SC443 POWER MANAGEMENT Features High Efficiency Integrated Boost Driver for 3-Strings of 30mA LEDs Description Wide input voltage range from 4.5V to 27V 42V maximum operating output voltage Programmable LED current for up to 30mA per string +/- 1% typical string-to-string current matching Greater than 90% efficiency Wide 0.4% to 100% PWM dimming range Integrated 1A power switch Programmable switching frequency for small size Low current sense voltage for high efficiency Adjustable OVP for cost- ef fec tive output cap selection LED open circuit protection Thermal protection with auto-recovery 3mm × 3mm ×0.6mm MLP-UT-16 lead-free package (WEEE and RoHS compliant) The SC443 is a high-efficiency multiple string WLED driver with an integrated boost converter. It operates over a wide input range from 4.5V to 27V with a maximum output voltage of 42V and a 1A internal power switch. It can drive 3 strings with current up to 30mA per string. The string-to-string current matching is within, typically, 1% and the overall efficiency is greater than 90% due to the low current sense voltage and a low-impedance internal power switch. The wide PWM dimming range boasts a ratio of 250: 1. The programmable switching frequency enables the user to optimize the external component sizes for high efficiency. When there are fewer LEDs in each string, users can use a lower output voltage protection level which yields an allowable reduction in associated costs, size and voltage ratings of the output capacitor. Applications The SC443 also features an open circuit LED protection function. It disables the corresponding strings with LED open while keeping other strings under normal operation. This feature allows LCD panels to remain viewable even under LED failure and wire disconnectons. The internal thermal shutdown protects the IC from overheating at abnormal conditions. The SC443 is available in a 3mm × 3mm × 0.6mm MLP-UT 16 Lead-free package. Small to Medium Size LCD Panel Notebook Display White LED Power Supplies Sub-Notebook and Tablet Computer Displays Portable Media Players LCD Monitors Digital Video Cameras Typical Application Circuit VIN SW VIN VOUT OVPIN COMP SS SC443 FSET Enable Dimming January 21, 2009 IO1- 3 3 IOSET EN PWM AGND IOGND PGND www.semtech.com SC443 FSET AGND IO1 Ordering Information EN Pin Configuration 16 15 14 13 12 IOGND 2 11 IO2 SW 3 10 IO3 VIN 4 9 T PWM 5 6 7 Package SC443ULTRT(1)(2) 3 x 3 x 0.6mm MLP-UT 16 SC443EVB Evaluation Board Notes: (1) Available in tape and reel only. A reel contains 3,000 devices. (2) Available in lead-free package only. Device is WEEE and RoHS compliant. COMP 8 IOSET PGND TOP VIEW SS 1 OVPIN VOUT Device θJA = 38°C MLP-UT-16 Marking Information 5 4 3 2 1 D D C C B B © 2009 Semtech Corp. www.semtech.com SC443 Absolute Maximum Ratings Recommended Operating Conditions VIN Pin: Supply Voltage …………………………… -0.3 to 30V Supply Input Voltage ……………………………… 4.5 to 27V Maximum Output Power ………………………………… 4W Maximum Output Voltage…………………………………42V FSET, IOSET Voltage ………………………………… -0.3 to 2V Maximum LED Current ………………………………… 30mA SW, VOUT, IO1~IO3, OVPIN Voltage ……………… 0.3V to 45V SS, COMP Voltage …………………………………… -0.3 to 3V Thermal Information EN, PWM Voltage …………………………… -0.3 to VIN +0.3V Junction to Ambient(1) ……………………………… 38°C/W PGND to AGND and IOGND………………………… ± 0.3V Maximum Junction Temperature……………………… 150°C Peak IR Reflow Temperature …………………………. 260°C Storage Temperature ………………………… -65 to +150°C 2kV Lead Temperature (Soldering) 10 sec ………………… 260°C ESD Protection Level(2) ………………………………… Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. NOTES-: (1) Calculated from package in still air, mounted to 3” x 4.5”, 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards. (2) Tested according to JEDEC standard JESD22-A114-B. Electrical Characteristics Unless otherwise noted, VIN = 12V, -40°C < TA = TJ < 85°C, RIOSET= 4.02kΩ, RFSET =40.2 kΩ Parameter Symbol Conditions Under-Voltage Lockout Threshold UVLO-TH VIN rising UVLO Hysteresis UVLO-H Min Typ Max Units 4.2 4.45 V Input Supply VIN Quiescent Supply Current IIN-Q No switching VIN Supply Current in Shutdown IIN-S EN / PWM = low Switching Frequency(1) fS RFSET = 40.2 kΩ Switching Frequency Range(1) fS 200 Maximum Duty Cycle(1) DMAX 90 Minimum Duty Cycle(1) DMIN 250 mV 3 mA 1 µA 0.96 MHz 1200 kHz Oscillator Minimum On-Time 0.64 0.8 % 0 TON-MIN 200 % ns Internal Power Switch Switch Current Limit ISW Switch Saturation Voltage VSAT ISW = 500mA 200 400 mV Switch Leakage Current IS-LEAK VSW = 12V 0.01 1 µA © 2009 Semtech Corp. 1 A www.semtech.com SC443 Electrical Characteristics (continued) Unless otherwise noted, VIN = 12V, -40°C < TA = TJ < 85°C, RIOSET= 4.02kΩ, RFSET = 40.2kΩ. Parameter Symbol Conditions Min Typ Max Units Sourcing Current IO-H VCOMP = 0.5V, TJ = 25 °C 7 µA Sinking Current IO-L VCOMP = 2V, TJ = 25 °C 5 µA EN, PWM High Voltage VEN_H VIN = 4.5V to 27V EN, PWM Low Voltage VEN_L VIN = 4.5V to 27V IEN_LEAK VEN = VPWM = 0V to 5.0V Compensation Control Signals EN, PWM Leakage Current PWM Dimming Frequency(1) fPWM 2 V 0.01 50 0.4 V 1 µA 50k Hz PWM Dimming Minimum Duty Cycle(1) DMIN_PWM 200Hz 0.4 % PWM Dimming Minimum Pulse Width TPWM_MIN 200Hz 20 µs SS Source Current ISS_H Vss = 0V 5 SS Sink Current ISS_L Vss = 2V at OVP or OTP 1.5 SS Switching Threshold VSS_TH TJ = 25°C SS End Voltage VSS_END 0.6 0.8 µA 1 2.6 V V Over-Voltage Protection OVPIN Threshold Voltage VOVPIN_TH OVPIN Leakage Current IOVPIN_L VOUT Internal Pull-Down in Over-Voltage Fault VOUT Leakage Current 1.45 OVPIN = VIN = 27V IOVP IVOUT_L VOUT = 40V IO1~IO3 EN, PWM = 1; TJ = 25°C 1.51 1.57 V 0.1 1 µA 0.8 mA 0.1 µA Current Source (IO1 ~ IO3) Backlight Current Accuracy LED Current Matching Overshoot Protection Threshold 27.8 EN, PWM = 1; TJ = 25°C IO1~IO3 Maximum LED Current IO_MAX IO Off Leakage Current IO_LEAK 29.9 32 mA +/-1 +/-3 % 0.95 V 35 EN = 0V, VIO1 ~ VIO3 = VIN mA 0.1 1 µA Over-Temperature Protection Thermal Shutdown Temperature Thermal Shutdown Hysteresis TOTP 150 °C TOTP_H 10 °C Notes: (1) Guaranteed by design. © 2009 Semtech Corp. www.semtech.com Switching Frequency(12Vin) SC443 Typical Characteristics UVLO Threshold vs. Temperature 4.35 Switching Frequency vs. Temperature 880 4.30 Switching Frequency (kHz) Rising VIN UVLO (V) 4.25 4.20 4.15 4.10 Falling IPK(4.5Vin,21Vin) 4.05 820 790 760 Comp(12Vin) 730 700 670 640 4.00 -40 25 -40 105 25 Temperature (°C) Switch Current Limit vs. Temperature COMP Current vs. Temperature 8.5 27VIN 21VIN COMP Current (µA) 2500 2300 5VIN 12VIN 2100 OVPIN 1900 12Vin 8.0 1700 Sourcing 7.5 7.0 6.5 SS 6.0 5.5 5.0 Sinking 4.5 4.0 1500 - 40 25 105 -40 25 Temperature (°C) 105 Temperature (°C) OVPIN Threshold vs. Temperature SS Current vs. Temperature 1.53 6.0 1.52 12Vin Sourcing 5.0 1.51 SS Current (µA) OVPIN Threshold Voltage (V) 105 Temperature (°C) 2700 Switch Current Limit (A) RFSET=40.2k 850 4.5VIN 21VIN 1.50 1.49 27VIN 1.47 25 Temperature (°C) © 2009 Semtech Corp. 3.0 2.0 1.48 -40 4.0 105 Sinking 1.0 -40 25 105 Temperature (°C) www.semtech.com Vout Discharge SC443 Typical Characteristics (continued) Efficiency vs. Input Voltage VOUT Pull Down Current (mA) VOUT Pull Down Current vs. Temperature VOUT=33V, D1=SS14, L1=B1000AS-220M, Co=10uF 0.92 92 0.88 90 Efficiency (%) 0.84 4.5VIN 0.80 IOSET(AVG) 0.76 21VIN 88 86 84 82 27VIN 0.72 80 0.68 78 -40 25 105 5 7 9 11 13 Temperature (°C) LED Current Setting (mA) 17 19 21 23 25 27 Input Voltage (V) PWM Dimming Duty Cycle LED Current Setting vs. Temperature 33 15 Vin=12V, L= 15uH, Cout= 10uF, 10*LED,3string, Ta=25°C, Switching Frequency=800KHz RISET=4.02k 90 800KHz ,B1000AS220M(22uH) , SS14, 3X10 per channel 30mA 50Hz 200Hz 80 LED Current (mA) 32 31 30 29 50KHz 70 60 50 40 30 20 10 28 0 -40 25 105 0 10 20 30 40 50 60 70 80 90 100 Duty Cycle(%) Temperature (°C) Vin=12V, L= 15uH; Cout= 10uF, Load=10*LED(30mA),3string, Ta=25°C,SF=800KHz Switching frequency setting ˅˃˃ ˄ˌ˃ ˄ˋ˃ ˄ˊ˃ ˄ˉ˃ ˄ˈ˃ ˥˙˦˘˧ʻ˞ʳʼ RFSET(kohm) ˄ˇ˃ ˄ˆ˃ ˄˅˃ ˄˄˃ ˄˃˃ ˌ˃ ˋ˃ ˊ˃ ˉ˃ ˈ˃ ˇ˃ ˆ˃ ˅˃ ˃ˁ˅ ˃ˁˆ ˃ˁˇ ˃ˁˈ ˃ˁˉ ˃ˁˊ ˃ˁˋ ˃ˁˌ ˄ˁ˃ ˄ˁ˄ ˄ˁ˅ ˦̊˼̇˶˻˼́˺ʳ˙̅˸̄̈˸́˶̌ʻˠ˛̍ʼ Switching Frequency (MHz) Switching Frequency Vs. RFSET © 2009 Semtech Corp. www.semtech.com SC443 Typical Characteristics (continued) Test condition: 5Vin, 3*10 LED, Start UP by Vin 30mA@per channel, 25̓̓C Shutdown by Vin VOUT VOUT 1V/DIV VIN 0.5V/DIV SSSS 10V/DIV 10V/DIV VIN VIN 0.5V/DIV Comp Comp SS Comp VOUT 1V/DIV 0.5V/DIV 0.5V/DIV Time(5ms/DIV) Condition: VIN=5V, Load=30mA x3 string, 10LEDs per string Time(0.1ms/DIV) Condition: VIN=12V, Load=30mA x3 string, 10LEDs per string Start up and Shutdown by EN Start up and Shutdown by PWM VIN 10V/DIV 10V/DIV VIN 2V/DIV SS VOUT VOUT 6V/DIV 1V/DIV EN 1V/DIV SS 6V/DIV PWM 0.7V/DIV Time(0.1s/DIV) Condition: VIN=12V, Load=30mA x3 string, 10LEDs per string Time(0.1ms/DIV) Condition: VIN=12V, Load=30mA x3 string, 10LEDs per string Main Power Switching Waveform LED OPEN 50mV/DIV VOUT IL 5V/DIV VOUT 1V/DIV SS 200mA/DIV 5V/DIV 1V/DIV 10V/DIV IO3 SW Time(1us/DIV) Condition: VIN=4.5V, Load=30mA x3 string, 10LEDs per string © 2009 Semtech Corp. VIN Time(20ms/DIV) Condition: VIN=12V, Load=30mA x3 string, 10LEDs per string www.semtech.com OTP Adaptor Plug-inSC443 Typical Characteristics (continued) Adaptor Plug-in OTP 10V/DIV VOUT SS 1V/DIV VIN 10V/DIV 1V/DIV VOUT IO3 0.5V/DIV 0.5/DIV IL 20V/DIV SW 20V/DIV Time(5ms/DIV) Condition: VIN=12V, Load=30mA x3 string, 10LEDs per string SW Time(10us/DIV) Condition: VIN=6V~18V, Load=30mA x3 string, 10LEDs per string PWM Dimming 2V/DIV VOUT 5V/DIV PWM 1A/DIV IL 20V/DIV SW Time(0.1s/DIV) Condition: VIN=12V, Load=30mA x3 string, 10LEDs per string © 2009 Semtech Corp. www.semtech.com SC443 Pin Descriptions Pin # Pin Name Pin Function 1 VOUT Output voltage pin and internal pull down current source in over voltage fault. Connect the output capacitor and the anode of the LED strings to this pin. 2 PGND Power ground 3 SW Collector of the internal power transistor – connect to the boost inductor and the rectifying diode. 4 VIN Input voltage supply for IC. Bypassed with capacitors close to the pin. 5 PWM PWM control pin for LED backlight strings, Connect to GND to disable the IO’s. 6 OVPIN Over-voltage input 7 SS 8 IOSET Current source IO value set pin – by selecting the resistor connected from this pin to GND, the corresponding maximum current on all 3 strings are set. 9 COMP The output of the internal transconductance error amplifier – this pin is used for loop compensation. 10 IO3 Provides constant source current to LED string 3. 11 IO2 Provides constant source current to LED string 2. 12 IOGND 13 IO1 14 AGND Analog ground 15 FSET External resistor setting switching frequency 16 EN T Thermal Pad Soft-start pin LED ground Provides constant current source to LED string 1. Enable the device including requlator and LED drivers. Thermal pad for heatsinking purposes — connect to ground plane using multiple vias — not connected internally. Table 1. EN STATUS 0 backlight disable 1 backlight enable Note: When EN = 0; the boost is turned OFF and disabled. © 2009 Semtech Corp. www.semtech.com SC443 Block Diagram 5 4 3 2 FSET SW HICCUP OSC S Q R ILIM IO2 - IO3 + ONE IO CHANNEL SHOWN LED OPEN CIRCUIT PROTECTION IO1 + DISABLE1 PGND - + LED CURRENT SETTING ISENSE CURRENT SOURCE COMP SS I to V IOSET IOGND PWM VOUT OVP OVPIN 0.8mA OVP Detect Fault Fault 3V3 HICCUP CONTROL LOGIC VIN HICCUP UVLO 5uA TSD SS UVLO & TSD Bandgap EN HICCUP AGND 1.5uA Figure 1. SC443 Block Diagram 5 © 2009 Semtech Corp. 4 3 2 www.semtech.com 10 SC443 Applications Information SC443 Detailed Description The SC443 contains a high frequency, current-mode boost regulator and three string LED current sources. The LED current for all strings is programmed by an external resistor and the PWM controller operates to maintain the output voltage at a level which will keep the current of each string regulated. A typical application would use 3-8 backlight LEDs for each string, driven with approximately 30mA. Operation The SC443 regulates the boost converter output voltage based on instantaneous requirements of the three string current sources. Therefore, only a single inductor and power switch is needed to provide power to the entire lighting subsystem, increasing efficiency and reducing part count. A logic interface to output control circuit has high-bandwidth, and supports PWM dimming with 50Hz to 50kHz dimming frequency while the entire supply current is reduced to 3mA (typical) when all LED strings are off. High frequency switching provides high output power using a 1.0mm height inductor, maximizing efficiency for space-constrained and cost-sensitive applications. Additionally, the converter and output capacitor are protected from open-LED conditions by programmable over-voltage protection. LED Current Programming The SC443 is a LED current programmable regulator. The LED current set point is chosen using an external resistor connected to the IOSET pin. The relationship between the programming resistor value and the LED current set point of each string can be described as follows: ILED=120/RIOSET Where, RIOSET is in kΩ. ILED is the output current of each string in mA. © 2009 Semtech Corp. Start-Up During start-up, when the VIN pin voltage reaches its UVLO threshold and both EN and PWM signals are set to high, the SS pin begins to source 5µA to the SS capacitor and its voltage begins to rise from 0V to its end value (2.6V). The output voltage of the internal error amplifier (COMP) is increased and clamped by the SS pin voltage. When the SS pin voltage reaches its switching threshold 0.8V(typical), the SC443 starts to switch and the output voltage increases. Each internal LED current source (IO1 ~ IO3) tries to regulate the LED current to its set point. While the output voltage increases, a suitable amount of error information will be generated on the internal error amplifier as the COMP pin voltage keeps rising. Once each LED current reaches its set point, the error information is not generated by the LED current source. The COMP pin voltage stays at a level which keeps the LED current at its set point. If the EN pin voltage is pulled below 0.4V and VIN reaches its UVLO, SC443 will stay at shutdown mode, drawing less than 1μA from the input power supply. If the PWM pin voltage is pulled below 0.4V when the EN pin is pulled high and VIN reaches its UVLO, the SC443 runs in standby mode, drawing 3mA (typical) from the input power supply. Under this condition, soft-start is initiated and the SS pin voltage is raised to its end value since the EN pin is pulled high. After that, when PWM signal goes high to enable the device, the COMP pin voltage will rise as quickly as it can since it is not being limited by the SS pin. A proper capacitance (10nF ~ 100nF) is required for the COMP pin and its external RC network in order to prevent output voltage overshoot. Shut Down When the VIN pin voltage falls below its UVLO or EN pin voltage goes low while SC443 is at normal operation, SC443 will run in shutdown mode. The internal switch and LED current sources will be immediately turned off. The SS capacitor is discharged by SS pin internal current source and the SS pin voltage decreases to 0V. The output voltage falls to the same level as the input voltage. www.semtech.com 11 SC443 Applications Information (continued) If PWM pin voltage goes low while the SC443 is at normal operation, the device will run in standby mode. The internal switcher and the LED current sources will be immediately turned off. The SS pin will not be affected by the PWM signal and remains at its final value. Main Power Stage Operation SC443 is a programmable frequency, peak current-mode boost switching regulator with an integrated 1A (typical), power transistor. The switching frequency is programmable at the FSET pin. Referring to the Block Diagram on Page 10, the clock from the oscillation section resets the latch and turns on the power transistor. Switch current is sensed with an integrated sense resistor. The sensed current is summed with the slope-compensating ramp and fed into the modulating ramp input of the PWM comparator. The latch is set and the power transistor conduction is terminated when the modulating ramp intersects the error amplifier output (COMP). The current-mode switching regulator is a dual-loop feedback control system. In the inner current loop, the EA output (COMP) controls the peak inductor current. In the outer loop, the error amplifier regulates the output voltage to keep the LED current in set point. The double reactive poles of the output LC filter are reduced to a single real pole by the inner current loop, allowing the simple loop compensation network to accommodate a wide range of input and output voltages. Over-Current Protection If the switch current exceeds 1A (the minimum currentlimit trip point), the current-limit comparator, ILIM will set the latch and immediately turn off the internal power switch. Due to separate pulse-width modulating and current limiting paths, the OCP trip point is not affected by slope compensation (i.e., trip point is not affected by switching duty cycle). Over-Voltage Protection (OVP) The SC443 includes an external programming over-voltage protection circuit to prevent damage to the IC and output capacitor in the event of an open-circuit condition. The output voltage of the boost converter is detected at © 2009 Semtech Corp. the OVPIN pin. If the voltage at the OVPIN pin exceeds 1.51V, the boost converter will shut off and a 0.8mA pull down current source will be applied to the VOUT pin to quickly discharge the over-voltage capacitor. This additional level of protection prevents a condition where the output capacitor and Schottky diode must endure high voltage for an extended period of time, which can pose a reliability risk for the system. The total resistance of the divider for the OVP protection should be more than 200kΩ. The output over-voltage trip point can be programmed by R2 and R4 resistor divider (see the schematic on page 16). The relationship can be described as follows: OVP_TRIP = VOVPIN_TH * R2 + R4 R4 Where OVPIN_TH is 1.51V typical. An OVP event causes a fault which disables the boost converter and enables the strong pull down, the soft-start capacitor is discharged. When the soft-start capacitor voltage falls below 0.5V and VOUT falls below 1V above VIN, the SC443 enters a soft-start process. The OVPIN pin is sensitive to noise and a proper decoupling cap, (1nF ~ 10nF) is required. LED Open-Circuit Protection If any LED is detected as open circuit, that string IO pinvoltage will be pulled low and less than 0.2V. When LED is opened, the output current is decreased at once ,but the COMP pin voltage can’t be pulled low at the same time, the boost converter duty cycle will be maintained causing VOUT to rise. Because of the open string, VOUT will continue to rise until it reaches the programmed OVP level. When OVP is reached, the voltages on the IO pins are monitored. If any IO voltage is less than 0.2V, that string will be identified as open and will be latched off. Only VIN falling below UVLO, recycle EN signal, and thermal shutdown will reset this latch. When a hiccup cycle is initiated the SS is discharged slowly with a 1.5μA cur- www.semtech.com 12 SC443 Applications Information (continued) rent source and a 1mA discharge path is turned on to pull down VOUT. When SS falls below 0.5V and VOUT falls below 1V above VIN, the OVP detection latches are reset and a new soft-start sequence is initiated to resume normal operation. millisecond or two, and in-rush current is reduced to less than 50mA. This dim time will vary based on the number of LEDs and the size of the output capacitor, but can be easily determined on the bench and programmed into the µC firmware. Thermal Shutdown (TSD) If the thermal shutdown temperature of 150°C is reached, a hiccup sequence is initiated where the boost converter and all IO current sources are turned off. SS is discharged by a 1.5µA current source, and a 1mA discharge path is turned on to pull down VOUT. As temperature falls below TSD trip point, the SC443 will retry once SS falls below 0.5V and VOUT falls below 1V above VIN. Parallel Operation When two or more SC443s are operating in parallel for a large-sized panel application, audible noise may be observed due to a non-synchronous switching frequency. The ripple voltage on the input voltage rail will be modulated by the beat frequency resulting in audible noise. This situation can be resolved by adding an input inductor between the input voltage rail and the VIN pin and can also be improved by adding more input decoupling capacitors. PWM Dimming The PWM input needs to be held high for normal operation. PWM dimming can be achieved by cycling the PWM input at a given frequency where a “low” on the PWM input turns off all IO current sources and a “high” turns on all IO current sources. The short and open detection latches are blanked for approximately 2µs as the PWM input transitions from low-to-high to prevent false fault detection during PWM dimming. The PWM pin can be toggled by external circuitry to allow PWM dimming. In a typical application, a microcontroller sets a register, or counter, that varies the pulse width on a GPIO pin. The SC443 allows dimming over two decades in frequency (50Hz-50kHz) in order to allow compatibility with a wide range of devices including the newest dimming strategies that avoid the audio band by using high frequency PWM dimming. In this manner, a wide range of illumination can be generated while keeping the instantaneous LED current at its peak value for high efficiency and color temperature. Furthermore, advanced lighting effects such as backlight dim-on can be implemented as the SC443 can resolve PWM from 10% to 90% duty at its highest frequency. Additionally, PWM dimming offers customers the ability to reduce in-rush current to the output capacitor. Simply apply the PWM signal to the device at 10% duty for a © 2009 Semtech Corp. Inductor Selection The inductance value of the inductor affects the converter’s steady state operation, transient response, and its loop stability. Special attention needs to be paid to three specifications of the inductor, its value, its DC resistance and saturation current. The inductor’s inductance value also determines the inductor ripple current. The converter can operate in either CCM or DCM depending on its working conditions. The inductor DC current or input current can be calculated as, ,,1 9287 ,287 9,1 Ș IIN - Input current; IOUT – Output current; VOUT – Boost output voltage; VIN – Input voltage; η – Efficiency of the boost converter. Then the boundary condition for CCM and DCM is, ' 9287 9,1 9' 9287 9' VD = Forward conduction drop of the output rectifying diode. www.semtech.com 13 SC443 Applications Information (continued) When the boost converter runs in DCM ( L < Lboundary), it takes advantage of small inductance and quick transient response while avoiding the bandwidth-limiting instability of the RHP zero found in CCM boost converters. The inductor peak current is, / / SHDN 9,1 ' )6 / The converter will work in CCM if L > Lboundary. Generally the converter has higher efficiency under CCM and the inductor peak current is, ,/ SHDN ,,1 9,1 ' )6 / For many applications an inductor with a value of 4.7µH to 22µH should be fine. The inductor peak current must be less than its saturation rating. When the inductor current is close to the saturation level, its inductance can decrease 20% to 35% from the 0A value depending on the vendor specifications. Using a small value inductor forces the converter into DCM, in which case the inductor current ramps down to zero before the end of each switching cycle. It also reduces the boost converter’s maximum output current and produces large input voltage ripple. An inductor with larger inductance will reduce the bandwidth of the feedback loop and possibly higher DC resistance (DCR). Inductor’s DCR plays a significant role for the total efficiency since the power transistor is integrated inside the SC443. Of course, there is a trade-off between the DCR and inductor size. Table 2 lists recommended inductors and their vendors. Table 2. Recommended Inductors Inductor DR74, 4.7μH ~ 15μH IHLP-2525CZ-01, 4.7μ ~ 10μH DS85LC, 6.8μH ~ 10μH © 2009 Semtech Corp. Website www.cooperet.com www.vishay.com www.tokoam.com Output Capacitor Selection The next task in design is targeting the proper amount of ripple voltage due to the constant-current LED loads. The two error amplifiers that control the PWM converter sense the delta between requested current and actual current in each output current regulator. On a cycleby-cycle basis, a small amount of output ripple ensures good sensing and tight regulation, while the output current regulators keep each LED current at a fixed value. Overall, this allows usage of small output caps while ensuring precision LED current regulation. Although the mechanics of regulation and frequency dependence may be complex, actual selection of the output capacitor can be simplified because this capacitor is mainly selected for the output ripple of the converter. Assume a ceramic capacitor is used. The minimum capacitance needed for a given ripple can be estimated by, C OUT (VOUT VIN ) x IOUT VOUT FS VRIPPLE VRIPPLE – Peak-to-peak output ripple; IOUT – Output current; VOUT – Boost output voltage; VIN – Input voltage; FS – Switching frequency. During load transient, the output capacitor supplies or absorbs additional current before the inductor current reaches its steady state value. Larger capacitance helps with the overshoot and undershoots during load transient, and loop stability. Recommended ceramic capacitor manufacturers are listed in Table 3. Table 3. Recommended Ceramic Capacitor Manufacturers Vendor Phone Website Kemet 408-986-0424 www.kemet.com Murata 814-237-1431 www.murata.com Taiyo Yuden 408-573-4150 www.t-yuden.com www.semtech.com 14 SC443 Applications Information (continued) Output Rectifying Diode Selection Schottky diodes are the ideal choice for SC443 due to their low forward voltage drop and fast switching speed. Table 4 shows several different Schottky diodes that work well with the SC443. Make sure that the diode has a voltage rating greater that the possible maximum ouput voltage. The diode conducts current only when the power switch is turned off. A diode of 1A will be sufficient for most designs. Since there is pad at the bottom of the SC443 for heat dissipation, a copper area right underneath the pad is used for better heat spreading. On the bottom layer of the board another copper area connected through vias to the top layer, is used for better thermal performance. The pad at the bottom of the SC443 should be tied to the analog ground. The analog ground should be connected to the power ground at one point for better noise immunity. Layout Guidelines The SC443 contains a boost converter and the placements of the power components outside. The SC443 should follow the layout guidelines of a general boost converter. The application circuit on page 16 will be used as an example. The layout illustration diagram is shown on page 16. R5 and C7 form a decoupling filter for the SC443. C7 should be placed as close as possible to the VIN and PGND to achieve the best performance. C6 is the input power filtering capacitor for the boost converter power train. L1 is the boost converter input inductor. D1 is the output rectifying diode and it is recommended that a Schottky diode be used for fast reverse recovery. To minimize switching noise for the boost converter, the output capacitor, C3, should be placed at the bottom, as displayed on page 17, so that the loop formed by C6, D1, and the internal switch, is the smallest. The output of the boost converter is used to power up the LEDs. R6 and C9 form the compensation network for the boost converter. C9 should return to analog ground. Table 4. Recommended Rectifying Diodes Part Vendor SS13 SS14 Vishay www.vishay.com C8 determines the soft-start time and should be connected to analog ground. R8 is the output current programming resistor for IO1 through IO3 and should return to analog ground. IOGND should also be connected to AGND. © 2009 Semtech Corp. www.semtech.com 15 SC443 5 4 3 2 1 Application Schematic D1 L1 22uH C6 D 2.2uF 10uF 1R C7 SS 8 I0SET IO2 11 R8 4.02K IO3 SC443 LED1 255K R3 R4 20K 10K LED11 LED21 CR4 1nF 1 17 EN 16 FSET 15 VOUT 3 U1 PAD AGND 14 IO1 13 IOGND 7 PGND OVPIN 9 C8 68nF 6 10 C PWM COMP 10R 5 SW 4 VIN 20K R10 2 2.2uF R9 D R2 VIN OPT R5 PWM C3 EN C R18 40.2K LED10 LED20 LED30 12 VIN D2 R12 R6 1R 1.5K B R13 B 1R C9 R14 22nF 1R Evaluation Board Bill of Materials A Item Quantity Reference 1 1 CR4 2 1 C3 5 4 3 3 2 C6, C7 Title Size A Date: A Part <Title> Document Number <Doc> 1nF 10uF, 50V, 1210 Tuesday, September 16, 2008 2 Sheet 2.2uF, 50V Rev <RevCod 1 1 of 4 1 C8 68nF 5 1 C9 22nF 6 1 D1 SS14 7 1 L1 22μH, TOKO B1000AS-220M 8 4 R5,R12,R13,R14 1R 9 1 R2 255K 10 1 R3, R9 20K 11 1 R4 10K 12 1 R6 1.5K 13 1 R8 4.02K 14 1 R10 10R 15 1 R18 40.2K 16 1 U1 SC443 17 30 LED1~30 SML-LX0603UWD © 2009 Semtech Corp. 1 www.semtech.com 16 SC443 Layout Illustration Diagrams 3. TOP PCB TOP 5. Components on TOP TOP COMPONENT © 2009 Semtech Corp. 4. BOTTOM PCB BOTTOM 6. Components on BOTTOM COMPONENT BOTTOM www.semtech.com 17 SC443 Outline Drawing - MLP-UT16 A D DIMENSIONS INCHES MILLIMETERS DIM MIN NOM MAX MIN NOM MAX B PIN 1 INDICATOR (LASER MARK) E A2 A aaa C C A1 SEATING PLANE A A1 A2 b D D1 E E1 e L N aaa bbb .024 .002 (.006) .007 .009 .012 .114 .118 .122 .061 .067 .071 .114 .118 .122 .061 .067 .071 .020 BSC .012 .016 .020 16 .003 .004 .020 .000 0.60 0.05 (0.152) 0.18 0.23 0.30 2.90 3.00 3.10 1.55 1.70 1.80 2.90 3.00 3.10 1.55 1.70 1.80 0.50 BSC 0.30 0.40 0.50 16 0.08 0.10 0.50 0.00 D1 e/2 LxN E/2 E1 2 1 N e D/2 bxN bbb C A B NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS. 3. DAP IS 1.90 x 1.90mm. © 2009 Semtech Corporation © 2009 Semtech Corp. www.semtech.com 18 SC443 Land Pattern - MLPQ-UT16 H R DIM (C) K G Y X P Z C G H K P R X Y Z DIMENSIONS INCHES MILLIMETERS (.114) .083 .067 .067 .020 .006 .012 .031 .146 (2.90) 2.10 1.70 1.70 0.50 0.15 0.30 0.80 3.70 NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. 3. THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD SHALL BE CONNECTED TO A SYSTEM GROUND PLANE. FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR FUNCTIONAL PERFORMANCE OF THE DEVICE. Contact Information Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805) 498-2111 Fax: (805) 498-3804 www.semtech.com © 2009 Semtech Corp. www.semtech.com 19