Four-String, White LED Driver for LCD Backlight Applications ADD5207 Data Sheet FEATURES GENERAL DESCRIPTION White LED driver based on inductive boost converter Integrated 40 V MOSFET with 1.5 A peak current limit Input voltage range: 6 V to 21 V Maximum output adjustable up to 36 V 600 kHz to 1 MHz adjustable operating frequency Typical 39 V fixed overvoltage protection (OVP) Built-in soft start for boost converter Drives up to 4 LED current strings LED current adjustable up to 25 mA for each channel Headroom control to maximize efficiency Fixed LED dimming frequency: 8 kHz LED open fault protection Brightness control with PWM input Dimming controls 4-channel operation: 90 degree phase shift between channels 3-channel operation: 120 degree phase shift between channels General Thermal shutdown Undervoltage lockout 14-lead, 4 mm × 3 mm LFCSP The ADD5207 is a white LED driver for backlight applications based on high efficiency, current mode, step-up converter technology. It is designed with a 0.15 Ω, 1.5 A internal switch and a pin-adjustable operating frequency between 600 kHz and 1 MHz. The ADD5207 contains four regulated current sources for uniform LED brightness. Each current source can drive up to 25 mA and the LED-driving current is pin adjustable by an external resistor. The ADD5207 drives up to four parallel strings of multiple series-connected LEDs with a ±1.5% current matching between strings. The ADD5207 provides phase shift PWM brightness control methods. LED dimming control is achieved through the PWM input. The device includes an 8 kHz LED-dimming oscillator for driving each current source. The ADD5207 operates over an input voltage range of 6 V to 21 V, but the device can function with a voltage as low as 5.6 V. The ADD5207 also has multiple safety protection features to prevent damage during fault conditions. If any LED is open, the device automatically disables the faulty current source. The internal soft start circuit prevents a high inrush current during startup. Thermal shutdown protection prevents thermal damage. APPLICATIONS The ADD5207 is available in a low profile, thermally enhanced, 4 mm × 3 mm × 0.75 mm, 14-lead, RoHS-compliant lead frame chip scale package (LFCSP) and is specified over the industrial temperature range of −25°C to +85°C. Notebook PCs, UMPCs, and monitor displays TYPICAL APPLICATION CIRCUIT VIN + – L1 10µH CIN 10µF CIN2 0.1µF D1 1 VIN COUT 4µF 14 13 SW OVP ADD5207 9 PWM RF 100kΩ 8 VDD FB1 4 FB2 5 2 FSLCT ISET FB3 6 COMP 3 RSET 180kΩ FB4 7 GND 11 RC 6.8kΩ 12 CC 2.2nF C2 OPEN 08350-101 OFF ON CBYPASS 1µF 10 SHDN Figure 1. Rev. A Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2009–2012 Analog Devices, Inc. All rights reserved. ADD5207 Data Sheet TABLE OF CONTENTS Features .............................................................................................. 1 Typical Performance Characteristics ..............................................9 Applications ....................................................................................... 1 Theory of Operation ...................................................................... 11 General Description ......................................................................... 1 Current Mode, Step-Up Switching Regulator Operation ..... 11 Typical Application Circuit ............................................................. 1 Internal 3.3 V Regulator ............................................................ 11 Revision History ............................................................................... 2 Boost Converter Switching Frequency .................................... 11 Functional Block Diagram .............................................................. 3 Dimming Frequency (fPWM) ...................................................... 11 Specifications..................................................................................... 4 Current Source ............................................................................ 11 Step-Up Switching Regulator Specifications............................. 4 PWM Dimming Mode .............................................................. 11 LED Current Regulation Specifications .................................... 5 Safety Features ............................................................................ 11 General Specifications ................................................................. 6 External Component Selection Guide ..................................... 12 Absolute Maximum Ratings ............................................................ 7 Layout Guidelines....................................................................... 13 Thermal Resistance ...................................................................... 7 Typical Application Circuits ......................................................... 15 ESD Caution .................................................................................. 7 Outline Dimensions ....................................................................... 16 Pin Configuration and Function Descriptions ............................. 8 Ordering Guide .......................................................................... 16 REVISION HISTORY 2/12—Rev. Sp0 to Rev. A Replaced Block Diagram with Typical Application Circuit ........ 1 Changes to Features Section and General Description Section . 1 Changes to Current Mode, Step-Up Switching Regulator Operation Section, Boost Converter Switching Frequency Section, PWM Dimming Mode Section, Phase Shift PWM Dimming Section, and Safety Features Section .......................... 11 Changes to Overvoltage Protection (OVP) Section .................. 11 Changes to Open-Loop Protection (OLP) Section, Undervoltage Lockout (UVLO) Section, and Thermal Protection Section .......................................................................... 12 Changes to Layout Guidelines Section ........................................ 13 7/09—Revision Sp0: Initial Version Rev. A | Page 2 of 16 Data Sheet ADD5207 FUNCTIONAL BLOCK DIAGRAM VIN VDD SHDN OVP SW 1 8 10 13 14 THERMAL SHUTDOWN LINEAR REGULATOR 500kΩ SHUTDOWN VOLTAGE REFERENCE VOUT_FB GND ADD5207 UVP COMP GND OVP REF UVP REF LL COMP ERROR AMP VOUT_FB LL REF R Q S PWM COMP gm COMP 11 OSC 2 FSLCT DREF DCOMP + + CURRENT SENSE SOFT START HEADROOM CONTROL 12 GND VDD LED OPEN/SHORT FAULT DETECTOR CURRENT SOURCE 1 4 FB1 CURRENT SOURCE 2 5 FB2 CURRENT SOURCE 3 6 FB3 CURRENT SOURCE 4 7 FB4 REF ISET 3 PWM DUTY EXTRACTOR PWM 9 CURRENT SOURCE DRIVER FPWM OSCILLATOR 08350-002 500kΩ GND Figure 2. Functional Block Diagram Rev. A | Page 3 of 16 ADD5207 Data Sheet SPECIFICATIONS STEP-UP SWITCHING REGULATOR SPECIFICATIONS VIN = 12 V, SHDN = high, TA = −25°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C. Table 1. Parameter SUPPLY Input Voltage Range BOOST OUTPUT Output Voltage SWITCH On Resistance Leakage Current Peak Current Limit OSCILLATOR Switching Frequency Maximum Duty Cycle SOFT START Soft Start Time OVERVOLTAGE PROTECTION Overvoltage Rising Threshold on OVP Pin Overvoltage Hysteresis on OVP Pin Symbol Test Conditions/Comments VIN Min Typ Max Unit 21 V 36 V 150 300 1 mΩ µA A 1000 90 1200 kHz % 6 VOUT RDS(ON) ILKG ICL VIN = 12 V, ISW = 100 mA Duty cycle (D) = DMAX 1.5 fSW DMAX RF = 97 kΩ RF = 97 kΩ 800 84 tSS 1.1 VOVPR VOVP_HYS Rev. A | Page 4 of 16 36.5 0.1 39 0.7 ms 40 1.4 V V Data Sheet ADD5207 LED CURRENT REGULATION SPECIFICATIONS VIN = 12 V, SHDN = high, TA = −25°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C. Table 2. Parameter CURRENT SOURCE ISET Pin Voltage Adjustable LED Current 1 Constant Current Sink of 20 mA 2 Minimum Headroom Voltage2 Current Matching Between Strings2 LED Current Accuracy2 Current Source Leakage Current FPWM GENERATOR Dimming Frequency LED FAULT DETECTION Open Fault Delay1 1 2 Symbol Test Conditions/Comments Min Typ Max Unit VSET ILED ILED20 VHR20 6 V ≤ VIN ≤ 21 V 1.14 0 19.4 1.18 1.22 25 20.6 0.9 +1.5 +3 1 V mA mA V % % µA 9.2 kHz 6.5 µs RSET = 180 kΩ RSET = 180 kΩ RSET = 180 kΩ RSET = 180 kΩ fPWM −1.5 −3 6.8 tD_OPENFAULT This electrical specification is guaranteed by design. Tested at TA = +25°C. Rev. A | Page 5 of 16 20 0.66 8.0 ADD5207 Data Sheet GENERAL SPECIFICATIONS VIN = 12 V, SHDN = high, TA = −25°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C. Table 3. Parameter SUPPLY Input Voltage Range Quiescent Current Shutdown Supply Current VDD REGULATOR VDD Regulated Output PWM INPUT PWM Voltage High PWM Voltage Low PWM Input Range THERMAL SHUTDOWN Thermal Shutdown Threshold 1 Thermal Shutdown Hysteresis1 UVLO VIN Falling Threshold VIN Rising Threshold SHDN CONTROL Input Voltage High Input Voltage Low SHDN Pin Input Current 1 Symbol Test Conditions/Comments VIN IQ ISD 6 V ≤ VIN ≤ 21 V, SHDN = high 6 V ≤ VIN ≤ 21 V, SHDN = low VVDD_REG 6 V ≤ VIN ≤ 21 V Min Typ Max Unit 3.5 21 7 2 V mA µA 3.3 3.5 V 5.5 0.8 10,000 V V Hz 6 VPWM_HIGH VPWM_LOW 3.1 2.0 100 TSD TSDHYS VUVLOF VUVLOR VIH VIL ISHDN 160 30 VIN falling VIN rising 4 4.2 5.0 2.5 SHDN = 3.3 V This electrical specification is guaranteed by design. Rev. A | Page 6 of 16 °C °C 5.6 5.5 0.5 6 V V V V µA Data Sheet ADD5207 ABSOLUTE MAXIMUM RATINGS TA = 25°C, unless otherwise noted. THERMAL RESISTANCE Table 4. Parameter VIN SW SHDN, PWM ISET, FSLCT, COMP VDD FB1, FB2, FB3, FB4 OVP Maximum Junction Temperature (TJ max) Operating Temperature Range (TA) Storage Temperature Range (TS) Reflow Peak Temperature (20 sec to 40 sec) Rating −0.3 V to +23 V −0.3 V to +40 V −0.3 V to +6 V −0.3 V to +3.5 V −0.3 V to +3.7 V −0.3 V to +40 V −0.3 V to +40 V 150°C −25°C to +85°C −65°C to +150°C 260°C θJA is specified for the worst-case conditions, that is, a device soldered in a circuit board for surface-mount packages. Table 5. Thermal Resistance Package Type 14-Lead LFCSP ESD CAUTION Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Rev. A | Page 7 of 16 θJA 33.24 θJC 2.42 Unit °C/W ADD5207 Data Sheet PIN CONFIGURATION AND FUNCTION DESCRIPTIONS SW VIN 1 14 FSLCT 2 13 OVP ISET 3 12 GND 11 COMP FB1 4 ADD5207 FB2 5 10 SHDN FB3 6 9 PWM FB4 7 8 VDD NOTES 1. CONNECT THE EXPOSED PADDLE TO GROUND. 08350-003 TOP VIEW (Not to Scale) Figure 3. Pin Configuration Table 6. Pin Function Descriptions Pin No. 1 2 3 4 5 6 7 8 9 10 11 Mnemonic VIN FSLCT ISET FB1 FB2 FB3 FB4 VDD PWM SHDN COMP 12 13 14 GND OVP SW EP Description Supply Input. Must be locally bypassed with a capacitor to ground. Frequency Select. A resistor from this pin to ground sets the boost switching frequency from 600 kHz to 1 MHz. Full-Scale LED Current Set. A resistor from this pin to ground sets the LED current up to 25 mA. Regulated Current Sink. Connect the bottom cathode of the LED string to this pin. Regulated Current Sink. Connect the bottom cathode of the LED string to this pin. Regulated Current Sink. Connect the bottom cathode of the LED string to this pin. Regulated Current Sink. Connect the bottom cathode of the LED string to this pin. If unused, connect FB4 to GND. Internal Linear Regulator Output. This regulator provides power to the ADD5207. PWM Signal Input. Shutdown Control for PWM Input Operation Mode. Active low. Compensation for the Boost Converter. Two capacitors and a resistor are connected in series between ground and this pin for stable operation. Ground. Overvoltage Protection. The boost converter output is connected to this pin directly. Drain Connection of the Internal Power FET. Exposed Paddle. Connect the exposed paddle to ground. Rev. A | Page 8 of 16 Data Sheet ADD5207 TYPICAL PERFORMANCE CHARACTERISTICS 90 20 LED CURRENT (mA) 88 86 84 82 15 10 5 80 78 5 10 0 20 15 INPUT VOLTAGE (V) 08350-007 08350-004 BOOST CONVERTER EFFICIENCY (V) 25 ILED = 20mA fSW = 800kHz BRIGHTNESS = 100% LEDs = 10 SERIES × 4 PARALLEL 5 10 15 INPUT VOLTAGE (V) 20 Figure 7. LED Current vs. Input Voltage (ILED = 20 mA) Figure 4. Boost Converter Efficiency vs. Input Voltage 28 1.2 LED CURRENT MATCHING (%) 24 22 20 18 16 14 12 10 0.9 0.6 0.3 0 –0.3 –0.6 –0.9 –1.2 08350-005 8 6 4 135 150 165 180 195 210 225 240 255 08350-008 LED CURRENT (mA) BRIGHTNESS = 100% LEDs = 10 SERIES × 4 PARALLEL ILED = 20mA 1.5 26 –1.5 6 270 8 10 12 14 16 18 20 22 INPUT VOLTAGE (V) RSET (kΩ) Figure 5. LED Current vs. RSET Figure 8. LED Current Matching vs. Input Voltage VOUT 20V/DIV 20 VSW 20V/DIV 15 0V SHDN 5V/DIV 10 0V IL 600mA/DIV 0A 08350-006 VIN = 12V BRIGHTNESS = 100% LEDs = 10 SERIES × 4 PARALLEL 96.48 91.41 86.33 81.25 76.17 71.09 66.02 60.94 55.86 50.78 45.70 40.63 35.55 30.47 25.39 20.31 15.23 10.16 0 0 5ms/DIV PWM DUTY CYCLE (%) Figure 6. LED Current vs. PWM Input Duty Cycle Figure 9. Start-Up Waveforms (Brightness = 100%) Rev. A | Page 9 of 16 08350-009 5 5.08 LED CURRENT (mA) 0V ADD5207 Data Sheet VOUT 100mV/DIV AC 0V PWM 2V/DIV 0V VSW 20V/DIV FB1 5V/DIV 0V 0V IL 500mA/DIV IFB1 10mA/DIV 0A VIN = 12V BRIGHTNESS = 1.5% LEDs = 10 SERIES × 4 PARALLEL 08350-010 VIN = 6V, fSW = 800kHz BRIGHTNESS = 100% LEDs = 10 SERIES × 4 PARALLEL 1µs/DIV 100µs/DIV Figure 10. Switching Waveforms (VIN = 6 V) 0V 08350-012 0A Figure 12. LED Current Waveforms (Brightness = 1.5%) VOUT 100mV/DIV AC 0V VSW 20V/DIV 0V FB1 7V/DIV FB2 7V/DIV FB3 7V/DIV 0V 0V IL 500mA/DIV 0A FB4 7V/DIV 1µs/DIV VIN = 12V BRIGHTNESS = 25% LEDs = 10 SERIES × 4 PARALLEL 50µs/DIV Figure 11. Switching Waveforms (VIN = 21 V) Figure 13. LED FBx Waveforms (Brightness = 25%) Rev. A | Page 10 of 16 08350-013 08350-011 0V VIN = 21V, fSW = 800kHz BRIGHTNESS = 100% LEDs = 10 SERIES × 4 PARALLEL Data Sheet ADD5207 THEORY OF OPERATION CURRENT SOURCE CURRENT MODE, STEP-UP SWITCHING REGULATOR OPERATION The ADD5207 uses a current mode PWM boost regulator to generate the minimum voltage needed to drive the LED string at the programmed LED current. The current mode regulation system allows a fast transient response while maintaining a stable output voltage. By selecting the proper resistor-capacitor network from COMP to GND, the regulator response is optimized for a wide range of input voltages, output voltages, and load conditions. The ADD5207 can provide a 36 V maximum output voltage and drive up to 10 LEDs (3.4 V/25 mA type of LEDs) for each channel. INTERNAL 3.3 V REGULATOR The ADD5207 contains a 3.3 V linear regulator that is used for biasing internal circuitry. The internal regulator requires a 1 μF bypass capacitor. Place this bypass capacitor between Pin VDD (Pin 8) and GND, as close as possible to Pin VDD. The ADD5207 contains an LED open fault protection circuit for each channel. If the headroom voltage of the current source remains below 150 mV while the boost converter output reaches the OVP level, the ADD5207 recognizes that the current source has an open-load fault for the current source, and the current source is disabled. If an application requires three LED strings, each LED string should be connected using FB1 to FB3. The unused FB4 pin should be tied to GND. The ADD5207 contains hysteresis to prevent the LED current change that is caused by a ±0.195% jitter of the PWM input. Programming the LED Current BOOST CONVERTER SWITCHING FREQUENCY The ADD5207 boost converter switching frequency is user adjustable, between 600 kHz to 1 MHz, by using an external resistor, RF. A frequency of 600 kHz is recommended to optimize the regulator for high efficiency, and a frequency of 1 MHz is recommended to minimize the size of external components. See Figure 14 for considerations when selecting a switching frequency and an adjustment resistor (RF). As shown in the Figure 2, the ADD5207 has an LED current set pin (ISET). A resistor (RSET) from this pin to ground adjusts the LED current up to 25 mA. LED current level can be set with following equation: ILED = 3600 ( A) RSET PWM DIMMING MODE The ADD5207 supports 8-bit resolution to control brightness. However, each current source has a minimum on time requirement for LED current regulation such that the dimming is in the range of 1.5% to 100%. Accordingly, even when the PWM input duty cycle is more than 0% and less than 1.5%, the LED duty cycle is held at 1.5%. 1000 900 800 Phase Shift PWM Dimming 700 There is a phase delay between each current source channel that is programmed by the number of current sources in operation. If the application requires four separate LED strings, each string has a 90 degree phase delay between channels. If three LED strings are connected at the FB1 to FB3 pins (FB4 = GND), each string has a 120 degree phase delay. 600 500 08350-014 SWITCHING FREQUENCY (kHz) The ADD5207 contains four current sources to provide accurate current sinking for each LED string. String-to-string tolerance is kept within ±1.5% at 20 mA. Each LED string current is adjusted up to 25 mA using an external resistor. 400 300 80 100 120 140 160 RF (kΩ) 180 200 220 SAFETY FEATURES The ADD5207 contains several safety features to provide stable and reliable operation. Figure 14. Switching Frequency vs. RF DIMMING FREQUENCY (fPWM) The ADD5207 contains an internal oscillator to generate the PWM dimming signal for LED brightness control. The LED dimming frequency (fPWM) is fixed at 8 kHz internally. Soft Start The ADD5207 contains an internal soft start function to reduce inrush current at startup. The soft start time is typically 1.1 ms. Overvoltage Protection (OVP) The ADD5207 contains OVP circuits to prevent boost converter damage if the output voltage becomes excessive for any reason. To keep a safe output level, the integrated OVP circuit monitors Rev. A | Page 11 of 16 ADD5207 Data Sheet the output voltage. When the OVP pin voltage reaches the OVP rising threshold, the boost converter stops switching, which causes the output voltage to drop. When the OVP pin voltage drops below the OVP falling threshold, the boot converter begins switching again, causing the output to rise. There is about 0.8 V hysteresis between the rising and falling thresholds. The OVP level is fixed at 39 V (typical). The inductor ripple current (ΔIL) in a steady state is: Open-Load Protection (OLP) Make sure that the peak inductor current (that is, the maximum input current plus half of the inductor ripple current) is less than the rated saturation current of the inductor. In addition, ensure that the maximum rated rms current of the inductor is greater than the maximum dc input current to the regulator. The ADD5207 contains a headroom control circuit to minimize power loss at each current source. Therefore, the minimum feedback voltage is achieved by regulating the output voltage of the boost converter. If any LED string is open circuit during normal operation, the current source headroom voltage (VHR) is pulled to GND. In this condition, OLP is activated if VHR is less than 150 mV until the boost converter output voltage rises up to the OVP level. Undervoltage Lockout (UVLO) An undervoltage lockout circuit is included with built-in hysteresis. The ADD5207 turns on when VIN rises above 5.0 V (typical) and shuts down when VIN falls below 4.2 V (typical). Thermal Protection Thermal overload protection prevents excessive power dissipation from overheating and damaging the ADD5207. When the junction temperature (TJ) exceeds 160°C, a thermal sensor immediately activates the fault protection, which shuts down the device and allows it to cool. The device self-starts when the junction temperature (TJ) of the die falls below 130°C. EXTERNAL COMPONENT SELECTION GUIDE Inductor Selection The inductor is an integral part of the step-up converter. It stores energy during the switch’s on time and transfers that energy to the output through the output diode during the switch’s off time. An inductor in the range of 4.7 µH to 22 µH is recommended. In general, lower inductance values result in higher saturation current and lower series resistance for a given physical size. However, lower inductance results in higher peak current, which can lead to reduced efficiency and greater input and/or output ripple and noise. Peak-to-peak inductor ripple current at close to 30% of the maximum dc input current typically yields an optimal compromise. The input (VIN) and output (VOUT) voltages determine the switch duty cycle (D), which, in turn, is used to determine the inductor ripple current. D= ∆I L = Solve for the inductance value (L): L= Use the duty cycle and switching frequency (fSW) to determine the on time. t ON = V IN × t ON ∆I L For duty cycles greater than 50% that occur with input voltages greater than half the output voltage, slope compensation is required to maintain stability of the current mode regulator. The inherent open-loop stability causes subharmonic instability when the duty ratio is greater than 50%. To avoid subharmonic instability, the slope of the inductor current should be less than half of the compensation slope. Inductor manufacturers include: Coilcraft, Inc., Sumida Corporation, and Toko. Input and Output Capacitor Selection The ADD5207 requires input and output bypass capacitors to supply transient currents while maintaining a constant input and output voltage. Use a low effective series resistance (ESR) 10 μF or greater capacitor for the input capacitor to prevent noise at the ADD5207 input. Place the input between VIN and GND, as close as possible to the ADD5207. Ceramic capacitors are preferred because of their low ESR characteristics. Alternatively, use a high value, medium ESR capacitor in parallel with a 0.1 μF low ESR capacitor as close as possible to the ADD5207. The output capacitor maintains the output voltage and supplies current to the load while the ADD5207 switch is on. The value and characteristics of the output capacitor greatly affect the output voltage ripple and stability of the regulator. Use a low ESR output capacitor; ceramic dielectric capacitors are preferred. For very low ESR capacitors, such as ceramic capacitors, the ripple current due to the capacitance is calculated as follows. Because the capacitor discharges during the on time (tON), the charge removed from the capacitor (QC) is the load current multiplied by the on time. Therefore, the output voltage ripple (ΔVOUT) is ∆VOUT = VOUT − V IN VOUT V IN × t ON L QC I ×t = L ON C OUT C OUT where: COUT is the output capacitance. IL is the average inductor current. D f SW Rev. A | Page 12 of 16 Data Sheet ADD5207 Using the duty cycle and switching frequency (fSW), users can determine the on time with the following equation: t ON = D f SW The input (VIN) and output (VOUT) voltages determine the switch duty cycle (D) with the following equation: D= VOUT − V IN VOUT Loop Compensation The external inductor, output capacitor, and the compensation resistor and capacitor determine the loop stability. The inductor and output capacitor are chosen based on performance, size, and cost. The compensation resistor (RC) and compensation capacitor (CC ) at COMP are selected to optimize control loop stability. For typical LED application of the ADD5207, a 6.8 kΩ compensation resistor in series with a 2.2 nF compensation capacitor at COMP is adequate. VOUT_FB Choose the output capacitor based on the following equation: I L × (VOUT − V IN ) RC f SW × VOUT × ∆VOUT Capacitor manufacturers include: Murata Manufacturing Co., Ltd., AVX, Sanyo, and Taiyo Yuden Co., Ltd. Diode Selection The output diode conducts the inductor current to the output capacitor and loads while the switch is off. For high efficiency, minimize the forward voltage drop of the diode. Schottky diodes are recommended. However, for high voltage, high temperature applications, where the Schottky diode reverse leakage current becomes significant and degrades efficiency, use an ultrafast junction diode. The output diode for a boost regulator must be chosen depending on the output voltage and the output current. The diode must be rated for a reverse voltage 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 decreases power dissipation and increases efficiency. The diode must be rated to handle the average output load current. Many diode manufacturers derate the current capability of the diode as a function of the duty cycle. Verify that the output diode is rated to handle the average output load current with the minimum duty cycle. The minimum duty cycle of the ADD5207 is: D MIN = C2 CC 08350-015 C OUT ≥ gm HEADROOM CONTROL VOUT − V IN_MAX VOUT where VIN_MAX is the maximum input voltage. For example, DMIN is 0.5 when VOUT is 30 V and VIN_MAX is 15 V. Schottky diode manufacturers include ON Semiconductor, Diodes Incorporated, Central Semiconductor Corp., and Sanyo. Figure 15. Compensation Components A step-up converter produces an undesirable right-half plane zero in the regulation feedback loop. Capacitor C2 is chosen to cancel the zero introduced by output capacitance ESR. Solving for C2, C2 = ESR × C OUT RC For low ESR output capacitance, such as with a ceramic capacitor, C2 is optional. LAYOUT GUIDELINES When designing a high frequency, switching, regulated power supply, layout is very important. Using a good layout can solve many problems associated with these types of supplies. The main problems are loss of regulation at high output current and/or large input-to-output voltage differentials, excessive noise on the output and switch waveforms, and instability. Using the following guidelines helps minimize these problems. Make all power (high current) traces as short, direct, and thick as possible. It is good practice on a standard printed circuit board (PCB) to make the traces an absolute minimum of 15 mil (0.381 mm) per ampere. The inductor, output capacitors, and output diode should be as close to each other as possible. This helps reduce EMI radiated by the power traces that carry high switching currents. Close proximity of the components also reduces lead inductance and resistance, which in turn reduce noise spikes, ringing, and resistive losses that produce voltage errors. Rev. A | Page 13 of 16 ADD5207 Data Sheet The grounds of the IC, input capacitors, output capacitors, and output diode (if applicable), should be connected close together, and directly to a ground plane. It is also a good idea to have a ground plane on both sides of the PCB. This reduces noise by reducing ground loop errors and by absorbing more of the EMI radiated by the inductor. Use the following general guidelines when designing PCBs: For multilayer boards of more than two layers, a ground plane can be used to separate the power plane (power traces and components) and the signal plane (feedback, compensation, and components) for improved performance. On multilayer boards, the use of vias is required to connect traces and different planes. If a trace needs to conduct a significant amount of current from one plane to the other, it is good practice to use one standard via per 200 mA of current. Arrange the components so that the switching current loops curl in the same direction. • • Due to how switching regulators operate, there are two power states: one state when the switch is on, and one when the switch is off. During each state, there is a current loop made by the power components currently conducting. Place the power components so that the current loop is conducting in the same direction during each of the two states. This prevents magnetic field reversal caused by the traces between the two half cycles and reduces radiated EMI. • Layout Procedure To achieve high efficiency, good regulation, and stability, a good PCB layout is required. It is recommended that the reference board layout be followed as closely as possible because it is already optimized for high efficiency and low noise. • • • • • • Keep CIN close to the VIN and GND leads of the ADD5207. Keep the high current path from CIN (through L1) to the SW and GND leads as short as possible. Keep the high current path from CIN (through L1), D1, and COUT as short as possible. Keep high current traces as short and as wide as possible. Keep nodes connected to SW away from sensitive traces, such as COMP, to prevent coupling of the traces. If such traces must be run near each other, place a ground trace between the two as a shield. Place the compensation components as close as possible to the COMP pin. Place the LED current setting resistors as close as possible to each pin to prevent noise pickup. Avoid routing noise-sensitive traces near high current traces and components, especially the LED current setting node (ISET). Use a thermal pad size that is the same dimension as the exposed pad on the bottom of the package. Heat Sinking When using a surface-mount power IC or external power switches, the PCB can often be used as the heat sink. This is done by using the copper area of the PCB to transfer heat from the device. Users should maximize this area to optimize thermal performance. Rev. A | Page 14 of 16 Data Sheet ADD5207 TYPICAL APPLICATION CIRCUITS L1 10µH VIN + – D1 CIN 10µF VIN 1 CIN2 0.1µF OFF ON CBYPASS 1µF 14 13 SW OVP COUT 4µF ADD5207 9 PWM 10 SHDN VDD 8 FB1 4 FB2 5 FSLCT 2 RF 100kΩ FB3 6 FB4 7 ISET COMP 3 11 GND 12 RC 6.8kΩ CC 2.2nF C2 OPEN 08350-016 RSET 180kΩ Figure 16. Typical Four-String Application Circuit L1 10µH – CIN 10µF VIN 1 CIN2 0.1µF CBYPASS 1µF 13 OVP COUT 4µF ADD5207 PWM 9 OFF ON 14 SW 10 SHDN VDD 8 FSLCT 2 RF 100kΩ ISET COMP 3 11 RSET 180kΩ RC 6.8kΩ CC 2.2nF FB1 4 FB2 5 FB3 6 FB4 7 GND 12 C2 OPEN 08350-017 + D1 Figure 17. Typical Three-String Application Circuit Rev. A | Page 15 of 16 ADD5207 Data Sheet OUTLINE DIMENSIONS 3.40 3.30 3.15 4.00 BSC 0.20 MIN 8 14 1.80 1.70 1.55 EXPOSED PAD 7 0.50 0.40 0.30 TOP VIEW 0.80 0.75 0.70 SEATING PLANE 0.30 0.25 0.20 0.50 BSC 0.05 MAX 0.02 NOM COPLANARITY 0.08 0.15 REF 1 BOTTOM VIEW PIN 1 INDICATOR (R 0.20) FOR PROPER CONNECTION OF THE EXPOSED PAD, REFER TO THE PIN CONFIGURATION AND FUNCTION DESCRIPTIONS SECTION OF THIS DATA SHEET. COMPLIANT TO JEDEC STANDARDS MO-220-WGED 052509-A 3.00 BSC PIN 1 INDICATOR Figure 18. 14-Lead Lead Frame Chip Scale Package [LFCSP_WD] 4 mm × 3 mm Body, Very Very Thin Dual (CP-14-1) Dimensions shown in millimeters ORDERING GUIDE Model1 ADD5207ACPZ-RL 1 Temperature Range −25°C to +85°C Package Description 14-Lead LFCSP_WD Z = RoHS Compliant Part. ©2009–2012 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D08350-0-2/12(A) Rev. A | Page 16 of 16 Package Option CP-14-1