LM3557 Step-Up Converter for White LED Applications General Description The LM3557 is a complete solution for white LED drive applications. With minimal external component count, no DC current leakage paths to ground, cycle-by-cycle current limit protection, and output over-voltage protection circuitry, the LM3557 offer superior performance and cost savings over standard DC/DC boost component implementations. The LM3557 switches at a fixed-frequency of 1.25 MHz, which allows for the use of small external components. Also, the LM3557 has a wide input voltage range to take advantage of multi-cell input applications. With small external components, high fixed frequency operation, and wide input voltage range, the LM3557 is the most optimal choice for LED lighting applications. 1.25 MHz Constant-Switching Frequency Output Over-Voltage Protection Input Under-Voltage Protection Cycle-By-Cycle Current Limit TRUE SHUTDOWN: No DC current paths to ground during shutdown n Low Profile Package: < 1 mm Height -8 Pin LLP n No External Compensation n n n n n Applications n White LED Display Lighting n Cellular Phones n PDAs Features n VIN Range: 2.7V–7.5V n Small External Components Typical Application Circuit 20131601 FIGURE 1. Backlight Configuration © 2004 National Semiconductor Corporation DS201316 www.national.com LM3557 Step-Up Converter for White LED Applications November 2004 LM3557 Connection Diagram Top View 20131602 8-Lead Thin Leadless Leadframe Package See NS Package Number SDA08A Ordering Information Order Number Package Marking Supplied As LM3557SD-2 L147B 1k Units, Tape and Reel LM3557SDX-2 L147B 4.5k Units, Tape and Reel Pin Description Pin # Name 1 Sw1 Drain Connection of the Internal Power Field Effect Transistor (FET) Switch (Figure 2: N1) Description 2 VIN Input Voltage Connection 3 NC No Connection 4 En Device Enable Connection 5 Ovp Over-Voltage Protection Input Connection 6 Fb 7 Sw2 Drain Connection of an Internal Field Effect Transistor (FET) Switch (Figure 2: N2) 8 Gnd Ground Connection DAP DAP Die Attach Pad (DAP), must be soldered to the printed circuit board’s ground plane for enhanced thermal dissipation www.national.com Feedback Voltage Connection 2 ESD Rating (Note 2) Human Body Model Machine Model If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN Pin −0.3V to +8V En Pin −0.3V to +8V Fb Pin −0.3V to +8V Sw2 Pin −0.3V to +8V Ovp Pin −0.3V to +30V Sw1 Pin −0.3V to +40V Continuous Power Dissipation Storage Temperature Range Operating Conditions (Notes 1, 6) Junction Temperature (TJ) Range −40˚C to +125˚C Ambient Temperature (TA) Range −40˚C to +85˚C Supply Voltage, VIN Pin 2.7V to 7.5V En Pin Internally Limited Maximum Junction Temperature (TJ-MAX) 2 kV 150V 0V to VIN +0.4V Thermal Properties (Notes 4, 7) +150˚C Junction-to-Ambient Thermal −65˚C to +150˚C 55˚C/W Resistance (θJA), Leadless Leadframe Package Electrical Characteristics (Notes 6, 8) 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 = 3.6V. Symbol Parameter Conditions Min VIN Input Voltage IQ Quiescent Current VEN = 0V (Shutdown) VEN = 1.8V; VOVP = 27V (Non-Switching) En Device Enable Threshold Device On Device Off 0.9 ICL Power Switch Current Limit (Note 10) VIN = 3V 0.4 0.55 RDS(ON) Power Switch ON Resistance ISw1 = 175 mA TC (RDS(ON)) RDS(ON) Temperature Coefficient OVP Over-Voltage Protection (Note 5) On Threshold Off Threshold UVP Under-Voltage Protection (Note 5) On Threshold Off Threshold IOVP Over-Voltage Protection Pin Bias Current (Note 3) IEN Enable Pin Bias Current (Note 3) VEN = 1.8V FS Switching Frequency VIN = 3V VFb-Sw2 Feedback Pin Voltage (Note 9) IFb Feedback Pin Bias Current (Note 3) DMAX Maximum Duty Cycle VIN = 3V ILSw1 Sw1 Pin Leakage Current (Note 3) VSw1 = 3.6V, Not Switching ILSw2 Sw2 Pin Leakage Current (Note 3) VSw2 = 3.6V, Not Switching ILOVP Ovp Pin Leakage Current (Note 3) VOvp = 3.6V, Not Switching RSw2 Sw2 Pin Switch Resistance ISw2 = 50 mA TC(RSw2) RSw2 Temperature Coefficient Typ Max Units 7.5 V 2 0.8 µA mA 0.3 V 0.8 0.8 1.1 1.02 A 800 1000 mΩ 2.7 0.01 0.55 0.5 22 21.5 26 25.5 %/C 28.5 28 2.2 2.3 V V 4 10 µA 0.8 3 µA 0.9 1.25 1.6 MHz 0.459 0.51 0.561 V 0.03 2 µA 85 90 % 0.002 2 µA 0.001 1 µA 2 8 0.5 nA 10 Ω %/C 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. 3 www.national.com LM3557 Absolute Maximum Ratings (Note 1) LM3557 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. Note 5: The on threshold indicates that the LM3557 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: Junction-to-ambient thermal resistance (θJA) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the JEDEC standard JESD51-7. The test board is a 4 layer FR-4 board measuring 102 mm x 76 mm x 1.6 mm with a 2 x 1 array of thermal vias. The ground plane on the board is 50 mm x 50 mm. Thickness of copper layers are 36 µm/18 µm/18 µm/36 µm (1.5 oz/1 oz/1 oz/1.5 oz). Ambient temperature in simulation is 22˚C, still air. Power dissipation is 1W. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues. For more information on these topics, please refer to Application Note 1187: Leadless Leadframe Package (LLP) and the Layout Guidelines section of this datasheet. Note 8: 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 9: Feedback pin voltage is with respect to the voltage at the Sw2 pin. Note 10: The Power Switch Current Limit is tested in open loop configuration. For closed loop application current limit please see the Current Limit vs Temperature performance graph. Block Diagram 20131603 FIGURE 2. Block Diagram reference voltage, the error amplifier outputs a signal that is translated into the correct amount of stored energy within the inductor that is required to put the feedback voltage back into regulation when the stored inductor energy is then transferred to the load. The aforementioned translation is a conversion of the error amplifier’s output signal to the proper on-time duration of the N1 power field effect transistor (FET). This conversion allows the inductor’s stored energy to increase, or decrease, to a sufficient level that when transferred to the load will bring the feedback voltage back into regulation. Operation The LM3557 is a current-mode controlled constantfrequency step-up converter optimized for the facilitation of white LED driving/current biasing. The LM3557’s operation can be best understood by the following device functionality explanation. For the following device functionality explanation, the block diagram in Figure 2 serves as a functional schematic representation of the underlying circuit blocks that make up the LM3557. When the feedback voltage falls below, or rises above, the internal www.national.com 4 OVER-VOLTAGE PROTECTION (Continued) When the output voltage exceeds the over-voltage protection (OVP) threshold, the LM3557’s internal power FET will be forcibly turned off until the output voltage falls below the over-voltage protection threshold minus the 500 mV hysteresis of the internal OVP circuitry. An increase in inductor current corresponds to an increase in the amount of stored energy within the inductor. Conversely, a decrease in inductor current corresponds to a decrease in the amount of stored energy. The inductor’s stored energy is released, or transferred, to the load when the N1 power FET is turned off. The transferred inductor energy replenishes the output capacitor and keeps the white LED current regulated at the designated magnitude that is based on the choice of the R2 resistor. When the N1 power FET is turned on, the energy stored within the inductor begins to increase while the output capacitor discharges through the series string of white LEDs, the R2 resistance, and N2 FET switch to ground. Therefore, each switching cycle consist of some amount of energy being stored in the inductor that is then released, or transferred, to the load to keep the voltage at the feedback pin in regulation at 510 mV above the Sw2 pin voltage. UNDER-VOLTAGE PROTECTION When the input voltage falls below the under-voltage protection (UVP) threshold, the LM3557’s internal power FET will be forcibly turned off until the input voltage is above the designated under-voltage protection threshold plus the 100 mV hysteresis of the internal UVP circuitry. TRUE SHUTDOWN When the LM3557 is put into shutdown mode operation there are no DC current paths to ground. The internal FET (Figure 2: N2) at the Sw2 pin turns off, leaving the white LED string open circuited. Features: THERMAL SHUTDOWN When the internal semiconductor junction temperature reaches approximately 150˚C, the LM3557’s internal power FET (Figure 2: N1) will be forcibly turned off. CYCLE-BY-CYCLE CURRENT LIMIT The current through the internal power FET (Figure 2: N1) is monitored to prevent peak inductor currents from damaging the part. If during a cycle (cycle = 1/switching frequency) the peak inductor current exceeds the current limit rating for the LM3557, the internal power FET would be forcibly turned off for the remaining duration of that cycle. Typical Performance Characteristics ( Circuit in Figure 1: L = DO1608C-223, D = SS16, and LED = LWT67C. Efficiency: η = POUT/PIN = [(VOUT – VFb) * IOUT]/[VIN * IIN]. TA= 25˚C, unless otherwise stated). IQ (SWITCHING) vs TEMPERATURE SWITCHING FREQUENCY vs TEMPERATURE 20131605 20131604 5 www.national.com LM3557 Operation LM3557 Typical Performance Characteristics ( Circuit in Figure 1: L = DO1608C-223, D = SS16, and LED = LWT67C. Efficiency: η = POUT/PIN = [(VOUT – VFb) * IOUT]/[VIN * IIN]. TA= 25˚C, unless otherwise stated). (Continued) En PIN CURRENT vs En PIN VOLTAGE CURRENT LIMIT vs TEMPERATURE 20131609 20131608 OVP PIN CURRENT vs TEMPERATURE RDS(ON) (Figure 2: N1) vs TEMPERATURE 20131610 20131611 RSw2(Figure 2: N2) vs TEMPERATURE ENABLE THRESHOLD vs TEMPERATURE 20131612 20131613 Application Information www.national.com 6 LM3557 Application Information (Continued) 20131616 FIGURE 3. Programmable Output Voltage adjust the OVP threshold of a given application. Instead of having the Ovp pin connected to the output voltage, it can be adjusted through a resistor divider circuit to only experience a fraction of the output voltage magnitude. The resistor divider circuit bias current should be at least 100 times greater than the Ovp pin bias current. Using Figure 3, the following equation can be used to adjust the output voltage: WHITE LED CURRENT SETTING For backlighting applications, the white LED current is programmed by the careful choice of the R2 resistor. Backlight: VEn≥ 0.9V 20131617 20131618 ILED: White LED Current. VFb-Sw2: Feedback Voltage. R2: Resistor. VOVP: VOUT: R3: R4: The feedback voltage is with respect to the voltage at the Sw2 pin, not ground. For example, if the voltage on the Sw2 pin were 0.1V then the voltage at the Fb pin would be 0.61V (typical). OVP Voltage Threshold. Maximum Output Voltage ( < 35V). Resistor. Resistor. ADJUSTING OVER-VOLTAGE PROTECTION If the over-voltage protection (OVP) threshold is too low for a particular application, a resistor divider circuit can be used to 7 www.national.com LM3557 Application Information (Continued) 20131619 FIGURE 4. Inductor Current Waveform CONTINUOUS AND DISCONTINUOUS MODES OF OPERATION Since the LM3557 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 LM3557 is in. The two operational modes of the LM3557 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’s 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’s current goes to and stays at zero for a significant amount of time during the switching cycle. Figure 4 illustrates the threshold between CCM and DCM operation. In Figure 4, 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 4, the R factor is equal to the average inductor current, IL(avg), divided by half the inductor ripple current, ∆iL. Using Figure 4, the following equation can be used to compute R factor: 20131622 20131623 VIN: VOUT: Eff: Fs: IOUT: L: D: ∆iL: IL(avg): Input Voltage. Output Voltage. Efficiency of the LM3557. 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: 20131624 20131625 20131620 tON: TS: Internal Power FET On-Time. Switching Period of Operation. D: Duty Cycle for CCM Operation. VOUT: Output Voltage. VIN: Input Voltage. 20131621 For DCM operation, the duty cycle can be computed with: www.national.com 8 D: (Continued) Duty Cycle for DCM Operation. IPeak: Peak Inductor Current. Some recommended inductor manufacturers are as follows: Coilcraft [www.coilcraft.com] Coiltronics [www.cooperet.com] TDK [www.tdk.com] 20131626 CAPACITOR SELECTION Multilayer ceramic capacitors are the best choice for use with the LM3557. 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 of the capacitor in your application. 20131627 tON: TS: D: VOUT: VIN: IOUT: Fs: Eff: L: Internal Power FET On-Time. Switching Period of Operation. Duty Cycle for DCM Operation. Output Voltage. Input Voltage. White LED Current/Load Current. Switching Frequency. Efficiency of the LM3557. Inductor Value/Inductance Magnitude. 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 LM3557. The reduction in input voltage ripple and noise helps ensure the LM3557’s proper operation, and reduces the effect of the LM3557 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, consult the capacitor manufacturer’s data curves to verify whether the minimum capacitance requirement is going to be achieved for a particular application. INDUCTOR SELECTION In order to maintain inductance, an inductor used with the LM3557 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 (continuous current mode) and DCM (discontinuous current mode). The cycle-by-cycle peak inductor current for CCM operation can be computed with: 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 LM3557 operates in overvoltage protection (OVP) mode operation. A minimum capacitance of 0.5 µF 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. 20131628 20131629 VIN: Eff: Fs: IOUT: L: D: IPeak: ∆iL: IL(avg): Input Voltage. Efficiency of the LM3557. Switching Frequency. White LED Current/Load Current. Inductance Magnitude/Inductor Value. Duty Cycle for CCM Operation. Peak Inductor Current. Inductor Ripple Current. Some recommended capacitor manufacturers are as follows: TDK [www.tdk.com] Murata [www.murata.com] Vishay [www.vishay.com] Average Inductor Current. The cycle-by-cycle peak inductor current for DCM operation can be computed with: 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). 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 20131630 VIN: Fs: L: Input Voltage. Switching Frequency. Inductance Magnitude/Inductor Value. 9 www.national.com LM3557 Application Information LM3557 Application Information The input capacitor, Cin, must be placed close to the LM3557. Placing Cin close to the device will reduce the metal trace resistance effect on input voltage ripple. The feedback current setting resistor R2 must be placed close to the Fb and Sw2 pins. The output capacitor, Cout, must be placed close to the Ovp and Gnd pin connections. Trace connections to the inductor should be short and wide to reduce power dissipation, increase overall efficiency, and reduce EMI radiation. The diode, like the inductor, should have trace connections that are short and wide to reduce power dissipation and increase overall efficiency. For more details regarding layout guidelines for switching regulators refer to Applications Note AN-1149. (Continued) 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 are as follows: Vishay [www.vishay.com] Diodes, Inc [www.diodes.com] On Semiconductor [www.onsemi.com] LAYOUT CONSIDERATIONS All components, except for the white LEDs, must be placed as close as possible to the LM3557. The die attach pad (DAP) must be soldered to the ground plane. www.national.com 10 LM3557 Step-Up Converter for White LED Applications Physical Dimensions inches (millimeters) unless otherwise noted 8-Lead Thin Leadless Leadframe Package NS Package Number SDA08A 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. 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