LM3502ITL-25

LM3502 Step-Up Converter for White LED Applications August 2006 LM3502 Step-Up Converter for White LED Applications General Description The LM3502 is a white LED driver for lighting applications. For dual display or large single white LED string backlighting applications, the LM3502 provides a complete solution. The LM3502 contains two internal white LED current bypass FET(Field Effect Transitor) switches that are ideal for controlling dual display applications. The white LED current can be adjusted with a PWM signal directly from a microcontroller without the need of an RC filter network. With no external compensation, cycle-by-cycle current limit, over-voltage protection, and under-voltage protection, the LM3502 offers superior performance over other application specific standard product step-up white LED drivers. Features n Drive up to 4, 6, 8 or 10 white LEDs for Dual Display Backlighting n > 80% efficiency n Output Voltage Options: 16V , 25V , 35V, and 44V n Input Under-Voltage Protection n Internal Soft Start Eliminating Inrush Current n 1 MHz Constant Switching Frequency n Wide Input Voltage: 2.5V to 5.5V n Small External Components n Low Profile Packages: < 1 mm Height -10 Bump MicroSMD -16 Pin LLP Applications n Dual Display Backlighting in Portable Devices n Cellular Phones and PDAs Typical Application 20131701 FIGURE 1. Blacklight Configuration with 10 White LEDs © 2006 National Semiconductor Corporation DS201317 www.national.com LM3502 Connection Diagrams 10-Bump Thin MicroSMD Package (TLP10) 16-Lead Thin Leadless Leadframe Package (SQA16A) 20131703 TOP VIEW 20131702 TOP VIEW Pin Descriptions/Functions Bump # A1 B1 C1 D1 D2 D3 C3 B3 A3 A2 Pin # 9 7 6 4 2 and 3 15 and 16 14 13 12 10 1 5 8 11 DAP Name Cntrl Fb VOUT2 VOUT1 Sw PGND AGND VIN En2 En1 NC NC NC NC DAP Shutdown Control Connection Feedback Voltage Connection Drain Connections of The NMOS and PMOS Field Effect Transistor (FET) Switches (Figure 2: N2 and P1) Over-Voltage Protection (OVP) and Source Connection of The PMOS FET Switch (Figure 2: P1) Drain Connection of The Power NMOS Switch (Figure 2: N1) Power Ground Connection Analog Ground Connection Supply or Input Voltage Connection NMOS FET Switch Control Connection PMOS FET Switch Control Connection No Connection No Connection No Connection No Connection Die Attach Pad (DAP), must be soldered to the printed circuit board’s ground plane for enhanced thermal dissipation. as possible, between the VOUT1 pin and ground plane. Also connect the Schottky diode as close as possible to the VOUT1 pin to minimize trace resistance and EMI radiation. Sw (Bump D2): Drain connection of the internal power NMOS FET switch. (Figure 2: N1) Minimize the metal trace length and maximize the metal trace width connected to this pin to reduce EMI radiation and trace resistance. PGND (Bump D3): Power ground pin. Connect directly to the ground plane. AGND (Bump C3): Analog ground pin. Connect the analog ground pin directly to the PGND pin. VIN (Bump B3): Supply or input voltage connection pin. The CIN capacitor should be as close to the device as possible, between the VIN pin and ground plane. En2 (Bump A3): Enable pin for the internal NMOS FET switch (Figure 2: N2) during device operation. When VEn2 is 2 Description Cntrl (Bump A1): Shutdown control pin. When VCntrl is ≥ 1.4V, the LM3502 is enabled or ON. When VCntrl is ≤ 0.3V, the LM3502 will enter into shutdown mode operation. The LM3502 has an internal pull down resistor on the Cntrl pin, thus the device is normally in the off state or shutdown mode of operation. Fb (Bump B1): Output voltage feedback connection. The white LED string network current is set/programmed using a resistor from this pin to ground. VOUT2 (Bump C1): Drain connections of the internal PMOS and NMOS FET switches. (Figure 2: P1 and N2). It is recommended to connect 100nF at VOUT2 for the LM3502-35V and LM3502-44 versions if VOUT2 is not used. VOUT1 (Bump D1): Source connection of the internal PMOS FET switch (Figure 2: P1) and OVP sensing node. The output capacitor must be connected as close to the device www.national.com LM3502 Pin Descriptions/Functions (Continued) ≤ 0.3V, the internal NMOS FET switch turns on and the SUB display turns off. When VEn2 is ≥ 1.4V, the internal NMOS FET switch turns off and the SUB display turns on. The En2 pin has an internal pull down resistor, thus the internal NMOS FET switch is normally in the on state of operation with the SUB display turned off. If VEn1 and VEn2 are ≤ 0.3V and VCntrl is ≥ 1.4V, the LM3502 will enter a low IQ shutdown mode of operation where all the internal FET switches are off. If VOUT2 is not used, En2 must be grounded or floating and use En1 along with Cntrl, to enable the device. En1 (Bump A2): Enable pin for the internal PMOS FET switch (Figure 2: P1) during device operation. When VEn1 is ≤ 0.3V, the internal PMOS FET switch turns on and the MAIN display is turned off. When VEn1 is ≥ 1.4V, the internal PMOS FET switch turns off and the MAIN display is turned on. The En1 pin has an internal pull down resistor, thus the internal PMOS FET switch is normally in the on state of operation with the MAIN display turned off. If VEn1 and VEn2 are ≤ 0.3V and VCntrl is ≥ 1.4V, the LM3502 will enter a low IQ shutdown mode of operation where all the internal FET switches are off. If VOUT2 is not used, En2 must be grounded and use En1 a long with Cntrl, to enable the device. Ordering Information Voltage Option 16 16 16 16 25 25 25 25 35 35 35 35 44 44 44 44 Order Number LM3502ITL-16 LM3502ITLX-16 LM3502SQ-16 LM3502SQX-16 LM3502ITL-25 LM3502ITLX-25 LM3502SQ-25 LM3502SQX-25 LM3502ITL-35 LM3502ITLX-35 LM3502SQ-35 LM3502SQX-35 LM3502ITL-44 LM3502ITLX-44 LM3502SQ-44 LM3502SQX-44 Package Marking SANB SANB L00048B L00048B SAPB SAPB L00049B L00049B SARB SARB L00044B L00044B SDLB SDLB L00050B L00050B Supplied As 250 units, Tape-and-Reel 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 3000 units, Tape-and-Reel 3 www.national.com LM3502 Absolute Maximum Ratings (Notes 6, 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN Pin Sw Pin Fb Pin Cntrl Pin VOUT1 Pin VOUT2 Pin En1 En2 Continuous Power Dissipation Maximum Junction Temperature (TJ-MAX) Storage Temperature Range −0.3V to +5.5V −0.3V to +48V −0.3V to +5.5V −0.3V to +5.5V −0.3V to +48V −0.3V to VOUT1 −0.3V to +5.5V −0.3V to +5.5V Internally Limited +150˚C −65˚C to +150˚C ESD Rating (Note 2) Human Body Model: Machine Model: 2 kV 200V Operating Conditions (Notes 1, 6) Junction Temperature (TJ) Range Ambient Temperature (TA) Range Input Voltage, VIN Pin Cntrl, En1, and En2 Pins −40˚C to +125˚C −40˚C to +85˚C 2.5V to 5.5V 0V to 5.5V Thermal Properties (Note 4) Junction-to-Ambient Thermal Resistance (θJA) Micro SMD Package Leadless Leadframe Package 65˚C/W 49˚C/W Preliminary Electrical Characteristics (Notes 6, 7) Limits in standard typeface are for TJ = 25˚C. Limits in bold typeface apply over the full operating junction temperature range (−40˚C ≤ TJ ≤ +125˚C). Unless otherwise specified, VIN = 2.5V. Symbol VIN IQ Parameter Input Voltage Non-Switching Switching Shutdown Low IQ Shutdown Feedback Voltage NMOS Power Switch Current Limit −16, −25, −35, −44, Fb Fb Fb Fb = = = = 0V 0V 0V 0V Fb > 0.25V Fb = 0V, Sw Is Floating Cntrl = 0V Cntrl = 1.5V, En1 = En2 = 0V 0.18 250 400 450 450 Conditions Min 2.5 0.5 1.9 0.1 6 0.25 400 600 750 750 64 0.8 ISw = 500 mA 0.55 1.1 Ω 1 Typ Max 5.5 1 3 3 15 0.3 650 800 1050 1050 500 1.2 Units V mA mA µA µA V VFb ICL mA IFb FS RDS(ON) Feedback Pin Bias Current (Note 8) Switching Frequency NMOS Power Switch ON Resistance (Figure 2: N1) Fb = 0.25V nA MHz RPDS(ON) PMOS ON Resistance IPMOS = 20 mA, En1 = 0V, En2 = 1.5V of VOUT1/VOUT2 Switch (Figure 2: N1) NMOS ON Resistance INMOS = 20 mA, En1 = 1.5V, En2 = 0V of VOUT2/Fb Switch (Figure 2: N2) Maximum Duty Cycle Cntrl Pin Input Bias Current (Note 3) Sw Pin Leakage Current (Note 3) Fb = 0V Cntrl = 2.5V Cntrl = 0V Sw = 42V, Cntrl = 0V VOUT1 VOUT1 VOUT1 VOUT1 = = = = 14V, 23V, 32V, 42V, Cntrl Cntrl Cntrl Cntrl = = = = 0V 0V 0V 0V (16) (25) (35) (44) 90 5 10 Ω RNDS(ON) 2.5 95 7 0.1 0.01 0.1 0.1 0.1 0.1 5 Ω % DMAX ICntrl ISw 14 µA µA 5 3 3 3 3 IVOUT1(OFF) VOUT1 Pin Leakage Current (Note 3) µA www.national.com 4 LM3502 Preliminary Electrical Characteristics (Notes 6, 7) Limits in standard typeface are for TJ = 25˚C. Limits in bold typeface apply over the full operating junction temperature range (−40˚C ≤ TJ ≤ +125˚C). Unless otherwise specified, VIN = 2.5V. (Continued) Symbol Parameter VOUT1 VOUT1 VOUT1 VOUT1 = = = = 14V, 23V, 32V, 42V, Conditions Cntrl Cntrl Cntrl Cntrl = = = = 1.5V 1.5V 1.5V 1.5V (16) (25) (35) (44) Min Typ 40 50 50 85 0.1 2.4 2.3 15.5 15 24 23 34 33 42 41 0.8 0.8 0.8 0.8 0.8 0.8 12 7 0.1 7 0.1 Max 80 100 100 140 3 2.5 16.5 16.0 25.5 24.5 35.0 34.0 43.5 42.0 0.3 V 0.3 V 0.3 16 14 14 Units IVOUT1(ON) VOUT1 Pin Bias Current (Note 3) µA IVOUT2 UVP OVP VOUT2 Pin Leakage Current (Note 3) Under-Voltage Protection Over-Voltage Protection (Note 5) Fb = 0V, Cntrl = 0V, VOUT2 = 42V On Threshold Off Threshold On Off On Off On Off On Off Threshold Threshold Threshold Threshold Threshold Threshold Threshold Threshold (16) (16) (25) (25) (35) (35) (44) (44) µA V 2.2 14.5 14.0 22.5 21.5 32.0 31.0 40.5 39.0 1.4 V VEn1 PMOS FET Switch Enabling Threshold (Figure 2: P1) NMOS FET Switch Enabling Threshold (Figure 2: N2) Device Enabling Threshold Shutdown Delay Time En1 Pin Input Bias Current En2 Pin Input Bias Current Off Threshold (Display Lighting) On Threshold (Display Lighting) Off Threshold (Display Lighting) On Threshold (Display Lighting) Off Threshold OnThreshold En1 = 2.5V En1 = 0V En2 = 2.5V En2 = 0V VEn2 1.4 VCntrl TSHDW IEn1 IEn2 1.4 8 V ms µA µA Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical characteristic specifications do not apply when operating the device outside of its rated operating conditions. Note 2: 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 3: Current flows into the pin. Note 4: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. See Thermal Properties 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. For more information on this topic, please refer to Application Note 1187: Leadless Leadframe Package (LLP) and Application Note 1112 (AN1112) for microSMD chip scale package. Note 5: The on threshold indicates that the LM3502 is no longer switching or regulating LED current, while the off threshold indicates normal operation. Note 6: All voltages are with respect to the potential at the GND pin. Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm. Note 8: Current flows out of the pin. 5 www.national.com LM3502 Block Diagram 20131704 FIGURE 2. Block Diagram www.national.com 6 LM3502 Detailed Description of Operation The LM3502 utilizes an asynchronous current mode pulsewidth-modulation (PWM) control scheme to regulate the feedback voltage over specified load conditions. The DC/DC converter behaves as a controlled current source for white LED applications. The operation can best be understood by referring to the block diagram in Figure 2 for the following operational explanation. At the start of each cycle, the oscillator sets the driver logic and turns on the internal NMOS power device, N1, conducting current through the inductor and reverse biasing the external diode. The white LED current is supplied by the output capacitor when the internal NMOS power device, N1, is turned on. The sum of the error amplifier’s output voltage and an internal voltage ramp are compared with the sensed power NMOS, N1, switch voltage. Once these voltages are equal, the PWM comparator will then reset the driver logic, thus turning off the internal NMOS power device, N1, and forward biasing the external diode. The inductor current then flows through the diode to the white LED load and output capacitor. The inductor current recharges the output capacitor and supplies the current for the white LED load. The oscillator then resets the driver logic again repeating the process. The output voltage of the error amplifier 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 and minimizing EMI radiation. The duty limit comparator is always operational, it prevents the internal NMOS power switch, N1, from being on for more than one oscillator cycle and conducting large amounts of current. The light load comparator allows the LM3502 to properly regulate light/small white LED load currents, where regulation becomes difficult for the LM3502’s primary control loop. Under light load conditions, the LM3502 will enter into a pulse skipping pulse-frequencymode (PFM) of operation where the switching frequency will vary with the load. The LM3502 has 2 control pins, En1 and En2, used for selecting which segment of a single white LED string network is active for dual display applications. En1 controls the main display (MAIN) segment of the single string white LED network between pins VOUT1 and VOUT2. En2 controls the sub display (SUB) segment of the single string white LED network between the VOUT2 and Fb. For a quick review of the LM3502 control pin operational characteristics, see Figure 3. When the Cntrl pin is ≥ 1.4V, the LM3502 will enter in low IQ state if both En1 and En2 ≤ 0.3V. At this time, both the P1 and N2 FETs will turn off. The output voltage will be a diode drop below the supply voltage and the soft-start will be reset limiting the peak inductor current at the next start-up. The LM3502 is designed to control the LED current with a PWM signal without the use of an external RC filter. Utilizing special circuitry, the LM3502 can operate over a large range of PWM frequencies without restarting the soft-start allowing for fast recovery at high PWM frequencies. Figure 4 reprsents a PWM signal driving the Cntrl pin where tL is defined as the low time of the signal. The following is true: • If tL < 12ms (typical): The device will stop switching during this time and the soft-start will not be reset allowing LED current PWM control. • If tL > 12ms (typical): The device will shutdown and the soft-start will reset to prevent high peak currents at the next startup. Both the N2 and P1 switches will turn off. The LM3502 has dedicated protection circuitry active during normal operation to protect the integrated circuit (IC) and external components. The thermal shutdown circuitry turns off the internal NMOS power device, N1, when the internal semiconductor junction temperature reaches excessive levels. The LM3502 has a under-voltage protection (UVP) comparator that disables the internal NMOS power device when battery voltages are too low, thus preventing an on state where the internal NMOS power device conducts large amounts of current. The over-voltage protection (OVP) comparator prevents the output voltage from increasing beyond the protection limit when the white LED string network is removed or if there is a white LED failure. OVP allows for the use of low profile ceramic capacitors at the output. The current though the internal NMOS power device, N1, is monitored to prevent peak inductor currents from damaging the IC. If during a cycle (cycle=1/switching frequency) the peak inductor current exceeds the current limit for the LM3502, the internal NMOS power device will be turned off for the remaining duration of that cycle. 7 www.national.com LM3502 20131705 FIGURE 3. Operational Characteristics Table 20131706 FIGURE 4. Control Signal Waveform www.national.com 8 LM3502 Typical Performance Characteristics ( Circuit in Figure 1: L = DO1608C-223 and D = MBRM140T3. Efficiency: η = POUT/PIN = [(VOUT – VFb) * IOUT]/[VIN * IIN]. TA= 25˚C, unless otherwise stated.) IQ (Non-Switching) vs VIN Switching Frequency vs Temperature 20131707 20131708 IQ (Switching) vs VIN IQ (Switching) vs Temperature 20131709 20131710 10 LED Efficiency vs LED Current 8 LED Efficiency vs LED Current 20131711 20131712 9 www.national.com LM3502 Typical Performance Characteristics ( Circuit in Figure 1: L = DO1608C-223 and D = MBRM140T3. Efficiency: η = POUT/PIN = [(VOUT – VFb) * IOUT]/[VIN * IIN]. TA= 25˚C, unless otherwise stated.) (Continued) 6 LED Efficiency vs LED Current 4 LED Efficiency vs LED Current 20131713 20131714 Cntrl Pin Current vs Cntrl Pin Voltage Maximum Duty Cycle vs Temperature 20131715 20131716 En1 Pin Current vs En1 Pin Voltage En2 Pin Current vs En2 Pin Voltage 20131717 20131718 www.national.com 10 LM3502 Typical Performance Characteristics ( Circuit in Figure 1: L = DO1608C-223 and D = MBRM140T3. Efficiency: η = POUT/PIN = [(VOUT – VFb) * IOUT]/[VIN * IIN]. TA= 25˚C, unless otherwise stated.) (Continued) VOUT1 Pin Current vs VOUT1 Pin Voltage Power NMOS RDS(ON) (Figure 2: N1) vs VIN 20131719 20131720 NMOS RDS(ON) (Figure 2: N2) vs VIN PMOS RDS(ON) (Figure 2: P1) vs VIN 20131721 20131722 Feedback Voltage vs Temperature 20131725 11 www.national.com LM3502 Typical Performance Characteristics ( Circuit in Figure 1: L = DO1608C-223 and D = MBRM140T3. Efficiency: η = POUT/PIN = [(VOUT – VFb) * IOUT]/[VIN * IIN]. TA= 25˚C, unless otherwise stated.) (Continued) Current Limit (LM3502-16) vs VIN Current Limit (LM3502-16) vs Temperature 20131754 20131755 Current Limit (LM3502-25) vs VIN Current Limit (LM3502-25) vs Temperature 20131756 20131757 Current Limit (LM3502-35/44) vs Temperature Current Limit (LM3502-35/44) vs VIN 20131758 20131729 www.national.com 12 LM3502 Application Information WHITE LED CURRENT SETTING The LED current is set using the following equation: 20131730 ILED: White LED Current. VFb: Feedback Pin Voltage. VFb = 0.25V, Typical. R1: Currrent Setting Resistor. brightness control. For best PWM duty cycle vs. white LED current linearity, the PWM frequency should be between 200Hz and 500Hz. Other PWM frequencies can be used, but the linearity over input voltage and duty cycle variation will not be as good as what the 200Hz to 500Hz PWM frequency spectrum provides. To minimize audible noise interference, it is recommended that a output capacitor with minimal susceptibility to piezoelectric induced stresses be used for the particular applications that require minimal or no audible noise interference. WHITE LED DIMMING For dimming the white LED string with a pulse-widthmodulated (PWM) signal on the Cntrl pin, care must taken to balance the tradeoffs between audible noise and white LED 20131735 FIGURE 5. If VOUT2 is not used , En2 must be grounded 13 www.national.com LM3502 Application Information (Continued) 20131736 FIGURE 6. Inductor Current Waveform CONTINUOUS AND DISCONTINUOUS MODES OF OPERATION Since the LM3502 is a constant frequency pulse-widthmodulated step-up regulator, care must be taken to make sure the maximum duty cycle specification is not violated. The duty cycle equation depends on which mode of operation the LM3502 is in. The two operational modes of the LM3502 are continuous conduction mode (CCM) and discontinuous conduction mode (DCM). Continuous conduction mode refers to the mode of operation where during the switching cycle, the inductor current never goes to and stays at zero for any significant amount of time during the switching cycle. Discontinuous conduction mode refers to the mode of operation where during the switching cycle, the inductor current goes to and stays at zero for a significant amount of time during the switching cycle. Figure 6 illustrates the threshold between CCM and DCM operation. In Figure 6, the inductor current is right on the CCM/DCM operational threshold. Using this as a reference, a factor can be introduced to calculate when a particular application is in CCM or DCM operation. R is a CCM/DCM factor we can use to compute which mode of operation a particular application is in. If R is ≥ 1, then the application is operating in CCM. Conversely, if R is < 1, the application is operating in DCM. The R factor inequalities are a result of the components that make up the R factor. From Figure 6, the R factor is equal to the average inductor current, IL(avg), divided by half the inductor ripple current, ∆iL. Using Figure 6 the following equation can be used to compute R factor: 20131739 20131740 VIN: VOUT: Eff: Fs: IOUT: L: D: ∆iL: IL(avg): Input Voltage. Output Voltage. Efficiency of the LM3502. Switching Frequency. White LED Current/Load Current. Inductance Magnitude/Inductor Value. Duty Cycle for CCM Operation. Inductor Ripple Current Average Inductor Current For CCM operation, the duty cycle can be computed with: 20131741 20131742 20131737 D: Duty Cycle for CCM Operation. VOUT: Output Voltage. VIN: Input Voltage. For DCM operation, the duty cycle can be computed with: 20131738 20131743 www.national.com 14 LM3502 Application Information (Continued) 20131748 D: 20131744 Duty Cycle. D: Duty Cycle for DCM Operation. VOUT: Output Voltage. VIN: Input Voltage. IOUT: White LED Current/Load Current. Fs: L: Switching Frequency. Inductor Value/Inductance Magnitude. D: 1–D. RDS(ON): NMOS Power Switch ON Resistance. Input Voltage. VIN: L: Inductance Magnitude/Inductor Value. This equation gives the value required to prevent subharmonic oscillations. The result of this equation and the inductor average and ripple current should be accounted for when choosing an inductor value. Some recommended inductor manufacturers included but are not limited to: CoilCraft DO1608C-223 DT1608C-223 www.coilcraft.com INDUCTOR SELECTION In order to maintain inductance, an inductor used with the LM3502 should have a saturation current rating larger than the peak inductor current of the particular application. Inductors with low DCR values contribute decreased power losses and increased efficiency. The peak inductor current can be computed for both modes of operation: CCM and DCM. The cycle-by-cycle peak inductor current for CCM operation can be computed with: 20131745 20131746 CAPACITOR SELECTION Multilayer ceramic capacitors are the best choice for use with the LM3502. Multilayer ceramic capacitors have the lowest equivalent series resistance (ESR). Applied voltage or DC bias, temperature, dielectric material type (X7R, X5R, Y5V, etc), and manufacturer component tolerance have an affect on the true or effective capacitance of a ceramic capacitor. Be aware of how your application will affect a particular ceramic capacitor by analyzing the aforementioned factors of your application. Before selecting a capacitor always consult the capacitor manufacturer’s data curves to verify the effective or true capacitance in your application. INPUT CAPACITOR SELECTION The input capacitor serves as an energy reservoir for the inductor. In addition to acting as an energy reservoir for the inductor the input capacitor is necessary for the reduction in input voltage ripple and noise experienced by the LM3502. The reduction in input voltage ripple and noise helps ensure the LM3502’s proper operation, and reduces the effect of the LM3502 on other devices sharing the same supply voltage. To ensure low input voltage ripple, the input capacitor must have an extremely low ESR. As a result of the low input voltage ripple requirement multilayer ceramic capacitors are the best choice. A minimum capacitance of 2.0 µF is required for normal operation, so consult the capacitor manufacturer’s data curves to verify whether the minimum capacitance requirement is going to be achieved for a particular application. OUTPUT CAPACITOR SELECTION The output capacitor serves as an energy reservoir for the white LED load when the internal power FET switch (Figure 2: N1) is on or conducting current. The requirements for the output capacitor must include worst case operation such as when the load opens up and the LM3502 operates in overvoltage protection (OVP) mode operation. A minimum capacitance of 0.5µF is required to ensure normal operation. Consult the capacitor manufacturer’s data curves to verify whether the minimum capacitance requirement is going to be achieved for a particular application. Some recommended capacitor manufacturers included but are not limited to: VIN: Eff: Fs: IOUT: L: D: IPEAK: ∆iL: IL(avg): Input Voltage. Efficiency of the LM3502. Switching Frequency. White LED Current/Load Current. Inductance Magnitude/Inductor Value. Duty Cycle for CCM Operation. Peak Inductor Current. Inductor Ripple Current. Average Inductor Current. The cycle-by-cycle peak inductor current for DCM operation can be computed with: 20131747 VIN: Fs: L: D: IPEAK: Input Voltage. Switching Frequency. Inductance Magnitude/Inductor Value. Duty Cycle for DCM Operation. Peak Inductor Current. The minimum inductance magnitude/inductor value for the LM3502 can be calculated using the following, which is only valid when the duty cycle is > 0.5: 15 www.national.com LM3502 Application Information Taiyo Yuden muRata TDK GMK212BJ105MD (0805/35V) (Continued) www.t-yuden.com GRM40-035X7R105K www.murata.com (0805/50V) C3216X7R1H105KT (1206/50V) C3216X7R1C475K (1206/16V) www.tdktca.com limit the peak inductor current at the next startup. When both En1 and En2 are less than 0.3V, the P1 PMOS and N2 NMOS switches will turn off. When Cntrl < 0.3V for more than 12ms, typicaly, the LM3502 will shutdown and the output voltage will be a diode drop below the supply voltage. If the Cntrl pin is low for more than 12ms, the soft-start will reset to limit the peak inductor current at the next startup. When Cntrl is < 0.3 but for less than 12ms, typically, the device will not shutdown and reset the soft-start but shut off the NMOS N1 Power Device to allow for PWM contrl of the LED current. THERMAL SHUTDOWN The LM3502 stops regulating when the internal semiconductor junction temperature reaches approximately 140˚C. The internal thermal shutdown has approximately 20˚C of hysteresis which results in the LM3502 turning back on when the internal semiconductor junction temperature reaches 120˚C. When the thermal shutdown temperature is reached, the softstart is reset to prevent inrush current when the die temperature cools. UNDER VOLTAGE PROTECTION The LM3503 contains protection circuitry to prevent operation for low input supply voltages. When Vin drops below 2.3V, typically the LM3502 will no longer regulate. In this mode, the output volage will be one diode drop below Vin and the softstart will be reset. When Vin increases above 2.4V, typically, the device will begin regulating again. OVER VOLTAGE PROTECTION The LM3502 contains dedicated circuitry for monitoring the output voltage. In the event that the LED network is disconnected from the LM3502, the output voltage will increase and be limited to 15.5V(typ.) for the 16V version , 24V(typ.) for the 25V version, 34V(typ.) for the 35V version and 42V(typ.) for the 44V version (see eletrical table for more details). In the event that the network is reconnected, regulation will resume at the appropriate output voltage. LAYOUT CONSIDERATIONS All components, except for the white LEDs, must be placed as close as possible to the LM3502. The die attach pad (DAP) must be soldered to the ground plane. The input bypass capacitor CIN, as shown in Figure 1, must be placed close to the IC and connect between the VIN and PGND pins. This will reduce copper trace resistance which effects 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, must be placed close to the IC and be connected between the VOUT1 and PGND 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, R1, 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 network should connect directly to the PGND pin. The AGND pin should be tied directly to the PGND pin. Trace connections made to the inductor should be minimized to reduce power dissipation and increase overall efficiency while reducing EMI radiation. For more details regarding layout guidelines for switching regulators, refer to Applications Note AN-1149. 16 AVX 08053D105MAT (0805/25V) 08056D475KAT (0805/6.3V) 1206ZD475MAT (1206/10V) www.avxcorp.com DIODE SELECTION To maintain high efficiency it is recommended that the average current rating (IF or IO) of the selected diode should be larger than the peak inductor current (ILpeak). At the minimum, the average current rating of the diode should be larger than the maximum LED current. To maintain diode integrity the peak repetitive forward current (IFRM) must be greater than or equal to the peak inductor current (ILpeak). Diodes with low forward voltage ratings (VF) and low junction capacitance magnitudes (CJ or CT or CD) are conducive to high efficiency. The chosen diode must have a reverse breakdown voltage rating (VR and/or VRRM) that is larger than the output voltage (Vout). No matter what type of diode is chosen, Schottky or not, certain selection criteria must be followed: 1. VR and VRRM > VOUT 2. IF or IO ≥ ILOAD or IOUT 3. IFRM ≥ ILpeak Some recommended diode manufacturers included but are not limited to: Vishay SS12(1A/20V) SS14(1A/40V) SS16(1A/60V) On Semiconductor MBRM120E (1A/20V) MBRS1540T3 (1.5A/40V) MBR240LT (2A/40V) Central Semiconductor CMSH1- 40M (1A/40V) www.centralsemi.com www.onsemi.com www.vishay.com SHUTDOWN AND START-UP On startup, the LM3502 contains special circuitry that limits the peak inductor current which prevents large current spikes from loading the battery or power supply. When Cntrl ≥ 1.4V and both the En1 and En2 signals are less than 0.3V, the LM3502 will enter a low IQ state and regulation will end. During this low IQ mode the output voltage is a diode drop below the supply voltage and the soft-start will be reset to www.national.com LM3502 Physical Dimensions inches (millimeters) unless otherwise noted 16-Lead Thin Leadless Leadframe Package NS Package Number SQA16A TLP10: 10-Bump Thin Micro SMD X1 = 1.958 mm X2 = 2.135 mm X3 = 0.6 mm NS Package No. TLP10 17 www.national.com LM3502 Step-Up Converter for White LED Applications Notes 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. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. BANNED SUBSTANCE COMPLIANCE National Semiconductor follows the provisions of the Product Stewardship Guide for Customers (CSP-9-111C2) and Banned Substances and Materials of Interest Specification (CSP-9-111S2) for regulatory environmental compliance. Details may be found at: www.national.com/quality/green. Lead free products are RoHS compliant. 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