LM3551SDX

LM3551/LM3552 1A White LED Driver with Flash Timeout Protection February 2006 LM3551/LM3552 1A White LED Driver with Flash Timeout Protection General Description The LM3551 and LM3552 are fixed frequency, current mode step-up DC/DC converters with two integrated NFETs that can be used for precision LED brightness control. The devices are capable of driving loads up to 1A from a single-cell Li-Ion battery. The LM3551 and LM3552 can drive one or more high current flash LEDs either in a high power Flash mode or a lower power Torch mode using the TORCH/FLASH pin. A programmable Timeout function on the FTO pin forces the internal NFETs to turn off after a certain user defined time. An external SD pin (LM3551) or EN pin (LM3552) is available to put the device into low power shutdown mode. During shutdown, the feedback resistors and the load are disconnected from the input to avoid leakage current paths to ground. User programmable soft-start circuitry has been integrated to eliminate large inrush currents at start-up. Over-voltage protection circuitry and a 1.25MHz switching frequency allow for the use of small, low-cost output capacitors with lower voltage ratings. The LM3551 and LM3552 are available in a low profile 14 pin LLP package. Features n n n n n n n n n Up to 1A total drive current Flash timeout protection Independent Torch/Flash/Shutdown modes LED disconnect in shutdown Programmable soft-start limits inrush current Over-voltage protection Wide voltage range 2.7 to 5.5V 1.25MHz constant switching frequency Small, low profile package, non-pullback LLP14 (4mm x 4mm) Applications n n n n n n White LED Camera Flash White LED Torch (Flashlight) DSC (Digital Still Camera) Flash Cellular Camera Phone Flash PDA Camera Flash Camcorder Torch (Flashlight) lamp Typical Application Circuits 20151205 © 2006 National Semiconductor Corporation DS201512 www.national.com LM3551/LM3552 Connection Diagram 14 Pin Dual LLP Package 20151202 NS Package Number SDA14B Pin Descriptions Pin 9 13 8 10 3 5 12 11 4 14 2 1,7,DAP 6 Name VIN T/F SW OVP VC SD(LM3551) EN(LM3552) FTO SS FB FET-T FET-F GND AGND Input Voltage. Input range: 2.7V to 5.5V. TORCH/FLASH Pin. Low = Torch Mode, High = Flash Mode Switch Pin Over Voltage Protection Pin Compensation network connection. Connected to the output of the voltage error amplifier. Shutdown pin logic input. High = Shutdown, Low = Enabled Enable pin logic input. High = Enabled, Low = Shutdown Flash Timeout. External capacitor determines max. duration allowed flash pulse Soft Start Pin Feedback Pin Torch FET Drain Flash FET Drain Ground Analog Ground. Connect the ground of the compensation components, CFTO and soft start cap to AGND. AGND must be connected to the GND pin through a low impedance connection. Function Ordering Information Order Number LM3551SD LM3551SDX LM3552SD LM3552SDX SD / EN SD SD EN EN Package Marking L3551SD L3551SD L3552SD L3552SD Supplied As 250 units, Tape-and-Reel 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 3000 units, Tape-and-Reel www.national.com 2 LM3551/LM3552 Absolute Maximum Ratings (Notes 1, 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN pin: Voltage to GND SW pin: Voltage to GND FB pin: Voltage to GND VC pin: Voltage to GND SD,T/F pins: Voltage to GND FET-T, FET-F: Voltage to GND Junction Temperature (TJ-MAX Storage Temperature Range Maximum Lead Temperature (Soldering) ESD Rating(Note 5) Human Body Model 7.5V 21V 7V 1.26V ± 0.3V 7.5V 6V 150˚C -65˚C to +150 (Note 4) 2.0kV Operating Ratings(Notes 1, 2) Input Voltage Range SW Voltage Max. (Note 6) Junction Temperature (TJ) Range Ambient Temperature (TA) Range (Note 7) 2.7V to 5.5V 20V -40˚C to +110˚C -40˚C to +85˚C Thermal Properties Junction-to-Ambient Thermal Resistance (θJA), SDA14B Package (Note 8) 37.3˚C/W Continuous Power Dissipation (Note 3) Internally Limited ) ESD Caution Notice National Semiconductor recommends that all integrated circuits be handled with appropriate ESD precautions. Failure to observe proper ESD handling techniques can result in damage to the device. Electrical Characteristics(Notes 2, 9) Limits in standard typeface are for TJ = +25˚ C. Limits in boldface type apply over the full operating junction temperature range (-40˚C ≤ TJ ≤ +110˚C). Unless otherwise noted, specifications apply to the LM3551 and LM3552 Typical Application Circuit (pg. 1) with: VIN = 3.6V, V(SD) = 0V for LM3551 and V(EN) = VIN for LM3552, ILOAD = 0A (Note 10) Symbol IQ Parameter Quiescent Current Conditions FB = VIN (Not Switching) V(SD) = VIN LM3551 V(EN) = 0V LM3552 VIN = 3.0V(Note 12) 1.2285 Min Typ 1.47 2.55 0.1 2.1 1.265 50 ∆I = 5µA 135 135 92.5 0.9 VSD = 0V VEN = 3.6V VT/F = 0V VT/F = VIN VL-SW = 20V ISW = 0.5A 1.25 3.0 3.0 26 22 0.07 0.165 0.1 0.98 0.1 0.36 Output High Output Low 1.2 0.3 8 1.6 6 6 1.2915 Max 2.0 5.0 µA 2.3 A V nA µmho V/V % MHz µA µA nA µA Ω µA Ω µA Ω V Units mA ISD Shutdown Current ICL(Note 11) VFB IFB(Note 13) gm AV DMAX fsw ISDPIN IENPIN IT/FPIN IL-SW RDSON-SW IL-T RDSON-T IL-F RDSON-F ThSD/EN Switch Current Limit Feedback Voltage Feedback Pin Bias Current Error Amp Transconductance Error Amp Voltage Gain Maximum Duty Cycle Switching Frequency Shutdown Pin Current (LM3551) Enable Pin Current (LM3552) T/F Pin Current SW Pin Leakage Current SW Pin RDSON FET-T Leakage Current FET-T RDSON FET-F Leakage Current FET-F RDSON Shutdown/Enable Pin Threshold 3 www.national.com LM3551/LM3552 Electrical Characteristics(Notes 2, 9) (Continued) Limits in standard typeface are for TJ = +25˚ C. Limits in boldface type apply over the full operating junction temperature range (-40˚C ≤ TJ ≤ +110˚C). Unless otherwise noted, specifications apply to the LM3551 and LM3552 Typical Application Circuit (pg. 1) with: VIN = 3.6V, V(SD) = 0V for LM3551 and V(EN) = VIN for LM3552, ILOAD = 0A (Note 10) Symbol Parameter T/F Pin Threshold Under Voltage Protection Thresholds Over Voltage Protection Thresholds Flash Timeout trip-point Flash Timeout Current Soft-Start Voltage Soft-Start Current Conditions Output High Output Low On Threshold Off Threshold On Threshold Off Threshold 2.25 2.43 11.3 9.2 0.99 1.12 1.18 10 2.48 2.58 12.4 10.6 1.16 1.4 1.25 11.5 Min 1.2 0.3 2.70 2.77 14 12 1.32 1.68 1.32 13 Typ Max Units V V V V µA V µA ThT/F UVP OVP VFTO IFTO VSS ISS Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the Electrical Characteristics tables. Note 2: All voltages are with respect to the potential at the GND pin. Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=140oC (typ.) and disengages at TJ=120oC (typ.). Note 4: For detailed soldering specifications and information, please refer to National Semiconductor Application Note: AN-1187 for Recommended Soldering Profiles. Note 5: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. (MIL-STD-883 3015.7) Note 6: Maximum recommended SW pin voltage when the OVP pin is grounded. Note 7: 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 = 110oC), 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 x PD-MAX). Note 8: Junction-to-ambient thermal resistance is highly application and board-layout dependent. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues in board design. Note 9: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical (Typ) numbers are not guaranteed, but do represent the most likely norm. Unless otherwise specified, conditions for Typ specifications are: VIN = 3.6V and TA = 25oC. Note 10: CIN and COUT,: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics Note 11: Duty cycle affects current limit due to ramp generator. Note 12: Current limit at 0% duty cycle. See TYPICAL PERFORMANCE section for Switch Current Limit vs. VIN Note 13: Bias current flows into FB pin. www.national.com 4 LM3551/LM3552 Typical Performance Characteristics Unless otherwise specified: TA = +25˚C; VIN = 3.6V; L = 4.7µH, (RC = 10kΩ, CC = 4.7nF, CIN = COUT = 10µF for Lumiled LED), (RC = 27kΩ, CC = 10nF, CIN = 10µF, COUT = 4.7µF for Sharp LED), CFTO = 1µF, CSS= 0.1µF. Current Limit vs. Input Voltage VOUT = 5V Current Limit vs. Input Voltage VOUT = 10V 20151220 20151223 Note 14: ICL measure when VOUT = 95% x VOUT (nominal) Note 15: ICL measure when VOUT = 95% x VOUT (nominal) Converter Efficiency vs. Input Voltage Lumiled Flash LED Converter Efficiency vs. Input Voltage Sharp Flash LED 20151211 20151212 5 www.national.com LM3551/LM3552 Typical Performance Characteristics Unless otherwise specified: TA = +25˚C; VIN = 3.6V; L = 4.7µH, (RC = 10kΩ, CC = 4.7nF, CIN = COUT = 10µF for Lumiled LED), (RC = 27kΩ, CC = 10nF, CIN = 10µF, COUT = 4.7µF for Sharp LED), CFTO = 1µF, CSS= 0.1µF. (Continued) Maximum IOUT vs. Input Voltage VOUT = 5V Maximum IOUT vs. Input Voltage VOUT = 10V 20151222 20151224 Note 16: IOUT measured at 95%x VOUT (nominal) Note 17: IOUT measured at 95%x VOUT (nominal) LED Torch Current vs. Input Voltage Lumiled Flash LED LED Flash Current vs. Input Voltage Lumiled Flash LED 20151218 20151219 www.national.com 6 LM3551/LM3552 Typical Performance Characteristics Unless otherwise specified: TA = +25˚C; VIN = 3.6V; L = 4.7µH, (RC = 10kΩ, CC = 4.7nF, CIN = COUT = 10µF for Lumiled LED), (RC = 27kΩ, CC = 10nF, CIN = 10µF, COUT = 4.7µF for Sharp LED), CFTO = 1µF, CSS= 0.1µF. (Continued) OVP Trip Voltage vs. Input Voltage Switching Frequency vs. Input Voltage 20151217 20151221 Start-Up Waveform Sharp LED Start-Up Waveform Lumiled LED 20151232 20151235 Ch1 = VSD, Ch3 = ILED, Ch4 = IIN Ch1 = VSD, Ch3 = ILED, Ch4 = IIN 7 www.national.com LM3551/LM3552 Typical Performance Characteristics Unless otherwise specified: TA = +25˚C; VIN = 3.6V; L = 4.7µH, (RC = 10kΩ, CC = 4.7nF, CIN = COUT = 10µF for Lumiled LED), (RC = 27kΩ, CC = 10nF, CIN = 10µF, COUT = 4.7µF for Sharp LED), CFTO = 1µF, CSS= 0.1µF. (Continued) Typical Switching Waveform 20151233 Sharp LED in Flash Mode Ch1 = VSW, Ch3 = ILED, Ch4 = IL www.national.com 8 LM3551/LM3552 Block Diagram 20151203 Circuit Description OVERVIEW The LM3551/2 are high power white LED flash drivers capable of delivering up to 1A of output current. The devices utilize a highly efficient inductive DC/DC boost converter to achieve the required output voltage. A current-mode PWM control scheme regulates the output current over a wide input voltage range. Both the LM3551 and the LM3552 have two low-side load disconnect FET’s allowing for a continuous low-power Torch-mode and a high-power, short duration Flash-mode. Several application specific safety features are integrated into the LM3551/2 design. A flash timeout circuit is present on-chip to prevent a failure in the flash LED caused by a timing violation. Over-Voltage Protection protects the output capacitor, inductor and main power switch in the event of an open circuit condition. Other safety features include inductor current limit, thermal shutdown, and an undervoltage lockout. CIRCUIT COMPONENTS Inductive DC/DC Boost Converter In order to achieve the output voltages required to power high power white LEDs, the LM3551 and LM3552 utilize a highly efficient inductive DC/DC converter. The boost converter utilizes a current-mode controlled, constant frequency (1.25MHz.), PWM architecture. This architecture creates a predictable noise spectrum that allows for easy filtering and low noise. A very low on-resistance power NFET(RDSON = 0.165Ω) and high value current limit (2.2A typ.) help efficiently provide a high power output (700mA@5V) over the entire lithium-ion voltage range. The feedback voltage for both the LM3551 and LM3552 is tightly regulated to 1.265V. SD/EN Pin The LM3551 and LM3552 provide two different options in regards to turn-on control logic. The LM3551 utilizes a shutdown pin (SD) that turns on the part when a voltage less than 0.3V is applied. An internal 1.2MΩ pull-up to VIN is provided to place the LM3551 into shutdown when no control signal is provided. The LM3552 utilizes a enable pin (EN) that turns on the part when a voltage greater than 1.2V is applied. An internal 1.2MΩ pull-down to GND is provided to place the LM3552 into shutdown when no control signal is provided. Low-side Load Disconnect FETs The LM3551 and LM3552 have two low-side load disconnect NFETs (FET-T and FET-F) that provide the physical mechanism of Torch Mode and Flash Mode. In Torch Mode, FET-T is enabled allowing current to flow through it. FET-T has an on-resistance of 0.98Ω and is capable of handling currents up to 200mA. In Flash Mode, both FET-T and FET-F are enabled. FET-F has an on-resistance of 0.36Ω and is capable of handling currents up to 500mA. The total Flash current is equal to the sum total of the current flowing through FET-T and FET-F. See the CURRENT SET EQUATIONS in the Application Informations section for more information regarding setting LED current . In shutdown mode, the LM3551/2 provide a true load disconnect helping to keep the total shutdown current to a minimum. Over-Voltage Protection (OVP) The over voltage protection (OVP) is engaged when a failure mode occurs (FB pin grounded, Flash LED becomes open or disconnected, etc.). In the event of a failure, OVP prevents the output voltage from exceeding 12.4V (typ). When the OVP level is reached, the switch FET shuts off preventing the output voltage from climbing higher. Once the FET has shut off, the output will droop at a rate determined by the value of the output capacitor and current leakage through the OVP pin and any other leakage path. When the output 9 www.national.com LM3551/LM3552 Circuit Description (Continued) To disable the timeout function, ground the FTO pin. Soft-Start The LM3551 and LM3552 have a soft-start pin that can be used to limit the inductor inrush current on start-up. The external SS pin is used to tailor the soft-start for a specific application but is not required for all applications and can be left open when not needed. When used, a current source charges the external soft-start capacitor, CSS, forcing the internal reference to ramp-up at a user determined rate. Typical Start-Up Times VIN = 3.6V, TA = +25˚C CSS (µF) Load (mA) Sharp LED @ 75mA Torch Sharp LED @ 250mA Flash 0.1 Lumiled LED @ 200mA Torch Lumiled LED @ 700mA Flash Sharp LED @ 75mA Torch Sharp LED @ 250mA Flash 0.47 Lumiled LED @ 200mA Torch Lumiled LED @ 700mA Flash Sharp LED @ 75mA Torch Sharp LED @ 250mA Flash 1.0 Lumiled LED @ 200mA Torch Lumiled LED @ 700mA Flash Start-Up Time (msec.) 3 8 1.6 6 12 35 6 35 25 75 30 70 voltage drops to 10.6V(typ), switching will resume. The LM3551 and LM3552 will go back into OVP if the failure is still present resulting in a pulsed output condition. Note: To disable OVP, ground to OVP pin. CAUTION: The LM3551 and LM3552 may be damaged if an OVP condition occurs and OVP is disabled. Under-Voltage Protection (UVP) Both the LM3551 and LM3552 have under-voltage protection circuitry (UVP). This protects the NMOS power device during startup and shutdown by preventing operation at voltages less than the minimum input voltage. The UVP protection is enabled at 2.48V(typ.) and will not disable until the input voltage rises above 2.58V(typ.) . Torch/Flash Pin (T/F) The TORCH/FLASH pin (T/F) controls whether the LM3551/2 is in continuous torch mode, or in flash mode. A logic ’0’ places the part into torch mode and a logic ’1’ places the part into flash mode. There are no pull-ups or pull-downs internally connected to T/F. When placed into torch mode, FET-T is enabled allowing the current set by RT to flow. In torch mode, FET-F is not enabled. Flash mode enables both FET-T and FET-F allowing the sum total of the current set by the two external resistors, RT and RF, to flow. Flash Timeout Protection (FTP) When SD is low(LM3551) or EN is high(LM3552), and T/F is high, a current is output to an external capacitor, CFTO. This causes the voltage on the capacitor to rise. If the voltage reaches Vtrip (1.16V(typ)), the timeout circuit forces the INTERNAL_EN signal to go low, which in turn shuts-off the low-side torch and flash FETs in addition to disabling the main power SW FET. At such time, the LED will be turned off. The part will remain disabled until SD is pulled high (LM3551) or EN is pulled low (LM3552) and/or T/F is pulled low. At that point, the part will return to normal operating mode. The diagram below shows a first pulse which exceeds the timeout period and internal_EN being driven low. The second FLASH pulse is shorter than the timeout period and therefore the voltage on CFTO never reaches Vtrip. For information on component selection, please see the FLASH TIMEOUT EQUATIONS below. Application Information LM3551 AND LM3552 FUNCTIONALITY TRUTH TABLE SD (LM3551) or EN (LM3552) 1 1 0 0 T/F 0 1 0 1 LM3551 Result LM3552 Result Shut-down Shut-down Torch Mode Flash Mode Torch Mode Flash Mode Shut-down Shut-down The LM3551 has a 1.2MΩ pull-up to VIN on SD and the LM3552 has a 1.2MΩ pull-down to GND on EN. 20151204 Flash Timeout Protection Diagram TFTO = CFTO x (∆VFTO ÷ IFTO) ∆VFTO = 1.16V and IFTO= 1.4µA TFTO = Desired Timeout Duration CFTO(µF) = TFTO(sec.) x 1.21(µA/V) www.national.com 10 CURRENT SET EQUATIONS The LM3551/2 utilize inline resistors to set the Torch and Flash LED currents. The Torch-Mode current (continuous) and the Flash-Mode current (pulsed) are programmed by placing the appropriately selected resistors between the feedback pin (FB) and FET-T (torch FET) and FET-F (flash FET) pins. Torch-mode is set by utilizes the current through LM3551/LM3552 Application Information (Continued) one resistor while Flash-mode is set by utilizes the currents though both current set resistors. The following equations are used to set the LED currents. RTORCH = (1.265V / ITORCH) - RDSON-T RFLASH = (1.265V / (IFLASH- ITORCH)) - RDSON-F RDSON-T= 0.98Ω and RDSON-F = 0.36Ω Note: Flash LEDs from different manufacturers can have very different continuous and pulse current ratings. See the manufacturers datasheets to ensure that the proper current levels are used to avoid damaging the flash LED. of the regulator is very close to the source output. The size will generally need to be larger for applications where the regulator is supplying nearly the maximum rated output or if large load steps are expected. A minimum value of 10µF should be used under normal operating condtions while a 10-22µF capacitor may be required for higher power and dynamic loads. Larger values and/or lower ESR may be needed if the application requires very low ripple on the input source voltage. Output Capacitor A minimum output capacitor value of 4.7µF (Sharp LED) and 10µF (Lumiled) is recommended and may be increased to a larger value. The ESR of the output capacitor is important because it determines the peak to peak output voltage ripple according to the approximate equation: ∆VOUT ) 2 x ∆ILx RESR (in Volts) After choosing the output capacitor you can determine a pole-zero pair introduced into the control loop by the following equations: INDUCTOR SELECTION Special care must be taken when selecting an inductor for use in LM3551/2 applications. The inductor should have a current saturation rating that is larger than the worst case peak inductor current of the application to ensure proper operation. Using an inductor with a lower saturation current rating than is required can cause a dramatic drop in the inductance and can derate the maximum output current levels severely. It is worth noting that the output voltage ripple is also affected by the total ripple current in the inductor. The following equations can help give a good approximation as to what the peak inductor current will be for a given application at room temperature (TA = +25˚C). IL(average) = [ILED x VOUT-MAX] ÷ [VIN-MIN x Eff.] ∆IL = [VIN x D] ÷ [L x FSW] IL(peak) = IL(ave) + [∆IL ÷ 2] VOUT-MAX Maximum Output Voltage. Maximum output voltage over temperature with OVP used is 11V (12.4V typically). Minimum Input Voltage. Recommended minimum VIN-MIN input voltage is 3.0V. The LM3551/2 will work down to 2.7V however, use at lower input voltages will required an inductor with a higher saturation current rating. Eff. Converter Efficiency (approx. 85% over input voltage range). D Duty Cycle = 1 - [VIN / VOUT] L Inductance. Recommended inductance value is 4.7µH. Switching Frequency = 1.25MHz FSW DIODE SELECTION The output diode for a boost regulator must be chosen correctly depending on the output voltage and output current. The output diode must have a reverse voltage rating equal to or greater than the output voltage used. The average current rating must be greater than the maximum load current expected, and the peak current rating must be greater than the peak inductor current. Using Schottky diodes with lower forward voltage drop will decrease power dissipation and increase efficiency. CAPACITOR SELECTION Input Capacitor An input capacitor is required to reduce the input ripple and noise for proper operation of the regulator. The size used is dependant on the application and board layout. If the regulator will be loaded uniformly, with very little load changes, and at lower current outputs, the input capacitor size can often be reduced. The size can also be reduced if the input The zero created by the ESR of the output capacitor is generally at a very high frequency if the ESR is small. If low ESR capacitors are used it can be neglected. The output capacitor pole information is useful in selecting the proper compensation components and is discussed in the COMPENSATION COMPONENTS section of the datasheet. Capacitor Properties Surface-mount multi-layer ceramic capacitors are recommended for both the input and output capacitors. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR < 20mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are not recommended for use with the LM3551/2 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM3551/2. These capacitors have tight capacitance tolerance (as good as ± 10%) and hold their value over temperature (X7R: ± 15% over -55˚C to 125˚C; X5R: ± 15% over -55˚C to 85˚C). Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM3551/2. Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -20%) and vary significantly over temperature (Y5V: +22%, -82% over -30˚C to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C range). Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1µF. Such deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM3551/2. The minimum voltage rating acceptable for the input capacitor is 6.3V (10V recommended) and 16V for the output capacitor. In applications that have DC operating points near the maximum voltage rating of the ceramic capacitor, larger capacitor values may be required to compensate for capaci11 www.national.com LM3551/LM3552 Application Information (Continued) tance loss due to capacitor voltage coefficient. See the capacitor manufacturer’s datasheet for DC bias performance. COMPENSATION DC Gain and Open-Loop Gain Since the control stage of the converter forms a complete feedback loop with the power components, it forms a closedloop system that must be stabilized to avoid positive feedback and instability. A value for open-loop DC gain will be required, from which you can calculate, or place, poles and zeros to determine the crossover frequency and the phase margin. A high phase margin (greater than 45˚) is desired for the best stability and transient response. For the purpose of stabilizing the LM3551/2, choosing a crossover point well below where the right half plane zero is located will ensure sufficient phase margin. To ensure a bandwidth of 1⁄2 or less of the frequency of the RHP zero, calculate the open-loop DC gain, ADC. After this value is known, you can calculate the crossover visually by placing a −20dB/decade slope at each pole, and a +20dB/ decade slope for each zero. The point at which the gain plot crosses unity gain, or 0dB, is the crossover frequency. If the crossover frequency is less than 1⁄2 the RHP zero, the phase margin should be high enough for stability. The equation for ADC is given below with additional equations required for the calculation: loop should be designed to have a bandwidth of less than 1⁄2 the frequency of the RHP zero. This zero occurs at a frequency of: where ILOAD is the maximum load current. Compensation Components The LM3551 and LM3552 provide a compensation pin (VC) to customize the voltage loop feedback. It is recommended that a series combination of RC and CC be used for the compensation network, as shown in the typical application circuit. For any given application, there exists a unique combination of RC and CC that will optimize the performance of the LM3551/2 circuit in terms of its transient response. The series combination of RC and CC introduces a pole-zero pair. The frequency of the pole created is determined by the equation: where RO is the output impedance of the error amplifier, approximately 900kΩ. Since RC is generally much less than RO, it has little effect on the above equation and can be neglected until a value is chosen to set the zero fZC. fZC is created to cancel the pole created by the output capacitor, fP1. The output capacitor pole will shift with different load currents as shown by the equation, so setting the zero is not exact. Determine the range of fP1 over the expected loads and then set the zero fZC to a point approximately in the middle. The frequency of this zero is determined by: mc ) 0.072 x fs (in V/s) Now RC can be chosen with the selected value for CC. Check to make sure that the pole fPC is still in the 10Hz to 500Hz range, and change each value slightly if needed to ensure both component values are in the recommended range. For both typical applications circuits shown on the front page, the Recommended value for CC is 4.7nF and RC = 10kΩ for Lumiled applications. 10nF and 27kΩ are recommended for Sharp applications. RECOMMENDED MINIMUM COMPONENT SPECIFICATIONS where RL is the minimum load resistance, fs is the switching frequency, VIN is the minimum input voltage, gm is the error amplifier transconductance and RDSON-S is the power switch on-resistance. The value for gm and RDSON-S are found in the Electrical Characteristics table. Right Half Plane Zero A current mode control boost regulator has an inherent right half plane zero (RHP zero). This zero has the effect of a zero in the gain plot, causing an imposed +20dB/decade on the rolloff, but has the effect of a pole in the phase, subtracting another 90˚ in the phase plot. This can cause undesirable effects if the control loop is influenced by this zero. To ensure the RHP zero does not cause instability issues, the control Component L1 CIN COUT CFTO Value 4.7µH 4.7µF (Sharp) 10µF (Lumiled) 4.7µF (Sharp) 10uF (Lumiled) User Determined Ratings 2.0A 30% ISAT Rating 10V X5R or X7R 16V X5R or X7R 6.3V X5R or X7R www.national.com 12 LM3551/LM3552 Application Information Component Value 4.7nF (Lumiled) 10nF (Sharp) 10kΩ (Lumiled) 27kΩ (Sharp) User Determined User Determined User Determined (Continued) Ratings 6.3V X5R or X7R Application Examples TYPICAL CONFIGURATIONS CC RC RFLASH RTORCH CSS Application Specific Application Specific 6.3V X5R or X7R 20151215 LUMILED LXCL-PWF1 RT = 5.6Ω, RF = 2.2Ω ITORCH = 200mA, IFLASH = 700mA Torch and Flash Resistor ratings are dependent upon the current through each resistor. The minimum ratings will vary depending upon the current selected on an applicaiton by application basis. Power Rating Minimum = (Desired Current)2 x Resistor Value. See the CURRENT SET EQUATIONS section to determine torch and flash currents. THERMAL PROTECTION Internal thermal protection circuitry disables the LM3551/2 when the junction temperature exceeds +140˚C (typ.). This feature protects the device from being damaged by high die temperatures that might otherwise result from excessive power dissipation. The device will recover and operate normally when the junction temperature falls below +120˚C (typ.). It is important that the board layout provide good thermal conduction to keep the junction temperature within the specified operating ratings. PCB LAYOUT CONSIDERATIONS The LLP is a leadframe based Chip Scale Package (CSP) with very good thermal properties. This package has an exposed DAP (die attach pad) at the center of the package measuring 2.6mm x 3.0mm. The main advantage of this exposed DAP is to offer lower thermal resistance when it is soldered to the thermal land pad on the PCB. For PCB layout, National highly recommends a 1:1 ratio between the package and the PCB thermal land. To further enhance thermal conductivity, the PCB thermal land may include vias to a ground plane. For more detailed instructions on mounting LLP packages, please refer to National Semiconductor Application Note AN-1187. 20151216 SHARP GM5BW05340A RT = 17Ω, RF = 6.5Ω ITORCH = 75mA, IFLASH = 250mA Note 18: Please refer to the RECOMMENDED MINIMUM COMPONENT section of the datasheet for more information. DUAL-MODE CONFIGURATION 20151209 R1 = 29.5kΩ,R2 = 10kΩ RF = 3Ω IFLASH = 500mA Using the Dual-Mode configuration with either the LM3551/2, a 5V, high current rail (approx. 700mA total) can be created while still allowing for a high flash with a true load disconnect. R1 and R2 setup the +5V following the equation: VOUT = 1.265 x (1 + R1/R2) . When the LM3551/2 is on, and the T/F pin is low (logic ’0’), the part will provide a regulated output voltage that can be used to provide a voltage rail 13 www.national.com LM3551/LM3552 Application Examples (Continued) within a system. By setting the T/F pin high, the LM3551/2 will allow the current to flow through the flash LED while still maintaining the fixed output voltage rail. The flash current is set by RF using the equation RF = [(VOUT - VLED) / ILED] - 0.36Ω , and should not exceed 500mA. The total usable output current is dependent upon the output voltage selected. If the dual-mode configuration is used, the FTO pin should be grounded to prevent the voltage rail from being shutdown at an unwanted time. www.national.com 14 LM3551/LM3552 1A White LED Driver with Flash Timeout Protection Physical Dimensions inches (millimeters) unless otherwise noted 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 manufactures products and uses packing materials that meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. Leadfree products are RoHS compliant. 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