LM3500TL-16

National Semiconductor is now part of Texas Instruments. Search http://www.ti.com/ for the latest technical information and details on our current products and services. LM3500 Synchronous Step-up DC/DC Converter for White LED Applications General Description Features The LM3500 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 (500mV), power efficiency of the LM3500 circuit has been optimized for lighting applications in wireless phones and other portable products (single cell Li-Ion or 3cell NiMH battery supplies). The LM3500 operates with a fixed 1MHz switching frequency. When used with ceramic input and output capacitors, the LM3500 provides a small, lownoise, low-cost solution. Two LM3500 options are available with different output voltage capabilities. The LM3500-21 has a maximum output voltage of 21V and is typically suited for driving 4 or 5 white LEDs in series. The LM3500-16 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 LM3500 uses internal protection circuitry on the output to prevent a destructive over-voltage event. A single external resistor is used to set the maximum LED current in LED-drive applications. The LED current can easily be adjusted using a pulse width modulated (PWM) signal on the shutdown pin. In shutdown, the LM3500 completely disconnects the input from output, creating total isolation and preventing any leakage currents from trickling into the LEDs. ■ Synchronous rectification, high efficiency and no external schottky diode required ■ Uses small surface mount components ■ Can drive 2-5 white LEDs in series (may function with more low-VF LEDs) ■ 2.7V to 7V input range ■ Internal output over-voltage protection (OVP) circuitry, ■ ■ ■ ■ ■ ■ with no external zener diode required LM3500-16: 15.5V OVP; LM3500-21: 20.5V OVP. True shutdown isolation Input undervoltage lockout Requires only small ceramic capacitors at the input and output Thermal Shutdown 0.1µA shutdown current Small 8-bump thin micro SMD package Applications ■ ■ ■ ■ ■ LCD Bias Supplies White LED Backlighting Handheld Devices Digital Cameras Portable Applications Typical Application Circuit 20065701 © 2007 National Semiconductor Corporation 200657 www.national.com LM3500 Synchronous Step-up DC/DC Converter for White LED Applications February 2005 LM3500 Connection Diagram Top View 20065702 8-bump micro SMD Ordering Information Maximum Output Voltage Order Number Package Type NSC Package Drawing Top Mark Supplied As 16V LM3500TL-16 micro SMD TL08SSA S18 250 Units, Tape and Reel 16V LM3500TLX-16 micro SMD TL08SSA S18 3000 Units, Tape and Reel 21V LM3500TL-21 micro SMD TL08SSA S23 250 Units, Tape and Reel 21V LM3500TLX-21 micro SMD TL08SSA S23 3000 Units, Tape and Reel Pin Description/Functions Pin Name A1 AGND Function 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 Power Ground. B3 FB Output voltage feedback connection. A3 NC No internal connection made to this pin. A2 SHDN Analog ground. Analog and Power supply input. Shutdown control pin. 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. NC(pin A3): No internal connection is made to this pin. The maximum allowable voltage that can be applied to this pin is 7.5V. 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 Operating Conditions If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN VOUT (LM3500-16)(Note 2) VOUT (LM3500-21)(Note 2) VSW(Note 2) FB, SHDN, and NC Voltages Maximum Junction Temperature Lead Temperature (Note 3) ESD Ratings (Note 4) Human Body Model Machine Model LM3500 Absolute Maximum Ratings (Note 1) Ambient Temperature (Note 5) Junction Temperature Supply Voltage −0.3V to 7.5V −0.3V to 16V −0.3V to 21V −0.3V to VOUT+0.3V −0.3V to 7.5V 150°C −40°C to +85°C −40°C to +125°C 2.7V to 7V Thermal Properties Junction to Ambient Thermal Resistance (θJA)(Note 6) 75°C/W 300°C 2kV 200V 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 specification apply to both LM3500-16 and LM3500-21. Symbol IQ Typ (Note 8) Max (Note 7) Quiescent Current, Device Not FB > 0.54V Switching 0.95 1.2 Quiescent Current, Device Switching FB = 0V 1.8 2.5 Parameter Conditions Shutdown SHDN = 0V VFB Feedback Voltage VIN = 2.7V to 7V ΔVFB Feedback Voltage Line Regulation VIN = 2.7V to 7V ICL Switch Current Limit (LM3500-16) VIN = 2.7V, Duty Cycle = 80% Switch Current Limit (LM3500-21) 0.1 2 µA 0.5 0.53 V 0.1 0.4 %/V 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 0.47 mA FB Pin Bias Current VIN Input Voltage Range RDSON NMOS Switch RDSON VIN = 2.7V, ISW = 300mA PMOS Switch RDSON VOUT = 6V, ISW = 300mA 2.7 0.43 1.1 2.3 Ω 80 87 Duty Cycle Limit (LM3500-21) FB = 0V 85 94 0.85 1.0 1.15 18 30 SHDN = 2.7V 9 16 µA SHDN = GND 0.1 Switch Leakage Current (LM3500-16) VSW = 15V 0.01 0.5 µA Switch Leakage Current (LM3500-21) 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 Switching Frequency ISD SHDN Pin Current (Note 10) UVP FB = 0.5V (Note 9) Duty Cycle Limit (LM3500-16) FB = 0V FSW IL Units mA IB DLimit Min (Note 7) SHDN = 5.5V 3 % MHz V www.national.com LM3500 Min (Note 7) Typ (Note 8) Max (Note 7) Output Overvoltage Protection ON Threshold (LM3500-16) OFF Threshold 15 15.5 16 14 14.6 15 Output Overvoltage Protection ON Threshold (LM3500-21) OFF Threshold 20 20.5 21 19 19.5 20 Symbol OVP IVout IVL SHDN Threshold Parameter Conditions VOUT Bias Current (LM3500-16) VOUT = 15V, SHDN = VIN 260 400 VOUT Bias Current (LM3500-21) VOUT = 20V, SHDN = VIN 300 460 PMOS Switch Leakage Current (LM3500-16) VOUT = 15V, VSW = 0V 0.01 3 PMOS Switch Leakage Current (LM3500-21) VOUT = 20V, VSW = 0V 0.01 3 0.65 0.3 V µA µA SHDN Low SHDN High Units 1.1 0.65 V 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 specification apply to both LM3500-16 and LM3500-21. Typ (Note 8) Max (Note 7) Quiescent Current, Device Not FB > 0.54V Switching 0.95 1.2 Quiescent Current, Device Switching FB = 0V 1.8 2.5 Symbol IQ Parameter Conditions Shutdown SHDN = 0V VFB Feedback Voltage VIN = 2.7V to 7V ΔVFB Feedback Voltage Line Regulation ICL Min (Note 7) Units mA 0.1 2 µA 0.5 0.53 V VIN = 2.7V to 7V 0.1 0.4 %/V Switch Current Limit (LM3500-16) VIN = 3.0V, Duty Cycle = 70% 400 Switch Current Limit (LM3500-21) VIN = 3.0V, Duty Cycle = 63% 670 IB FB Pin Bias Current FB = 0.5V (Note 9) 45 VIN Input Voltage Range RDSON NMOS Switch RDSON VIN = 2.7V, ISW = 300mA PMOS Switch RDSON VOUT = 6V, ISW = 300mA DLimit 1.1 94 SHDN Pin Current (Note 10) 200 nA 7.0 V 0.43 87 Switching Frequency OVP 2.7 Duty Cycle Limit (LM3500-21) FB = 0V ISD UVP mA Duty Cycle Limit (LM3500-16) FB = 0V FSW IL 0.47 2.3 Ω % 1.0 1.2 18 30 SHDN = 2.7V 9 16 µA SHDN = GND 0.1 Switch Leakage Current (LM3500-16) VSW = 15V 0.01 0.5 µA Switch Leakage Current (LM3500-21) 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 Output Overvoltage Protection ON Threshold (LM3500-16) OFF Threshold 15 15.5 16 14 14.6 15 Output Overvoltage Protection ON Threshold (LM3500-21) OFF Threshold 20 20.5 21 19 19.5 20 www.national.com 0.8 SHDN = 5.5V 4 MHz V V IVout IVL SHDN Threshold Parameter Conditions Min (Note 7) Typ (Note 8) Max (Note 7) VOUT Bias Current (LM3500-16) VOUT = 15V, SHDN = VIN 260 400 VOUT Bias Current (LM3500-21) VOUT = 20V, SHDN = VIN 300 460 PMOS Switch Leakage Current (LM3500-16) VOUT = 15V, VSW = 0V 0.01 3 PMOS Switch Leakage Current (LM3500-21) VOUT = 20V, VSW = 0V 0.01 3 0.65 0.3 µA µA SHDN Low SHDN High Units 1.1 V 0.65 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: For more detailed soldering information and specifications, please refer to National Semiconductor Application Note 1112: Micro SMD Wafer Level Chip Scale Package (AN-1112), available at www.national.com. Note 4: 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 5: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = 125ºC), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX). Note 6: 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 7: 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 8: Typical numbers are at 25°C and represent the most likely norm. Note 9: Feedback current flows out of the pin. Note 10: Current flows into the pin. Typical Performance Characteristics Switching Quiescent Current vs VIN Non-Switching Quiescent Current vs VIN 20065755 20065756 5 www.national.com LM3500 Symbol LM3500 2 LED Efficiency vs LED Current L = Coilcraft DT1608C-223, Efficiency = 100*(PIN/(2VLED*ILED)) 2 LED Efficiency vs LED Current L = TDK VLP4612T-220MR34, Efficiency = 100*(PIN/(2VLED*ILED)) 20065779 20065757 3 LED Efficiency vs LED Current L = Coilcraft DT1608C-223, Efficiency = 100*(PIN/(3VLED*ILED)) 3 LED Efficiency vs LED Current L = TDK VLP4612T-220MR34, Efficiency = 100*(PIN/(3VLED*ILED)) 20065780 20065758 4 LED Efficiency vs LED Current L = Coilcraft DT1608C-223, Efficiency = 100*(PIN/(4VLED*ILED)) 4 LED Efficiency vs LED Current L = TDK VLP4612T-220MR34, Efficiency = 100*(PIN/(4VLED*ILED)) 20065759 www.national.com 20065781 6 LM3500 2 LED Efficiency vs VIN L = Coilcraft DT1608C-223, Efficiency = 100*(PIN/(2VLED*ILED)) 3 LED Efficiency vs VIN L = Coilcraft DT1608C-223, Efficiency = 100*(PIN/(3VLED*ILED)) 20065769 20065770 4 LED Efficiency vs VIN L = Coilcraft DT1608C-223, Efficiency = 100*(PIN/(4VLED*ILED)) SHDN Pin Current vs SHDN Pin Voltage 20065761 20065773 Output Power vs VIN: LM3500-16 (L = Coilcraft DT1608C-223) Output Power vs Temperature: LM3500-16 (L = Coilcraft DT1608C-223) 20065785 20065784 7 www.national.com LM3500 Switch Current Limit vs VIN: LM3500-16 Switch Current Limit vs Temperature LM3500-16, VOUT=8V 20065762 20065763 Switch Current Limit vs Temperature LM3500-16, VOUT=12V Switch Current Limit vs VIN: LM3500-21 20065791 20065776 Switch Current Limit vs Temperature LM3500-21, VOUT=8V Switch Current Limit vs Temperature LM3500-21, VOUT=12V 20065792 www.national.com 20065793 8 LM3500 Switch Current Limit vs Temperature LM3500-21, VOUT=18V Oscillator Frequency vs VIN 20065764 20065794 VOUT DC Bias vs VOUT Voltage: LM3500-21 VOUT DC Bias vs VOUT Voltage: LM3500-16 20065765 20065795 FB Voltage vs Temperature FB Voltage vs VIN 20065766 20065767 9 www.national.com LM3500 NMOS RDSON vs VIN (ISW = 300mA) PMOS RDSON vs Temperature 20065775 20065774 Typical VIN Ripple Start-Up: LM3500-16 20065771 20065768 3 LEDs, RLED = 22Ω, VIN = 3.0V 1) SHDN, 1V/div, DC 2) IL, 100mA/div, DC 3) ILED, 20mA/div, DC T = 100µs/div LM3500-16, 3 LEDs, RLED = 22Ω, VIN = 3.0V 1) SW, 10V/div, DC 3) IL, 100mA/div, DC 4) VIN, 100mV/div, AC T = 250ns/div Start-Up: LM3500-21 SHDN Pin Duty Cycle Control Waveforms 20065796 3 LEDs, RLED = 22Ω, VIN = 3.0V 1) SHDN, 1V/div, DC 4) IL, 100mA/div, DC 2) VOUT, 10/div, DC T = 200µs/div VCONT = 2.7V www.national.com 20065772 LM3500-16, 3 LEDs, RLED = 22Ω, VIN = 3.0V, SHDN frequency = 200Hz 1) SHDN, 1V/div, DC 2) IL, 100mA/div, DC 3) ILED, 20mA/div, DC 4) VOUT, 10V/div, DC T = 1ms/div 10 Typical VOUT Ripple, OVP Functioning: LM3500-21 20065797 20065782 VOUT open circuit and equals approximately 15V DC, VIN = 3.0V 3) VOUT, 200mV/div, AC T = 1ms/div VOUT open circuit and equals approximately 20V DC, VIN = 3.0V 1) VOUT, 200mV/div, AC T = 400µs/div 11 www.national.com LM3500 Typical VOUT Ripple, OVP Functioning: LM3500-16 LM3500 Operation 20065704 FIGURE 1. LM3500 Block Diagram The LM3500 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 LM3500 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 1. 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 out- www.national.com put capacitor and supplies the current for the white LED branches. The oscillator then sets the driver logic again repeating the process. 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 LM3500 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 LM3500 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(LM3500-16) and 20.5V(LM3500-21) when the primary white LED network is removed or if there is an LED failure, allowing the use of small (16V for LM3500-16 and 25V for LM3500-21) ceramic capacitors at the output. This comparator has hysteresis that will regulate the output voltage between 15.5V and 14.6V typically for the LM3500-16, and between 20.5V and 19.5V for the LM3500-21. The LM3500 features a shutdown mode that reduces the supply current to 0.1uA and isolates the input and output of the converter. 12 ADJUSTING LED CURRENT The White LED current is set using the following equation: RELIABILITY AND THERMAL SHUTDOWN The maximum continuous pin current for the 8 pin thin micro SMD package is 535mA. When driving the device near its power output limits the VSW pin can see a higher DC current than 535mA (see INDUCTOR SELECTION section for average switch current). To preserve the long term reliability of the device the average switch current should not exceed 535mA. The LM3500 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 LM3500 will return to normal operation. The LED current can be controlled using a PWM signal on the SHDN pin with frequencies in the range of 100Hz (greater than visible frequency spectrum) to 1kHz. For controlling LED currents down to the µA levels, it is best to use a PWM signal frequency between 200-500Hz. The LM3500 LED current can be controlled with PWM signal frequencies above 1kHz 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. LED-DRIVE CAPABILITY The maximum number of LEDs that can be driven by the LM3500 is limited by the output voltage capability of the LM3500. When using the LM3500 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 LM3500 feedback pin (VFB-MAX = 0.53V), and the minimum output over-voltage protection level of the chosen LM3500 option (LM3500-16: OVPMIN = 15V; LM3500-21: OVPMIN = 20V). For the circuit to function properly, the following inequality must be met: (NMAX × VF-MAX) + 0.53V ≤ OVPMIN When inserting a value for maximim LED VF, LED forward voltage variation over the operating temperature range should be considered. The table below provides maximum LED voltage numbers for the LM3500-16 and LM3500-21 in the typical application circuit configuration (with 3, 4, 5, 6, or 7 LEDs placed in series between the VOUT and FB pins). INDUCTOR SELECTION The inductor used with the LM3500 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. The minimum inductor value required for the LM3500-16 can be calculated using the following equation: The minimum inductor value required for the LM3500-21 can be calculated using the following equation: Maximum LED VF # of LEDs (in series) LM3500-16 LM3500-21 3 4.82V 6.49V 4 3.61V 4.86V 5 2.89V 3.89V 6 X 3.24V 7 X 2.78V For 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 22µH 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: For the LM3500 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 LM3500 contains dedicated circuitry for monitoring the output voltage. In the event that the primary LED network is disconnected from the LM3500-16, the output voltage will increase and be limited to 15.5V (typ.). There is a 900mV 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 1µF ceramic output capacitors creating an overall small solution for white LED applications. In the event that the primary LED network is disconnected from the LM3500-21, the output voltage will increase and be where VOUT is the voltage at pin C1. 13 www.national.com LM3500 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 1µF ceramic output capacitors. Application Information LM3500 Typical Peak Inductor Currents (mA) # LEDs (in series) 15 mA 20 mA 30 mA 40 mA 50 mA 60 mA 2 3 4 5 82 118 142 191 100 138 174 232 134 190 244 319 160 244 322 413 204 294 X X 234 352 X X 3.3 2 3 4 5 76 110 132 183 90 126 158 216 116 168 212 288 136 210 270 365 172 250 320 446 198 290 X X 4.2 2 3 4 5 64 102 122 179 76 116 146 206 96 148 186 263 116 180 232 324 142 210 272 388 162 246 318 456 VIN (V) 2.7 Coilcraft DT1608C series Coilcraft DO1608C series TDK VLP4612 series TDK VLP5610 series TDK VLF4012A series LED Current 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 1µF ceramic output capacitor is sufficient. Local bypassing for the input is needed on the LM3500. Multilayer X7R or X5R ceramic capacitors with low ESR are a good choice for this as well. A 1µF ceramic capacitor is sufficient for most applications. However, for some applications at least a 4.7µF ceramic capacitor may be required for proper startup of the LM3500. Using capacitors with low ESR decreases input voltage ripple. For additional bypassing, a 100nF ceramic capacitor can be used to shunt high frequency ripple on the input. Some recommended capacitors include but are not limited to: CIN = COUT = 1 µF L = 22 µH, 160 mΩ DCR max. Coilcraft DT1608C-223 2 and 3 LED applications: LM3500-16 or LM3500-21; LED VF = 3.77V at 20mA; TA = 25°C 4 LED applications: LM3500-16 or LM3500-21; LED VF = 3.41V at 20mA; TA = 25°C 5 LED applications: LM3500-21 only; LED VF = 3.28V at 20mA; TA = 25°C TDK C2012X7R1C105K Taiyo-Yuden EMK212BJ105 G LAYOUT CONSIDERATIONS The input bypass capacitor CIN, as shown in Figure 1, 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 100nF 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 LM3500 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 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 LM3500 and 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 2 and Figure 3 for an example of a good layout as used for the LM3500 evaluation board. The typical cycle-by-cycle peak inductor current can be calculated from 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 maximum output current capability of the LM3500 can be estimated with the following equation: where ICL is the current limit. Some recommended inductors include but are not limited to: www.national.com 14 LM3500 20065777 FIGURE 2. Evaluation Board Layout (2X Magnification) Top Layer 20065778 FIGURE 3. Evaluation Board Layout (2X Magnification) Bottom Layer (as viewed from the top) 15 www.national.com LM3500 20065709 FIGURE 4. 2 White LED Application 20065754 FIGURE 5. Multiple 2 LED String Application www.national.com 16 LM3500 20065783 FIGURE 6. Multiple 3 LED String Application 20065790 FIGURE 7. LM3500-21 5 LED Application 17 www.national.com LM3500 www.national.com 18 LM3500 Physical Dimensions inches (millimeters) unless otherwise noted 8-Bump Micro SMD Package (TL) For Ordering, Refer to Ordering Information Table NS Package Number TLA08SSA X1 = 1.92mm (±0.03mm), X2 = 1.92mm (±0.03mm), X3 = 0.6mm (±0.075mm) 19 www.national.com LM3500 Synchronous Step-up DC/DC Converter for White LED Applications Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. 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