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| XRP7604 1A 29V Non-Sync. Buck High Power LED Driver March 2009 Rev. 1.0.0 GENERAL DESCRIPTION The XRP7604 is a non-synchronous voltage mode PWM step down converter with integrated high side FET optimized to drive high power LEDs at up to 1A of continuous current. A wide 4.5V to 29V input voltage range allows for single supply operations from industry standard 5V, 12V or 24V power rails. A 1.2MHz constant operating frequency allows for small external components selection while an internal type II control loop compensation reduces the overall component count and solution footprint. A low 200mV feedback reference voltage minimizes power dissipation in the system while efficiency is mazimized via a 100% duty cycle capability. Dimming and shutdown mode is provided via an enable function when required. An adjustable over current and under voltage lock out protection insures safe operations under abnormal operating conditions. The XRP7604 is pin compatible with Exar's XRP7603 and SP7600, non synchronous buck high power led drivers respectively rated at 500mA and 2A. The XRP7604 is offered in a compact thermally enhanced RoHS compliant "green"/halogen free 8-pin SO package. APPLICATIONS * General Lighting and Displays * Architectural and Accent Lighting * Medical and Industrial Instrumentation * Video Projectors FEATURES * 1A Continous Output Current Capable * 4.5V to 29V Single Rail Input Voltage * 1.2MHz Constant Switching Frequency * Internal Control Loop Compensation * 0.2V Feedback Reference Voltage * 2.5% Output Voltage Accuracy * Built-in Soft Start * PWM & Analog Dimming Capability * Adjustable Over-Current Protection * Pin Compatible with 500mA rated XRP7603 & 2A rated SP7600 * Thermally Enhanced Package * RoHS Compliant "Green"/Halogen Free 8-pin SO Package TYPICAL APPLICATION DIAGRAM Fig. 1: XRP7604 Application Diagram Exar Corporation 48720 Kato Road, Fremont CA 94538, USA www.exar.com Tel. +1 510 668-7000 - Fax. +1 510 668-7001 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver ABSOLUTE MAXIMUM RATINGS These are stress ratings only and functional operation of the device at these ratings or any other above those indicated in the operation sections of the specifications below is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability. Input Voltage ............................................. -0.3V to 30V Lx................................................................-2V to 30V FB .....................................................-0.3V to VIN+0.3V Storage Temperature .............................. -65C to 150C Power Dissipation (Note 1) ................... Internally Limited Lead Temperature (Soldering, 10 sec) ................... 300C ESD Rating (Lx, ISET) ....................................1KV - HBM ESD Rating (All other pins) .............................2KV - HBM OPERATING RATINGS Input Voltage Range VIN ................................4.5V to 29V Junction Temperature Range ....................-40C to 125C Thermal Resistance JA ...................................... 59C/W ELECTRICAL SPECIFICATIONS Specifications with standard type are for an Operating Junction Temperature of TJ = 25C only; limits applying over the full Operating Junction Temperature range are denoted by a "*". Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25C, and are provided for reference purposes only. Unless otherwise indicated, VIN = 4.5V to 29V, CIN = 1F, TJ = -40C to 125C. Parameter UVLO Turn-On Threshold UVLO Turn-Off Threshold UVLO Hysteresis Operating Input Voltage Range Operating Input Voltage Range Operating VCC Current Standby VCC Current Reference Voltage Reference Voltage Switching Frequency Minimum On-Pulse Duration Minimum Duty Cycle Maximum Duty Cycle VDR Voltage Over-Current Threshold ISET Pin Input Current OFF Interval During Hiccup SHDN Threshold SHDN Threshold Hysteresis Switch On Resistance Switch Leakage 0.8 100 4.5 250 25 300 33 100 1.0 100 95 3 5 1.2 5.5 350 40 186 960 4.5 7 2 0.6 200 200 1250 40 214 1550 100 0 Min. 4.0 3.8 Typ. 4.2 4.0 0.2 29 29 5 1 Max. 4.5 4.3 Units V V V V V mA mA mV mV kHz ns % % V mV A ms V mV m A * Apply voltage to FB * * Measure VIN-VDR VIN > 7V Measure VIN-Lx VIN=VLx * * * VFB=0.1V, not switching VFB=1.2V, not switching 0C TJ 125C Conditions 0C TJ 125C 0C TJ 125C Note 1: All parameters tested at TA=25C. Specifications over temperature are guaranteed by design. (c) 2009 Exar Corporation 2/13 Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver BLOCK DIAGRAM Fig. 2: XRP7604 Block Diagram PIN ASSIGNEMENT Fig. 3: XRP7604 Pin Assignement (c) 2009 Exar Corporation 3/13 Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver PIN DESCRIPTION Name FB GND VDR PVIN SVIN LX ISET Pin Number 1 2 3 4,5 6 7 8 Description Regulator feedback input. A current setting resistor is connected to LED's cathode and FB on one side and to ground on the other side. This pin can be also used for dimming control. By connecting a diode between this pin and a >2V signal the LED can be pulsed at up to 1kHz Ground pin Power supply for the internal driver. This voltage is internally regulated to about 5V below VIN. Place a 0.1uF decoupling capacitor between VDR and Vin as close as possible to the IC. Connection to the FET source Input power supply for the regulator. Place input decoupling capacitor as close as possible to this pin. This is the Vin connection for the regulator and is not tied to the high-side FET. Connect to the output inductor. This is the P-Channel FET Drain This pin is used as a current limit input for the internal current limit comparator. Connect to LX through an optional resistor. Internal threshold is pre-set to 350mV nominal and can be decreased by changing the external resistor based on the following formula: VTRSHLD = 350mV - 33uA * R Can be connected to inductor LX node for a thermal PAD - see Layout suggestions section. Power PAD 9 ORDERING INFORMATION Part Number Temperature Range -40CTJ+125C Marking XRP7604E YYWWF LOT# XRP7604E YYWWF LOT# Package HSOICN-8 Exp. PAD HSOICN-8 Exp. PAD Packing Quantity Bulk Note 1 RoHS Compliant Green-Halogen Free RoHS Compliant Green/Halogen Free Note 2 XRP7604EDB-F XRP7604EDBTR-F XRP7604EVB -40CTJ+125C 2.5K/Tape & Reel XRP7604 Evaluation Board "YY" = Year - "WW" = Work Week - "F" = Green/Halogen Free designator - "LOT#" = Lot Number (c) 2009 Exar Corporation 4/13 Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver TYPICAL PERFORMANCE CHARACTERISTICS The typical performance characteristics follow and begin with an illustration of the efficiencies that can be obtained with the XRP7604 driving 1 or 6 white LEDs in series for up to 500mA output current. For the 6 LED applications with a 24V input, the duty cycle is high and an efficiency of 94% can be obtained. For 12V input and 1 LED at 1A output, the duty cycle is much lower, but the efficiency is still over 80%. Note: to improve line regulation a small 22pF ceramic capacitor C6 should be placed from VFB to GND to filter out any noise obtained on the sensitive FB pin. Scope photos of output ripple are shown for the typical application circuit for 6V input at 150mVpp ripple and at 29V input with over 400mVpp output ripple, both shown with no output capacitor. For comparison, an output ripple scope photo is shown with only 70mVpp when a 1uF capacitor is used at the output. For applications sensitive to output ripple, adding this relatively small 1206 sized 1uF 50V ceramic capacitor to the output provides a very good reduction in output ripple but since the value is only 1uF the circuit will still provide good PWM output response. Vin startup scope photos are shown for 6V, 12V and 29V input with no problems in startup as shown in the Vout, VFB and the outpt current Io. The last scope photos are for the output short circuit which causes a hiccup mode. The output can be shorted which causes a controlled automatic reset or hiccup mode of about 50 to 100msec period. All data taken at VIN = 12V, TA = 25C, unless otherwise specified - Schematic and BOM from Application Information section of this datasheet. Efficiency versus Vin at Iout = 750mA 100 1 LED 2 LED 3 LED 6 LED Iout versus Vin 0.800 1 LED 2 LED 3 LED 6 LED 90 Efficiency (%) Iout (mA) 0.775 80 0.750 70 0.725 60 5 10 15 Vin (V) 20 25 30 0.700 5 10 15 Vin (V) 20 25 30 Fig. 4: Efficiency vs Input Voltage Fig. 5: Output Current vs Input Voltage (c) 2009 Exar Corporation 5/13 Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver VFB versus Vin at Iout = 750mA 0.210 1 LED 2 LED 0.205 VFB(V) 0.200 0.195 Ch1: Lx Ch2: Vout(AC) Ch4: Io 0.75A/div 0.190 5 10 15 Vin(V) 20 25 30 Fig. 6: Feedback Voltage vs Input Voltage Fig. 7: No Cout, Output Ripple=104mVpp, Vin=6V 1 LED, Vf=3.3V @ 0.75A Ch1: Lx Ch2: Vout(AC) Ch4: Io 0.75A/div Ch1: Lx Ch2: Vout(AC) Ch4: Io 0.75A/div Fig. 8: No Cout, Output Ripple=284mVpp, Vin=29V 1 LED, Vf=3.3V @ 0.75A Fig. 9: Cout=1uF, Output Ripple=164mVpp, Vin=29V 1 LED, Vf=3.3V @ 0.75A Ch1: Ch2: Ch3: Ch4: Vin Vout VFB Io 0.5A/div Ch1: Ch2: Ch3: Ch4: Vin Vout VFB Io 0.5A/div Fig. 10: 5V Vin Startup 1 LED, Vf=3.3V @ 0.75A Fig. 11: 12V Vin Startup 1 LED, Vf=3.3V @ 0.75A (c) 2009 Exar Corporation 6/13 Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver Ch1: Ch2: Ch3: Ch4: Vin Vout VFB Io 2A/div Ch1: Ch2: Ch3: Ch4: Lx Vout VFB Io 0.5A/div Fig. 12: 29V Vin Startup 1 LED, Vf=3.3V @ 0.75A Fig. 13: Output Overcurrent Hiccup mode with Vin=29V (c) 2009 Exar Corporation 7/13 Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver THEORY OF OPERATION The XRP7604 is a fixed frequency, Voltagemode, non-synchronous buck PWM regulator optimized for driving LEDs. Constant LED current is achieved using resistor RFB as shown in the page 1 schematic. A low 0.2V reference voltage minimizes power dissipation in RFB. A tight reference voltage tolerance of 3%, over full operating conditions, helps accurately program the LED current. High switching frequency of 1.2MHz (nominal) reduces the size of passive components. Dimming and power sequencing is achieved using a logic-level PWM signal applied to FB pin via a diode. Overcurrent protection (OCP) is based on high-side MOSFET's Rds(on) and is programmable via a resistor placed at LX node. DIM = L. The DIM signal needs to be greater than 600mV minimum to turn-off the XRP7604 and less than 200mV to fully turn-on the XRP7604. It is recommended to use a signal with DIM = 1V or more for OFF and 0V for ON. The user should note that the logic is reversed relative to many other PWM controlled LED drivers. In other words a logic level high at 20% duty cycle will result in approximately an 80% duty cycle for the LED. Recommended modulation frequencies are from 100Hz to 200Hz with 10 - 90% duty cycle, 500Hz with 10 - 80% duty cycle, and 1000Hz with 10 70% duty cycle. Figures 15 & 16 show the output response at the maximum PWM DIM signal of 1000Hz. See figure 17 for 100Hz to 1000Hz duty cycle response for two Luxeon K2 LEDs in parallel at 0.75A total current. PROGRAMMING THE LED CURRENT Use the following equation to program the LED current: Equ.1: RFB = 0.2V I LED Ch1: DIM Signal Ch2: VFB - 0.75A IOUT/div The output voltage will adjust as needed to ensure average ILED is supplied. For example if the output current has to be set at 0.35A then RFB=0.57 Ohm. If the output LED has a corresponding Vf of 3.5V then XRP7604 will step down the VIN to 3.5V. If two of these LEDs are placed in series then XRP7604 will step down the voltage to 7V. Superimposed on ILED there is a current ripple that is equal in magnitude to inductor current ripple. Current ripple will be nominally set to 10% of ILED by proper sizing of inductor. Note that throughout this datasheet ILED and IO will be used interchangeably. Fig. 14: 1.1KHz, 10% Duty Cycle Dimming Signal Dimming Signal is ~70% LED Duty Cycle DIMMING SIGNAL A logic-level PWM signal applied through a small-signal diode to the feed-back (FB) pin can be used for dimming control of the LED. This external signal we call DIM turns the MOSFET gate drive on/off, thereby modulating the average current delivered to the LED. The DIM signal connects to the VFB pin through a 1N4148 diode and will shutdown the XRP7604 when DIM = H and turn-on the XRP7604 when (c) 2009 Exar Corporation 8/13 Ch1: DIM Signal Ch2: VFB - 0.75A IOUT/div Fig. 15: 1KHz, 70% Duty Cycle Dimming Signal is ~10% LED Duty Cycle Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver LED current versus PWM Dimming Duty 100 100Hz 90 80 200Hz 500Hz 1kHz after FB is set at the high state (>1.2V). The regulator is now in standby and once Vin has reached steady-state then FB is transitioned from a high to a low state. The regulator then starts operating at nominal frequency. Another benefit of using power sequencing for power up is that it ensures all internal circuitry is alive and fully operational before the device is required to regulate the current through the LEDs. Since the regulator was "Off" before power was applied, it is unlikely the LED is under any type of thermal stress. EXAR does not recommend using the XRP7604 in applications where dimming of the LED is achieved by PWM'ing the actual input power as is common in automotive dimming applications. LED current (%) 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 (1-D) PWM Dimming Duty (%) Fig. 16: Linearity, LED current vs (1-D) PWM Dimming Duty Cycle, Vin=12V, Io=0.75A, 2 LED in series BUCK OPERATION WITHOUT OUTPUT CAPACITOR In order to be able to apply the aforementioned dimming signal to the LED, the output filter capacitor that is normally used with a buck converter has to be removed from the circuit. Thus the LED current ripple equals the inductor current ripple. As a rule of thumb current ripple should be limited to 10% of ILED. Allowing for a higher current ripple, up to 30%, while staying within LED manufacturer ripple guidelines, will reduce inductance and possibly inductor size. MODULATOR OPERATION AND POWER SEQUENCING The XRP7604 has a unique modulator design which improves the device's ability to operate at very high duty cycle. While seamless in operation as the duty cycle is increasing (input voltage falling), when the duty cycle is decreased (input voltage rising), the user will observe the switching frequency increasing in distinct fractions of the switching frequency. If the device is operating at 100% duty cycle, a unique advantage of using a p-channel pass device, and then the input voltage is increased, the frequency will start at 300kHz, then 600kHz, and then finally 1.2MHz. The frequency will tend to increase to the next higher fraction once the duty cycle reaches 75 to 65 percent. This is the normal operation of the device and should be expected. There is no impact on the LED current accuracy. If PWM dimming is being used as the input voltage is increased, one will see the frequency increasing when the duty cycle is < 90%. When power is initially applied the device will begin operating as if the input voltage is increasing and may start operation at one of the fractional operating frequencies. Many users will prefer to have the device start operating at the nominal operating frequency, thus it is recommended that Vin be applied (c) 2009 Exar Corporation 9/13 OVERCURRENT PROGRAMMING Resistor Rs can be used to program Overcurrent Protection (OCP). Use the following equation for calculating the Rs value. 0.35V - (1.5 x 1.15 x I OCP x Rds(on)) Rs = 33A Where Iocp is the programmed overcurrent and is generally set 50% above nominal output current, and Rds(on) = 135mohms. Maximum value of Rs that can be used for programming OCP is 4k. Equ.2: INDUCTOR SELECTION Select the inductor L1 for inductance, Irms and Isat. Calculate inductance from Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver Equ.3: L = Vo x (Vin - Vo) Vin x f x I L Where Vin is converter input voltage and Vo is converter output voltage. Since voltage across Rset is small, Vo approximately equals Vf (for a string of series connected LEDs Vo equals total Vf) IL is inductor current ripple (nominally set to 30% of Io) Inductor Isat and Irms must allow sufficient safeguard over output current Io. As a rule of thumb these parameters should be 50% higher than Io. Where high efficiency is required a low DCR inductor should be used. Ceramic capacitors are recommended for input filtering due to their low Equivalent Series Resistance (ESR), Equivalent Series inductance (ESL) and small form factor. SCHOTTKY RECTIFIER SELECTION Select the Schottky D1 for Voltage rating VR and current rating If. Recommended schottky diode voltage rating for 12V and 24V applications is 30V and 40V respectively. Current rating can be calculated from: Equ.5 : If 1 - Vo x Io Vin INPUT CAPACITOR SELECTION Select the input capacitor for capacitanceand ripple current rating. Use the capacitances listed in the table 1 as a starting point and if needed increase Cin. IO(A) <0.7 0.71 to 1.2 >1.2 Table 1: Cin Selection Cin (F) 2.2 4.7 2 x 4.7 Note that in applications where duty cycle is low, Schottky losses comprise a larger percentage of converter losses. In order to improve the efficiency in these applications choose a Schottky that meets the calculated current rating and has a lower Vf. FEEDBACK RESISTOR RFB R2 is part of XRP7604 loop compensation network. Use a 30k R2 for Vin of 20V and larger. Use R2 of 60K for Vin less than 20V. CAPACITOR C5 This is the decoupling capacitor for the power supply for the internal driver. Use a 0.1uF and place as closely to VDR and SVIN pins as possible. Calculate the ripple current requirement from: Equ.4: Irip = Io x Where D = D x (1 - D) Vo Vin (c) 2009 Exar Corporation 10/13 Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver DESIGN EXAMPLE Design a drive circuit for a string of 5 LED at 0.75A with a 24V input voltage. Nominal LED voltage is 3.3V. Resistor RFB calculation Schottky current rating IF IF 1- (5 x 3.3) x 0.75 A = 0.42 A 24V RFB = 0.2V = 0.27 0.75 A Voltage rating should be 30V. B340A rated at 30V/3A or equivalent can be used for its ample current rating and low forward voltage. Rs calculation A standard value of 0.27ohm 0805 is selected. Inductor L1 calculation Rs = 0.3V - (1.5 x 1.15 x 1.5 x 0.75 A x 0.095) = 3.5k 33A L1 = (5 x 3.3V ) x (24V - (5 x 3.3V )) = 19.1H 24V x 1.2 MHz x (0.3 x 0.75 A) Use standard resistor value for Rs of 3.4k. Use a 22uH standard inductor. Input capacitor A 4.7F CIN (C1) is needed (refer to table 1). From Equ.4, the ripple current rating of CIN is a fraction of 0.75A. A 4.7uF/25V ceramic capacitor easily meets this requirement and offers low ESR and ESL. Fig. 17: Circuit for design example LAYOUT CONSIDERATION i) Place the bypass capacitors C4 and C5 as close as possible to the XRP7604 IC. See figure 5 for details. ii) Create a pad under the IC that connects the power pad (pin 9) to the inductor. Duplicate this pad through the pcb layers if present, and on the bottom side of the PCB. Use multiple vias to connect these layers to aid in heat dissipation. Do not oversize this pad - since the LX node is subjected to very high dv/dt voltages, the stray capacitance formed between these islands and the surrounding circuitry will tend to couple switching noise iii) Connect the Schottky diode cathode as close as possible to the LX node and inductor input side. Connect the anode to a large diameter trace or a copper area that connects the input ground to the output ground. iv) The output capacitor, if used, should be placed close to the load. Use short wide copper regions to connect output capacitors to load to minimize inductance and resistances. v) Keep other sensitive circuits and traces away from the LX node in particular and away from the power supply completely if possible. For more detail on the XRP7604 layout see the XRP7604EVB Evaluation Board Manual available on our web site. Each layer is shown in detail as well as a complete bill of materials. Fig. 18: XRP7604 Eval Board Component Side Lay (c) 2009 Exar Corporation 11/13 Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver PACKAGE SPECIFICATION 8-PIN HSOICN (c) 2009 Exar Corporation 12/13 Rev. 1.0.0 XRP7604 1A 29V Non-Sync. Buck High Power LED Driver REVISION HISTORY Revision 1.0.0 Date 03/17/2009 First release of data sheet Description FOR FURTHER ASSISTANCE Email: Exar Technical Documentation: customersupport@exar.com http://www.exar.com/TechDoc/default.aspx? EXAR CORPORATION HEADQUARTERS AND SALES OFFICES 48720 Kato Road Fremont, CA 94538 - USA Tel.: +1 (510) 668-7000 Fax: +1 (510) 668-7030 www.exar.com NOTICE EXAR Corporation reserves the right to make changes to the products contained in this publication in order to improve design, performance or reliability. EXAR Corporation assumes no responsibility for the use of any circuits described herein, conveys no license under any patent or other right, and makes no representation that the circuits are free of patent infringement. Charts and schedules contained here in are only for illustration purposes and may vary depending upon a user's specific application. While the information in this publication has been carefully checked; no responsibility, however, is assumed for inaccuracies. EXAR Corporation does not recommend the use of any of its products in life support applications where the failure malfunction of the product can reasonably be expected to cause failure of the life support system or to significantly affect safety or effectiveness. Products are not authorized for use in such applications unless EXAR Corporation receives, writing, assurances to its satisfaction that: (a) the risk of injury or damage has been minimized; (b) the user assumes such risks; (c) potential liability of EXAR Corporation is adequately protected under the circumstances. Reproduction, in part or whole, without the prior written consent of EXAR Corporation is prohibited. or its in all (c) 2009 Exar Corporation 13/13 Rev. 1.0.0 |
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