501-21) FB = 0V FSW Switching Frequency ISD SHDN Pin Current (Note 9) ICNTRL IL UVP OVP CNTRL Pin Current (Note 9) IVL Typ (Note 7) Max (Note 6) 0.43 1.3 2.3 Ω % 94 0.8 1.0 1.2 SHDN = 5.5V 1.8 4 SHDN = 2.7V 1 2.5 SHDN = GND 0.1 VCNTRL = 2.7V 10 20 VCNTRL = 1V 4 15 0.01 0.5 Switch Leakage Current (LM3501-21) VSW = 20V 0.01 2.0 Input Undervoltage Lockout ON Threshold 2.4 2.5 2.6 OFF Threshold 2.3 2.4 2.5 ON Threshold 15 15.5 16 Output Overvoltage Protection (LM3501-16) Units 87 VSW = 15V OFF Threshold 14 14.6 15 ON Threshold 20 20.5 21 OFF Threshold 19 19.5 20 260 400 300 460 0.01 3 0.01 3 VOUT Leakage Current (LM3501-16) VOUT = 15V, SHDN = 1.5V VOUT Leakage Current (LM3501-21) VOUT = 20V, SHDN = 1.5V PMOS Switch Leakage Current (LM3501-16) VOUT = 15V, VSW = 0V PMOS Switch Leakage Current (LM3501-21) VOUT = 20V, VSW = 0V CNTRL Threshold SHDN Threshold Min (Note 6) Switch Leakage Current (LM3501-16) Output Overvoltage Protection (LM3501-21) IVout Conditions µA µA µA V V µA µA LED power off 75 LED power on 125 SHDN low 0.65 SHDN High MHz 1.1 mV 0.3 0.65 V Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: This condition applies if VIN < VOUT. If VIN > VOUT, a voltage greater than VIN + 0.3V should not be applied to the VOUT or VSW pins. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200 pF capacitor discharged directly into each pin. Note 4: The maximum allowable power dissipation is a function of the maximum operating junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. See the Thermal Properties section for the thermal resistance. The maximum allowable power dissipation at any ambient temperature is calculated using: PD (MAX) = (TJ(MAX) − TA)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature. Note 5: Junction-to-ambient thermal resistance (θJA) is highly application and board-layout dependent. The 75oC/W figure provided was measured on a 4-layer test board conforming to JEDEC standards. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues when designing the board layout. Note 6: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are production tested, guaranteed through statistical analysis or guaranteed by design. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL). Note 7: Typical numbers are at 25˚C and represent the most likely norm. Note 8: Feedback current flows out of the pin. Note 9: Current flows into the pin. 5 www.national.com LM3501 Electrical Characteristics LM3501 Typical Performance Characteristics Switching Quiescent Current vs. VIN Non-Switching Quiescent Current vs. VIN 20065356 20065355 3 LED Efficiency vs. Load Current L = Coilcraft DT1608C-223, Efficiency = 100*(PIN/(3VLED*ILED)) 2 LED Efficiency vs. Load Current L = Coilcraft DT1608C-223, Efficiency = 100*(PIN/(2VLED*ILED)) 20065357 20065358 4 LED Efficiency vs. Load Current L = Coilcraft DT1608C-223, Efficiency = 100*(PIN/(4VLED*ILED)) Output Power vs. VIN (LM3501-16, L = Coilcraft DT1608C-223) 20065359 www.national.com 20065386 6 LM3501 Typical Performance Characteristics (Continued) Output Power vs. Temperature (LM3501-16, L = Coilcraft DT1608C-223) FB Pin Current vs. Temperature 20065387 20065360 SHDN Pin Current vs. SHDN Pin Voltage CNTRL Pin Current vs. CNTRL Pin Voltage 20065378 20065377 Switch Current Limit vs. VIN (LM3501-16) FB Voltage vs. CNTRL Voltage 20065379 20065362 7 www.national.com LM3501 Typical Performance Characteristics (Continued) Switch Current Limit vs. Temperature (LM3501-16, VOUT = 8V) Switch Current Limit vs. Temperature (LM3501-16, VOUT = 12V) 20065376 20065363 Switch Current Limit vs. Temperature (LM3501-21, VOUT = 8V) Switch Current Limit vs. VIN (LM3501-21) 20065332 20065331 Switch Current Limit vs. Temperature (LM3501-21, VOUT = 18V) Switch Current Limit vs. Temperature (LM3501-21, VOUT = 12V) 20065345 20065333 www.national.com 8 LM3501 Typical Performance Characteristics (Continued) VOUT DC Bias vs. VOUT Voltage (LM3501-16) Oscillator Frequency vs. VIN 20065364 20065365 FB Voltage vs. Temperature FB Voltage vs. Temperature 20065380 20065382 NMOS RDSON vs. VIN (ISW = 300 mA) FB Voltage vs. VIN 20065381 20065374 9 www.national.com LM3501 Typical Performance Characteristics (Continued) PMOS RDSON vs. Temperature Typical VIN Ripple 20065368 3 LEDs, RLED = 22Ω, VIN = 3.0V, CNTRL = 2.7V 1) SW, 10 V/div, DC 20065375 3) IL, 100 mA/div, DC 4) VIN, 100 mV/div, AC T = 250 ns/div Start-Up (LM3501-16) SHDN Pin Duty Cycle Control Waveforms 20065371 20065346 3 LEDs, RLED = 22Ω, VIN = 3.0V, CNTRL = 2.7V LM3501-16, 3 LEDs, RLED = 22Ω, VIN = 3.0V, SHDN frequency = 200 Hz 1) SHDN, 1 V/div, DC 1) SHDN, 1 V/div, DC 2) IL, 100 mA/div, DC 2) IL, 100 mA/div, DC 3) ILED, 20 mA/div, DC 3) ILED, 20 mA/div, DC T = 100 µs/div 4) VOUT, 10 V/div, DC T = 1 ms/div www.national.com 10 (Continued) Typical VOUT Ripple, OVP Functioning (LM3501-16) Typical VOUT Ripple, OVP Functioning (LM3501-21) 20065383 20065347 VOUT open circuit and equals approximately 15V DC, VIN = 3.0V 3) VOUT, 200 mV/div, AC VOUT open circuit and equals approximately 20V DC, VIN = 3.0V 1) VOUT, 200 mV/div, AC T = 1 ms/div T = 400 µs/div 11 www.national.com LM3501 Typical Performance Characteristics LM3501 Operation 20065304 FIGURE 2. LM3501 Block Diagram The LM3501 utilizes a synchronous Current Mode PWM control scheme to regulate the feedback voltage over almost all load conditions. The DC/DC controller acts as a controlled current source ideal for white LED applications. The LM3501 is internally compensated preventing the use of any external compensation components providing a compact overall solution. The operation can best be understood referring to the block diagram in Figure 2. At the start of each cycle, the oscillator sets the driver logic and turns on the NMOS power device conducting current through the inductor and turns off the PMOS power device isolating the output from the VSW pin. The LED current is supplied by the output capacitor when the NMOS power device is active. During this cycle, the output voltage of the EAMP controls the current through the inductor. This voltage will increase for larger loads and decrease for smaller loads limiting the peak current in the inductor minimizing EMI radiation. The EAMP voltage is compared with a voltage ramp and the sensed switch voltage. Once this voltage reaches the EAMP output voltage, the PWM COMP will then reset the logic turning off the NMOS power device and turning on the PMOS power device. The inductor current then flows through the PMOS power device to the white LED load and output capacitor. The inductor current recharges the output capacitor and supplies the current for the white LED branches. The oscillator then sets the driver logic again repeating the process. www.national.com The Duty Limit Comp is always operational preventing the NMOS power switch from being on more than one cycle and conducting large amounts of current. The LM3501 has dedicated protection circuitry active during normal operation to protect the IC and the external components. The Thermal Shutdown circuitry turns off both the NMOS and PMOS power devices when the die temperature reaches excessive levels. The LM3501 has a UVP Comp that disables both the NMOS and PMOS power devices when battery voltages are too low preventing an on state of the power devices which could conduct large amounts of current. The OVP Comp prevents the output voltage from increasing beyond 15.5V (LM3501-16) and 20.5V (LM350121) when the primary white LED network is removed or if there is an LED failure, allowing the use of small (16V for LM3501-16 and 25V for LM3501-21) ceramic capacitors at the output. This comparator has hysteresis that will regulate the output voltage between 15.5V and 14.6V typically for the LM3501-16, and between 20.5V and 19.5V for the LM350121. The LM3501 features a shutdown mode that reduces the supply current to 0.1 uA and isolates the input and output of the converter. The CNTRL pin can be used to change the white LED current. A CNTRL voltage above 125 mV will enable power to the LEDs and a voltage lower than 75 mV will turn off the power to the LEDs. 12 ADJUSTING LED CURRENT The maximum White LED current is set using the following equation: The LED current can be controlled using an external DC voltage. The recommended operating range for the voltage on the CNTRL pin is 0V to 2.7V. When CNTRL is 2.7V, FB = 0.515V (typ.) The FB voltage will continue to increase if CNTRL is brought above 2.7V (not recommended). The CNTRL to FB voltage relationship is: Maximum LED VF # of LEDs (in series) LM3501-16 LM3501-21 3 4.82V 6.49V 4 3.61V 4.86V 5 2.89V 3.89V 6 X 3.24V 7 X 2.78V For the LM3501 to operate properly, the output voltage must be kept above the input voltage during operation. For most applications, this requires a minimum of 2 LEDs (total of 6V or more) between the FB and VOUT pins. OUTPUT OVERVOLTAGE PROTECTION The LM3501 contains dedicated circuitry for monitoring the output voltage. In the event that the primary LED network is disconnected from the LM3501-16, the output voltage will increase and be limited to 15.5V (typ.). There is a 900 mV hysteresis associated with this circuitry which will cause the output to fluctuate between 15.5V and 14.6V (typ.) if the primary network is disconnected. In the event that the network is reconnected regulation will begin at the appropriate output voltage. The 15.5V limit allows the use of 16V 1 µF ceramic output capacitors creating an overall small solution for white LED applications. The LED current can be controlled using a PWM signal on the SHDN pin with frequencies in the range of 100 Hz (greater than visible frequency spectrum) to 1 kHz. For controlling LED currents down to the µA levels, it is best to use a PWM signal frequency between 200-500 Hz. The LM3501 LED current can be controlled with PWM signal frequencies above 1 kHz but the controllable current decreases with higher frequency. The maximum LED current would be achieved using the equation above with 100% duty cycle, ie. the SHDN pin always high. Applying a voltage greater than 125 mV to the CNTRL pin will begin regulating current to the LEDs. A voltage below 75 mV will prevent application or regulation of the LED current. In the event that the primary LED network is disconnected from the LM3501-21, the output voltage will increase and be limited to 20.5V (typ.). There is a 1V hysteresis associated with this circuitry which will cause the output to fluctuate between 20.5V and 19.5V (typ.) if the primary network is disconnected. In the event that the network is reconnected regulation will begin at the appropriate output voltage. The 20.5V limit allows the use of 25V 1 µF ceramic output capacitors. LED-DRIVE CAPABILITY The maximum number of LEDs that can be driven by the LM3501 is limited by the output voltage capability of the LM3501. When using the LM3501 in the typical application configuration, with LEDs stacked in series between the VOUT and FB pins, the maximum number of LEDs that can be placed in series (NMAX) is dependent on the maximum LED forward voltage (VF-MAX), the voltage of the LM3501 feedback pin (VFB-MAX = 0.545V), and the minimum output overvoltage protection level of the chosen LM3501 option (LM3501-16: OVPMIN = 15V; LM3501-21: OVPMIN = 20V). For the circuit to function properly, the following inequality must be met: (NMAX x VF-MAX) + 0.545V ≤ OVPMIN When inserting a value for maximum LED VF, LED forward voltage variation over the operating temperature range should be considered. The table below provides maximum LED voltage numbers for the LM3501-16 and LM3501-21 in the typical application circuit configuration (with 3, 4, 5, 6, or 7 LEDs placed in series between the VOUT and FB pins). RELIABILITY AND THERMAL SHUTDOWN The maximum continuous pin current for the 8 pin thin micro SMD package is 535 mA. When driving the device near its power output limits the VSW pin can see a higher DC current than 535 mA (see INDUCTOR SELECTION section for average switch current). To preserve the long term reliability of the device the average switch current should not exceed 535 mA. The LM3501 has an internal thermal shutdown function to protect the die from excessive temperatures. The thermal shutdown trip point is typically 150˚C. There is a hysteresis of typically 35˚C so the die temperature must decrease to approximately 115˚C before the LM3501 will return to normal operation. INDUCTOR SELECTION The inductor used with the LM3501 must have a saturation current greater than the cycle by cycle peak inductor current (see Typical Peak Inductor Currents table below). Choosing inductors with low DCR decreases power losses and increases efficiency. 13 www.national.com LM3501 Application Information LM3501 Application Information The typical cycle-by-cycle peak inductor current can be calculated from the following equation: (Continued) The minimum inductor value required for the LM3501-16 can be calculated using the following equation: where IOUT is the total load current, FSW is the switching frequency, L is the inductance and η is the converter efficiency of the total driven load. A good typical number to use for η is 0.8. The value of η can vary with load and duty cycle. The average inductor current, which is also the average VSW pin current, is given by the following equation: The minimum inductor value required for the LM3501-21 can be calculated using the following equation: For both equations above, L is in µH, VIN is the input supply of the chip in Volts, RDSON is the ON resistance of the NMOS power switch found in the Typical Performance Characteristics section in ohms and D is the duty cycle of the switching regulator. The above equation is only valid for D greater than or equal to 0.5. For applications where the minimum duty cycle is less than 0.5, a 22 µH inductor is the typical recommendation for use with most applications. Bench-level verification of circuit performance is required in these special cases, however. The duty cycle, D, is given by the following equation: The maximum output current capability of the LM3501 can be estimated with the following equation: where ICL is the current limit. Some recommended inductors include but are not limited to: Coilcraft DT1608C series Coilcraft DO1608C series TDK VLP4612 series TDK VLP5610 series TDK VLF4012A series where VOUT is the voltage at pin C1. CAPACITOR SELECTION Choose low ESR ceramic capacitors for the output to minimize output voltage ripple. Multilayer X7R or X5R type ceramic capacitors are the best choice. For most applications, a 1 µF ceramic output capacitor is sufficient. Typical Peak Inductor Current (mA)(Note 10) LED Current VIN (V) # LEDs (in series) 2.7 2 82 100 134 160 204 234 3 118 138 190 244 294 352 4 142 174 244 322 X X 5 191 232 319 413 X X 2 76 90 116 136 172 198 3 110 126 168 210 250 290 4 132 158 212 270 320 X 5 183 216 288 365 446 X 2 64 76 96 116 142 162 3 102 116 148 180 210 246 4 122 146 186 232 272 318 5 179 206 263 324 388 456 3.3 4.2 15 mA 20 mA 30 mA 40 mA 50 mA 60 mA Local bypassing for the input is needed on the LM3501. Multilayer X7R or X5R ceramic capacitors with low ESR are a good choice for this as well. A 1 µF ceramic capacitor is sufficient for most applications. However, for some applications at least a 4.7 µF ceramic capacitor may be required for proper startup of the LM3501. Using capacitors with low ESR decreases input voltage ripple. For additional bypassing, a 100 nF ceramic capacitor can be used to shunt high frequency ripple on the input. Some recommended capacitors include but are not limited to: TDK C2012X7R1C105K Taiyo-Yuden EMK212BJ105 G LAYOUT CONSIDERATIONS The input bypass capacitor CIN, as shown in Figure 2, must be placed close to the device and connect between the VIN and GND pins. This will reduce copper trace resistance which effects the input voltage ripple of the IC. For additional input voltage filtering, a 100 nF bypass capacitor can be placed in parallel with CIN to shunt any high frequency noise to ground. The output capacitor, COUT, should also be placed close to the LM3501 and connected directly between the VOUT and GND pins. Any copper trace connections for the COUT capacitor can increase the series resistance, which directly effects output voltage ripple and efficiency. The current setting resistor, RLED, should be kept close to the FB pin Note 10: CIN = COUT = 1 µF L = 22 µH, 160 mΩ DCR max. Coilcraft DT1608C-223 2 and 3 LED applications: LM3501-16 or LM3501-21; LED VF = 3.77V at 20mA; TA = 25˚C 4 LED applications: LM3501-16 or LM3501-21; LED VF = 3.41V at 20mA; TA = 25˚C 5 LED applications: LM3501-21 only; LED VF = 3.28V at 20mA; TA = 25˚C www.national.com 14 limit its current driving capability. Trace connections made to the inductor should be minimized to reduce power dissipation, EMI radiation and increase overall efficiency. It is good practice to keep the VSW routing away from sensitive pins such as the FB pin. Failure to do so may inject noise into the FB pin and affect the regulation of the device. See Figure 3 and Figure 4 for an example of a good layout as used for the LM3501 evaluation board. (Continued) to minimize copper trace connections that can inject noise into the system. The ground connection for the current setting resistor should connect directly to the GND pin. The AGND pin should connect directly to the GND pin. Not connecting the AGND pin directly, as close to the chip as possible, may affect the performance of the LM3501 and 20065384 FIGURE 3. Evaluation Board Layout (2X Magnification) Top Layer 20065385 FIGURE 4. Evaluation Board Layout (2X Magnification) Bottom Layer (as viewed from the top) 15 www.national.com LM3501 Application Information LM3501 Application Information (Continued) 20065309 FIGURE 5. 2 White LED Application 20065366 FIGURE 6. Multiple 2 LED String Application www.national.com 16 LM3501 Application Information (Continued) 20065367 FIGURE 7. Multiple 3 LED String Application 20065369 FIGURE 8. LM3501-21 5 LED Application 17 www.national.com Synchronous Step-up DC/DC Converter for White LED Applications Physical Dimensions inches (millimeters) unless otherwise noted 8-Bump micro SMD Package (TL) For Ordering, Refer to Ordering Information Table NS Package Number TLA08A X1 = 1.92 mm ( ± 0.03 mm), X2 = 1.92 mm ( ± 0.03 mm), X3 = 0.6 mm ( ± 0.075 mm) National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. 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