SC441A High Efficiency Integrated Driver for 4-Strings of 150mA LEDs POWER MANAGEMENT Features Description Wide input voltage range from 4.5V to 21V 36V maximum output voltage Drives up to 40 WLEDs in 4 strings Programmable LED current for up to 150mA per string +/- 2% string-to-string current matching Up to 90% efficiency Wide 0.2% to 100% PWM dimming range Possible analog dimming Integrated 2.5A power switch 700KHz switching frequency for small size Adjustable OVP for cost-effective output cap selection Open/short LED protection Thermal protection with auto-recovery Thermally enhanced TSSOP-20 EDP package Fully WEEE and RoHS compliant The SC441A is a high-efficiency multiple string WLED driver with an integrated boost converter. It operates over a wide input range from 4.5V to 21V with a maximum output voltage of 36V and a 2.5A internal power switch. It can drive up to forty WLEDs in 4 strings with current up to 150mA per string. The string-to-string current matching is 2% typical, 3% maximum. 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 500: 1. The 700KHz switching frequency enables the user to optimize the external component sizes for efficiency. When there are fewer than 10 LEDs in each string, users can adjust the output voltage protection yielding an allowable reduction in associated costs, size and voltage ratings of the output capacitor. The SC441A also features comprehensive open and shortcircuit LED protection functions. It disables the corresponding strings with LED open or LED short conditions while maintaining normal operation of other, unaffected LED strings. This feature allows LCD panels to remain viewable even under LED failure, wire disconnect, or short-circuit conditions. The internal thermal shutdown protects the IC from overheating at abnormal conditions. Applications Medium-sized LCD panel Notebook Display Automotive Car Navigation Display Sub-Notebook and Tablet Computer Displays Portable Media Players The SC441A is available in a thermally-enhanced TSSOP20 EDP package. Typical Application Circuit Vin (4.5V -21V) 1 2 VIN SW SW VOUT EN 4 Strings OVPIN OVPIN FFLAG PWM OVPRTN SC441A COMP IO1-4 IO14 4 10 IOSET IOSET AGND AGND IOGND SS EDP July 23, 2009 PGND PGND PGND www.semtech.com SC441A Pin Configuration 5 IO2 D Ordering Information 4 3 1 20 IO1 IO3 AGND SW SS SW PGND EN OVPIN Package SC441ATETRT (1,2) TSSOP-20 EDP SC441AEVB Evaluation Board Notes: (1) Available in tape and reel only. A reel contains 2,500 devices. (2) Available in lead-free package only. Device is WEEE and RoHS C compliant. OVPRTN IOSET VIN PWM VOUT Device D PGND COMP B 1 IO4 IOGND C 2 10 11 B FFLAG θJA = 39º C/W (TSSOP-20 EDP) A 5 4 3 A 2 1 Marking Information SC441A : Part Number yyww = Date (Example: 0952) xxxxxx = Semtech Lot# (Example: A94A01) © 2009 Semtech Corporation www.semtech.com SC441A Absolute Maximum Ratings Recommended Operating Conditions VIN Pin: Supply Voltage …………………………… -0.3 to 25V Input Voltage Range ……………………………… 4.5V~21V Maximum Output Power ……………………………… 22W Output Voltage …………………………………… Up to 36V SW, OVPIN, OVPRTN, VOUT, IO1~IO4 Voltage …… -0.3 to 40V LED Current …………………………………… Up to 150mA IOSET Voltage ……………………………………… -0.3 to 2V SS, COMP Voltage ………………………………… Thermal Information -0.3 to 4V EN, PWM, FFLAG Voltage …………………… -0.3 to VIN +0.3V Junction to Ambient(1) ……………………………… 39°C/W PGND to AGND ……………………………………… ± 0.3V Maximum Junction Temperature ……………………… 150°C Peak IR Reflow Temperature …………………………… 260°C Storage Temperature ………………………… -65 to +150°C ESD Protection Level ……………………………… Lead Temperature (Soldering) 10 sec ………………… 260°C (2) 2.5kV 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 specified: VIN =12V, -40°C < TA = TJ < 105°C, RIOSET=1.74kW. Parameter Symbol Conditions Under-Voltage Lockout Threshold UVLO-TH VIN rising UVLO Hysteresis UVLO-H Min Typ Max Units 4.3 4.45 V Input Supply VIN Quiescent Supply Current IIN_Q No switching VIN Supply Current in Shutdown IIN_S EN / PWM = low 250 mV 3 mA 1 µA 0.84 MHz Oscillator FS 0.56 0.7 Switch Current Limit ISW 2.5 3.32 Switch Saturation Voltage VSAT Switching Frequency Internal Power Switcher Switch Leakage Current IS_LEAK Maximum Duty Cycle DMAX Minimum Duty Cycle DMIN Minimum On-Time (1) ISW = 1A 88 A 200 350 mV 0.1 1 µA 93 % 0 TON_MIN % 100 ns Compensation Sourcing Current IEA_SOURCE VCOMP = 0.5V 5 µA Sinking Current IEA_SINK VCOMP = 2V 6 µA © 2009 Semtech Corporation www.semtech.com SC441A Electrical Characteristics (continued) Parameter Symbol Conditions Min Typ Max Units Control Signals EN, PWM High Voltage VEN_H, VPWM_H EN, PWM Low Voltage VEN_L, VPWM_L IEN, IPWM EN, PWM Leakage Current VEN, VPWM = 5V FDimming PWM Dimming Frequency (1) PWM Dimming Minimum Duty Cycle PWM Dimming Minimum Pulse-Width 2 DMIN_Dimming (2) V 0.1 50 FDimming = 200Hz TMIN_Dimming TMIN_Off FFLAG Voltage VFFLAG IFFLAG = 2 mA 0.25 ISS_SOURCE VSS = 0V 4.5 ISS_SINK VSS = 2V at OVP or OTP 1 VSS_Switching VIN = 12V, TJ = 25 °C VSS_END VIN = 12V VOVPIN_TH OVPIN - AGND IOVPIN OVPRTN Saturation Voltage OVPRTN Leakage Current SS Sink Current SS Switching Threshold SS End Value V 1 µA 50k Hz 0.2 PWM Dimming Minimum Off Time SS Source Current 0.4 200 0.5 0.7 % 5 µs 300 ns V µA 0.85 2.5 V V Over-Voltage Protection OVPIN Threshold OVPIN Leakage Current VOUT Internal Pull-down Current Source VOUT Leakage Current 1.43 1.52 1.58 V VOVPIN = 20V 0.1 1 µA VOVPRTN IOVPRTN = 100 µA 60 mV IOVPRTN VOVPRTN = 20V 0.1 µA IOVP VOUT = VIN + 3V 0.9 mA IVOUT_Leak VOUT = 40V 0.1 µA IO1~IO4 TJ = 25 °C Current Source (IO1 ~ IO4) Current Accuracy 140 TJ = 25 °C Current Matching Maximum LED Current IOMAX LED Short-Circuit Protection VIO_SCP TJ = 25 °C, VIO1 ~ VIO4 Leakage Current IIO_LEAK EN = 0, VIO1 ~ VIO4 = VIN 150 160 mA +/- 2 +/-3 % 200 TJ = 25 °C, VIO1 ~ VIO4 2.2 mA 2.35 2.55 V 0.1 1 µA 0.963 1.07 1.177 V 0.9095 1.07 1.2305 V Overshoot Protection Threshold VIO1~VIO4 Overshoot Protection Hysteresis Any of IO1~IO4 100 mV TOTP 150 °C TOTP_H 30 °C Over-Temperature Protection Thermal Shutdown Temperature Thermal Shutdown Hysteresis Notes: (1) Guaranteed by design. (2) For achievable PWM dimming minimum pulse-width in applications, see the corresponding curves in Typical Characteristics. © 2009 Semtech Corporation www.semtech.com SC441A Typical Characteristics UVLO Threshold vs. Temperature 4.36 3.20 247 246 SS SINK/SOURCE 245 UVLO Hysteresis (mV) 4.34 4.33 4.32 4.31 VIN Quiescent Supply Current (mA) Comp SINK SOURCE 4.35 VIN UVLO (V) VIN Quiescent Supply Current vs. Temperature UVLO Hysteresis vs. Temperature 244 243 242 241 240 239 4.30 3.10 3.00 2.90 2.80 2.70 2.60 238 4.29 2.50 237 25 105 -40 25 Temperature (oC) 175 3.0 5.50 2.0 5.00 SOURCE SINK 1.0 4.50 165 VIN = 4.5V 160 155 VIN = 12V 150 VIN = 21V 145 140 4.00 0.0 -40 25 105 -40 Temperature ( C) 2.410 0.91 2.405 0.87 0.85 0.83 VIN = 21V 0.79 VOUT = VIN + 3V Average LED Current Source Setting vs. Temperature 150.7 VIN = 12V 150.6 2.400 2.395 2.390 2.385 2.380 2.375 2.370 25 Temperature (oC) © 2009 Semtech Corporation 105 150.5 150.4 150.3 150.2 150.1 150.0 149.9 VIN = 12V RIOSET = 1.78kW 149.8 149.7 2.360 -40 105 Temperature ( C) 2.365 0.75 25 o LED Current Source Setting (mA) LED Current Source SCP Threshold (V) VIN = 4.5V 0.77 -40 LED Current Source SCP Threshold vs. Temperature 0.93 0.81 135 105 o VOUT Pull Down Current Source vs. Temperature 0.89 25 Temperature ( C) o VOUT Pull down Current Source (mA) 170 SOURCE 4.0 6.00 SOURCE SW Saturation Voltage at 1A vs. Temperature VIN = 12V 5.0 SINK 6.50 SINK 105 Temperature (oC) SS SINK / SOURCE Current vs. Temperature 6.0 VIN = 12V 7.00 25 Temperature (oC) COMP SINK / SOURCE Current vs. Temperature 7.50 -40 105 SW Saturation Voltage at 1A (mV) -40 -40 25 Temperature (oC) 105 -40 25 105 Temperature (oC) www.semtech.com SC441A Typical Characteristics (continued) OVPRTN Saturation Voltage vs. Temperature LED Current Source Saturation Voltage vs. LED Current IOVPRTN = 100uA 70 700 600 500 400 300 200 1.56 65 1.54 60 1.52 55 1.50 50 1.48 45 100 0 20 40 60 80 100 120 140 160 -40 25 LED Current (mA) -40 105 PWM Dimming Minimum Pulse Width(uS) 94 IFFLAG = 2mA VIN = 21V 92 90 Efficiency (%) 220 200 180 86 160 84 140 82 120 80 25 105 VIN = 12V 88 VIN = 5V 80 160 240 320 400 480 560 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 600 0.2 PWM Dimming Minimum Pulse Width vs. PWM Dimming Frequency 4.0 3.5 3.0 2.5 2.0 0.5 1 2 5 10 20 30 40 50 20 18 16 14 12 10 8 6 4 2 10 20 30 40 50 12 10 8 6 4 2 0 0 0.2 0.5 1 2 5 10 20 30 40 50 PWM Dimming Frequency (kHz) PWM Dimming Frequency (kHz) Condition: VIN = 12V, VOUT = 30V / 150mA x 4 strings Condition: VIN = 12V, VOUT = 30V / 20mA x 4 strings © 2009 Semtech Corporation 5 14 PWM Dimming Minimum Duty Cycle(%) PWM Dimming Minimum Duty Cycle(%) 4.5 2 PWM Dimming Minimum Duty Cycle vs. PWM Dimming Frequency 22 5.0 1 PWM Dimming Frequency (kHz) PWM Dimming Minimum Duty Cycle vs. PWM Dimming Frequency 5.5 0.5 Condition: VIN = 12V, VOUT = 30V / 20mA x 4 strings Condition: VOUT = 36V PWM Dimming Minimum Pulse Width(uS) 8.5 Boost Section Output Current (mA) Temperature (oC) 105 PWM Dimming Minimum Pulse Width vs. PWM Dimming Frequency Efficiency ( PBOOST_OUTPUT / PINPUT ) 260 -40 25 Temperature (oC) Temperature (oC) FFLAG Saturation Voltage vs. Temperature 240 VIN = 12V 1.46 40 10 FFLAG Saturation Voltage (mV) 1.58 75 TA = 25 oC 800 OVPRTN Saturation Voltage (V) LED Current Source Saturation Voltage (mV) 900 OVPIN Threshold Voltage vs. Temperature 0.5 1 2 5 10 20 30 40 50 PWM Dimming Frequency (kHz) Condition: VIN = 12V, VOUT = 30V / 150mA x 4 strings www.semtech.com 150mA SC441A 10mA Typical Characteristics (continued) 10mA PWM Dimming Minimum Pulse Width vs. PWM Dimming Frequency ˄ˉ PWM Dimming Minimum Pulse Width(uS) PWM Dimming Minimum Pulse Width(uS) PWM Dimming Minimum Pulse Width vs. PWM Dimming Frequency ˄ˈ ˄ˇ ˄ˆ ˄˅ ˄˄ ˄˃ ˌ ˋ ˊ ˃ˁ˄ ˃ˁ˅ ˃ˁˈ ˄ ˅ ˈ ˄˃ ˅˃ ˆ˃ ˇ˃ ˈ˃ PWM Dimming Frequency (kHz) Condition: VIN = 12V, VOUT = 34.5V / 10mA x 4 strings © 2009 Semtech Corporation ˄˄ ˄˃ ˌ ˋ ˊ ˉ ˈ ˇ ˆ ˅ ˄ ˃ˁ˄ ˃ˁ˅ ˃ˁˈ ˄ ˅ ˈ ˄˃ ˅˃ ˆ˃ ˇ˃ ˈ˃ PWM Dimming Frequency (kHz) Condition: VIN = 12V, VOUT = 34.5V / 150mA x 4 strings www.semtech.com SC441A Typical Characteristics (continued) Start up Shut Down VIN (5V/DIV) VIN (10V/DIV) VSS (2V/DIV) VSS (2V/DIV) VSW (20V/DIV) VSW (20V/DIV) VOUT (20V/DIV) VOUT (20V/DIV) Time (20mS/DIV) Time (200mS/DIV) Conditions: VIN = 19V, Output = 30V / 150mA x 4 LED strings Conditions: VIN = 12V, Output = 27V / 20mA x 4 LED strings Main Power Switching Waveform VSW (20V/DIV) Main Power Switching Waveform VSW (10V/DIV) VOUT Accoupling (200mV/DIV) VOUT Accoupling (100mV/DIV) Time (1uS/DIV) Time (400nS/DIV) Conditions: VIN = 12V, Output = 36V / 150mA x 4 LED strings Conditions: VIN = 12V, Output = 27V / 20mA x 4 LED strings © 2009 Semtech Corporation www.semtech.com SC441A Typical Characteristics (continued) LED Short Circuit Protection OTP and OTP Recovery VIN = 12V VIN (10V/DIV) VSS (2V/DIV) VOUT (20V/DIV) VSW (20V/DIV) IO1 (5V/DIV) VOUT (20V/DIV) IO2 (1V/DIV) Time (10mS/DIV) Time (40mS/DIV) Conditions: VIN = 12V, Output = 36V / 150mA x 4 LED strings Conditions: VIN = 12V, IO1 has one LED short circuit, Output = 36V / 150mA x 4 LED strings LED Open Circuit Protection PWM Dimming VIN = 12V VIN (5V/DIV) PWM (2V/DIV) VOUT (20V/DIV) SW (20V/DIV) IO1 (10V/DIV) IO2 (1V/DIV) IO1 (5V/DIV) Time (10mS/DIV) Conditions: VIN = 12V, IO2 LED String is open circuit, Output = 36V / 150mA x 3 LED strings © 2009 Semtech Corporation Time (10mS/DIV) Conditions: VIN = 12V, Output = 36V / 50mA x 4 LED strings www.semtech.com SC441A Pin Descriptions Pin # Pin Name 1 IO2 Provides constant current source to LED string 2. 2 IO1 Provides constant current source to LED string 1. 3 IOGND Constant current source ground. 4 AGND Analog ground 5 SS 6 COMP 7 EN 8 IOSET Current source IO value set pin. By selecting the resistor connected from this pin to GND, the corresponding maximum current on all 4 strings are set. 9 PWM PWM dimming control pin for LED strings. 10 VOUT Internal pull down current source in over voltage fault. Connect this pin directly to Boost output. 11 FFLAG Power failure signal output with open collector. Held low under normal operation. 12 VIN 13 OVPRTN 14 OVPIN Over-Voltage Protection sense signal input. 15,18 PGND Power ground 16,17 SW Collector of the internal power switch. 19 IO4 Provides constant current source to LED string 4. Connect to VIN for 3 strings operation. 20 IO3 Provides constant current source to LED string 3. EDP Pin Function Soft-start pin The output of the internal trans-conductance error amplifier. Enable the device including regulator and LED drivers. Power input voltage pin. Bypassed with capacitors close to the pin. Over-Voltage Protection sense signal return path pin. Solder to the ground plane of the PCB. Note: Any unused IO pin should be pulled up to VIN. EN STATUS 0 backlight disable 1 backlight enable Note: When EN = 0; the boost is turned OFF and disabled. © 2009 Semtech Corporation www.semtech.com 10 SC441A 5 4 3 2 Block Diagram FFLAG SW HICCUP DISABLE2 OSC SW S Q R FAULT-1 + LED OPEN/SHORT/OVERSHOOT CIRCUIT PROTECTION ILIM - ONE IO CHANNEL SHOWN ISENSE PGND - DISABLE1 + IO1 PGND + CURRENT SOURCE COMP - LED CURRENT SETTING SS IOSET IOGND PWM VOUT OVP OVPIN OVP Detect OVPRTN Fault Fault HICCUP 0.9mA 3V3 CONTROL LOGIC VIN HICCUP UVLO 4.5uA TSD SS UVLO & TSD EN Bandgap HICCUP AGND 1uA Figure 1. SC441A Block Diagram 5 © 2009 Semtech Corporation 4 3 2 www.semtech.com 11 SC441A Applications Information SC441A Detailed Description The SC441A contains a high frequency, current-mode boost regulator and four programmable current sources. The LED current source value is set using an external resistor while the PWM controller maintains the output voltage at a level keeping the current regulated through the LEDs. Since the SC441A receives feedback from all of the LED current sources, all LED strings can be turned on at any given time. A typical application would use 3-10 backlight LEDs for each string, driven up to 150mA. Operation The SC441A controls the boost converter set point based on instantaneous requirements of four 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 digital interface to output control is high-bandwidth, supporting digital PWM dimming at 50Hz to 50kHz dimming frequency, while aggressively shutting the entire supply current down to 3mA (typical), when all LED strings are turned off. High frequency switching provides high output power using a tiny 1.0mm high inductor, maximizing efficiency for space-constrained and cost-sensitive applications. Additionally, both converter and output capacitor are protected from open-LED conditions by over-voltage protection. LED Current Programming The SC441A features programmable LED current regulators. The LED current set points are chosen using external resistors tied to the IOSET pin. The relationship between the programming resistor value and the output current set point can be described as follows: RIOSET = (0.261V) / ILED Where RIOSET is in kΩ. ILED is the LED current in Amperes. The four output channels have the same output current. Start-Up During start-up, when the VIN pin voltage reaches its UVLO threshold and both the EN and PWM signals are set to high, the SS pin begins to source 4.5µA as its voltage begins to rise from 0V to its end value of 2.5V. The output voltage of the internal trans-conductance error amplifier (COMP), increases and clamps to the SS pin voltage. When the SS pin voltage reaches its switching threshold, output © 2009 Semtech Corporation voltage increases. Proper decoupling is required on the VIN pin, especially for a lower input voltage condition. A 22µF, 6.3V rated X5R ceramic capacitor is recommended for a 5V input system. The internal LED current source (IO1 ~ IO4) helps 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. The COMP pin voltage keeps rising and once the LED current reaches its set point, the error information will not be generated by the LED current source. The COMP pin voltage stays level while keeping the LED current in its set point. If the EN pin voltage is pulled below 0.4V, the SC441A will stay in shutdown mode drawing less than 1µA from its input power supply. During the normal operation, when PWM pin is pulled below 0.4V, the device operates in standby mode, drawing 3mA (typical) current from the input. Under this condition, since the EN pin is pulled high, soft-start is initiated and the SS pin voltage is raised to its end value. Following this, when the PWM signal goes high to enable the SC441A, the COMP pin voltage will rise quickly since it is not limited by the SS pin. A proper capacitance (10nF ~ 100nF) is required to prevent output voltage overshoot on the COMP pin and its external RC network. Shut Down If the VIN pin voltage falls below its UVLO, or the voltage on the EN pin goes low, the device will run in shutdown mode as the internal switch and the LED current sources will immediately turn off. The SS capacitor is discharged by the internal current source of the SS pin. The SS pin voltage decreases to 0V while the output voltage falls to the same level as the input voltage. If the PWM pin voltage goes low while SC441A is in normal operation, then the SC441A will run in standby mode. The Internal switcher and the LED current source will immediately turn off. NOTE– The PWM signal does not affect the SS pin nor its final value. Main Power Operation SC441A is a 700KHz fixed-frequency, peak current-mode step-up switching regulator with an integrated 2.5A (minimum) power transistor. www.semtech.com 12 SC441A Applications Information (continued) Referring to the Block Diagram, Figure 1, 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 with 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 at setting 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. It is well known that, in Boost converter, Vo is greater than or equal to Vin. In normal continuous conduction mode (CCM) operation, Vo Vin 1 1 D Where, D is the duty ratio of the PWM power switch. As Vin increases, in order to regulate Vo to a given constant value, D decreases. When Vin approaches Vo, D surely leads to 0. In practice, due to the minimum on-time of the PWM power switch, D usually could not approach 0 with infinitely small granularity. At some point, it either produces one pulse with minimum on-time or generates 0 by skipping the pulse. Such point could be theoretically calculated for SC441A as follows. For CCM: Vin ≥ 0.92 Vo. For DCM (Discontinuous conduction mode): Vin t 2 1 1 1.6 *102 * Ro L Vo Where, Ro is the Boost equivalent output resistance (=Vo/ Io), L is the Boost inductor (in uH). In many Boost converter designs and operations, pulse skipping is normally allowed at light load conditions. Some designers even purposely let the Boost power converter enter the pulse skipping in order to save power at light load conditions. If some designers do not want pulse skipping mode, based on the conditions provided above, © 2009 Semtech Corporation there are some choices. 1) Leave some room between Vin range and Vo. 2) Operate the Boost converter at normal load (less Ro) 3) Increases the Boost inductance (L). Over-Current Protection SC441A provides cycle-by-cycle current limiting for its internal switch. If the switch current exceeds 3.32A (the typical current-limit trip point), then the current-limit comparator ILIM, will set the latch immediately turning off internal power. All LED current sources keep operating in an over-current condition. 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) SC441A 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 boost converter’s output voltage is detected at the OVPIN pin. If the voltage at the OVPIN pin exceeds 1.52V (typical), the boost converter will shut down and a 0.9mA pull-down current will be applied to the VOUT pin to quickly discharge the output capacitor. This added protection prevents a condition where the output capacitor and Schottky diode must endure high voltage for an extended time, which can pose a reliability risk for the user’s system. Refer to evaluation application circuit in page15. The output over voltage trip point can be programmed by R5 and R7 resistor divider. The relationship can be described as follows: OVP _ trip OVPIN _ TH u R5 R7 R7 Where OVPIN_TH is 1.52V typical. An OVP event causing a fault could disable the boost converter enabling the device to a strong pull-down. This event would cause the FFLAG pin to go high and the softstart capacitor to discharge. When the soft-start capacitor voltage falls below 0.5V, and the output voltage falls to VIN, SC441A enters a soft-start process. The OVP detection circuitry provides a disconnect function during the shutdown state to prevent any leakage from the output. The external OVP resistor divider should be connected between VOUT and OVPRTN with the central www.semtech.com 13 SC441A Applications Information (continued) tap connected to OVPIN. Note: If this disconnect function is not desired, bypass the OVPRTN pin and connect the end of the OVP resistor divider directly to GND. The OVPIN pin is sensitive to noise, and a proper decoupling capacitor (1nF ~ 10nF) is required. The combined impedance of the resistor divider for OVPIN should be greater than 200kΩ. LED Short-Circuit Protection If one or more LEDs are detected as short-circuit, that string will be latched off. Voltage is monitored if it exceeds 2.35V on the internal LED current source (IO pins). (The IO voltage on an abnormal LED string will rise earlier than other normal LED strings). If the voltage exceeds 2.35V on any IO pin, the IO current source will latch off and the FFLAG will go high. The latch is reset if VIN falls below UVLO or it will recycle the EN signal. Other LED strings operate normally. If all IO pin voltages reach 1.07V then the internal main switch will be off until any of the IO voltages is lower than 970mV. LED Open-Circuit Protection If any LED string is detected as an open-circuit, that string will latch off. If any given string is open, the IO current source will go to deep saturation; the COMP and FFLAG pins will be driven high and the boost converter duty cycle will increase causing VOUT to rise. At some point VOUT will rise high enough to cause all the IO pin voltages of the intact strings to reach the shorted LED detection level and latch off those strings. Because of the LED open string VOUT will continue to rise until it reaches the programmed OVP level. When OVP is reached, the voltage on the IO pins are monitored and if any IO voltage is less than 0.2V that string will be identified as open and will latch off. Only when VIN falls below UVLO, or an EN signal is recycled, and if thermal shutdown occurs, can this latch be reset. A hiccup cycle is then initiated and the SS pin is discharged slowly with a 1µA current source and a 0.9mA discharge path (turned on to pull down VOUT). When SS falls below 0.5V and VOUT falls below to VIN, the shorted LED detection latches are reset and a new soft-start sequence is initiated to resume normal operation. Thermal Shutdown (TSD) © 2009 Semtech Corporation 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 1µA current source, and a 0.9mA discharge path is turned on to pull down VOUT. As temperature falls below the TSD trip point, SC441A will retry when SS falls below 0.5V and VOUT falls to VIN. PWM Dimming The PWM input needs to be held high for normal operation. PWM dimming can be done 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 a 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 SC441A 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 luminescent efficiency and color purity. Furthermore, advanced lighting effects such as backlight dim-on can be implemented as the SC441A can resolve 10µs (minimum), PWM dimming pulse-width. As far as the maximum PWM dimming pulse-width, it depends on the PWM dimming frequency. Clearly, it is trivial to get 100% LED brightness by pulling PWM pin “High” constantly. When the user tries to dim the LED brightness using PWM signal from 100% down, he or she needs to observe the following. When the PWM dimming signal is actively switching from “High” to “Low” and to “High”, there is a minimal OFF time (T_off_min, 200ns, guaranteed by design) requirement of the PWM dimming signal with this IC. Such minimal OFF time sets the maximum PWM duty ratio before hitting to 100% in the following way. Dmax 1 Toff _ min f PWM For example, if the PWM dimming frequency f_ PWM=200Hz, the D_max=99.996%. If f_PWM=25kHz, the D_max=99.5%. With most practical dimming interfaces, www.semtech.com 14 SC441A Applications Information (continued) the needed dimming steps and resolutions, it is uncommon to run into the above D_max before reaching 100%. While most applications will not run into D_max, the designer should be aware of possible parasitic elements from PWM dimming interface to the PWM pin of SC441A. Usually, simply checking signal D_max at PWM pin of SC441A is sufficient. 5 4 3 2 1 Linear Dimming The linear dimming control is available for SC441A D by applying an external control voltage on IOSET pin through an external resistor-like circuit (shown below). External environment brightness compensation can also be achieved when the control voltage is generated by a light sensor circuit. C V_EXT B A IOSET R_EXT R_IOSET The IOSET voltage is 0.5V when linear dimming is used and the minimum IOSET current must be higher than 27µA (i.e. 15mA per LED string). The external control voltage slew rate must slow at 1V/10ms. LED Strings Connection Generally, LED strings are connected to IO1 ~ IO4 pins through a mechanical connector which, generally, cannot support an electrical connection thereby resulting in 5 4 3 2 1 significant noise. Consequently, the SC441A LED shortcircuit protection may false trip when the noise level is large. Certain ceramic decoupling capacitor on pins IO1 ~ IO4 to GND are useful to prevent the SC441A from noise influence. As a general guideline, the decoupling capacitance should be limited as follows. Cdcple 0.6uS I LED * Vo Where, I_LED is the LED current per string, Vo is the Boost output voltage and C_dcple is the suggested decoupling capacitor value. For example, if I_LED=10mA, Vo=13.5V, the calculated upper bound of C_dcple is about 444pF. One could use 390pF or less in the circuit. If I_LED=100mA, Vo=13.5V, the calculated upper bound of C_dcple is about 4.44nF. © 2009 Semtech Corporation One may use 3.9nF or less in the circuit. In some applications, circuit designers tend to select the decoupling capacitors in the range of (100pF ~ 1nF). For some low LED current (e.g. 10mA) applications, it is recommended to add 1M-10Mohm resistor from IO pin to GND in order to reduce IO pin voltage during PWM dimming. Parallel Operation When two or more SC441As are operating in parallel for a large-sized panel application, audible noise may be D observed due to 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 input voltage rail and the SC441A VIN pin. This situation can also be improved by adding more input C decoupling capacitors. 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 B 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 A 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 duty ratio is, ' 9287 9,1 9' 9287 9' VD – Forward conduction drop of the output rectifying diode When the boost converter runs in DCM ( L < Lboundary), it takes the advantages of small inductance and quick transient response while avoiding the bandwidth limiting instability of the RHP zero found in CCM boost converters. www.semtech.com 15 SC441A Applications Information (continued) The inductor peak current is, I L − peak = VIN ⋅ D FS ⋅ L The converter will work in CCM if L > Lboundary. Generally the converter has higher efficiency under CCM and the inductor peak current is, ,/ SHDN C OUT 9 ' ,,1 ,1 )6 / For many applications, an inductor with value of 4.7µH to 22µH should be fine, such as for the typical case shown on page 1. 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 under DCM in which case the inductor current ramps down to zero before the end of each switching cycle. It 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, possibly higher DC resistance (DCR). Inductor’s DCR plays a significant role for the total efficiency since the power transistor is integrated inside the SC441A. 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 Output Capacitor Selection Website www.cooperet.com www.vishay.com www.tokoam.com The next task in SC441A 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 cycle-by-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 © 2009 Semtech Corporation the mechanics of regulation and frequency dependence may be complex, actual selection of 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, (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 Output Rectifying Diode Selection Schottky diodes are the ideal choice for SC441A due to their low forward voltage drop and fast switching speed. Table 4 shows several different Schottky diodes that work well with the SC441A. 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 2A will be sufficient for most designs. Table 4. Recommended Rectifying Diodes Part Vendor SS23 SS24 Vishay www.vishay.com www.semtech.com 16 SC441A Applications Information (continued) Layout Guidelines The SC441A contains a boost converter and the placements of the power components outside the SC441A should follow the layout guidelines of a general boost converter. The evaluation application circuit on page 17 will be used as an example. C2 and C3 are input decoupling capacitor for SC441A VIN pin and main power input. C2,C3 should be placed as close as possible to the VIN pin to achieve the best decoupling performance. To minimize the switching noise, The switching loop formed by input decoupling capacitors, internal switch, output Schottky diode and output capacitors must be minimized. The LED current programming resistor(R6), compensation network (R9,C5,C7) and soft start capacitor (C6) should be placed as close as possible to SC441A. The C14~C17 are decoupling capacitors for LED current source which prevent IO pins from noise influence. C14~C17 should be placed close to each corresponding IO pin. Use an isolated local AGND plane underneath the controller and tie it to the negative side of output capacitor through R14 for better noise immunity. © 2009 Semtech Corporation www.semtech.com 17 SC441A 5 4 3 2 1 Evaluation Application Circuit VIN VOUT R1 0R 10K R8 VIN 1R C2 C3 10uF/25V N.P. C4 L1 6.8uH C1 1nF R7 10K 2.2uF/25V R5 220K VOUT D1 C8 C9 4.7uF/50V 4.7uF/50V IO3 IO4 R10 1R FFLAG VOUT 10 12 VIN PWM 9 13 OVPRTN IOSET 8 14 OVPIN EN 7 15 PGND COMP 6 16 SW SS 5 17 SW AGND 4 18 PGND IOGND 3 19 IO4 IO1 2 20 IO3 IO2 1 R12 1R SC441A 21 VOUT 11 EDP R3 C14 1nF C15 1nF D R2 10K R4 10K PWM R6 RIOSET EN C R9 1.5K C7 N.P. C6 100nF C5 47n R11 1R R13 1R U1 C16 1nF C17 1nF IO1 IO2 B R14 0R Evaluation Board Bill of Materials Item Reference Quantity Description Package Part Vendor 1 C1, C14, C15, C16, C17 5 50V ceramic capacitor, X7R SM_0603 1nF Panasonic 2 C2 1 25V ceramic capacitor, X5R SM_1206 10uF Panasonic 35 C8, C9 2 3 50V ceramic capacitor, X5R SM_12062 4.7uF 1 Panasonic 4 C4 1 25V ceramic capacitor, X5R SM_0805 2.2uF Panasonic 5 C5 1 6.3V ceramic capacitor, X7R SM_0603 47nF Panasonic 6 C6 1 6.3V ceramic capacitor, X7R SM_0603 100nF Panasonic 7 D1 1 40V, 3A Schottky diode SMA B340 Diodes or Any 8 L1 1 6.8μH, 6.6A DR125 6.8μH Copper or Any 9 R1, R14 2 1% SMD resistor SM_0603 0R Any 10 R2, R3, R4, R7 4 5% SMD resistor SM_0603 10K Any 11 R5 1 1% SMD resistor SM_0603 220K Any 12 R6 1 1% SMD resistor SM_0603 RIOSET Any 13 R9 1 5% SMD resistor SM_0603 1.5K Any 14 R8, R10, R11, R12, R13 5 5% SMD resistor SM_0603 1R Any 15 U1 1 Controller EDP TSSOP-20 SC441A SEMTECH © 2009 Semtech Corporation 4 www.semtech.com 18 A SC441A Outline Drawing - TSSOP-20 EDP A D e N DIM 2X E/2 E1 PIN 1 INDICATOR ccc C 2X N/2 TIPS 123 E e/2 B aaa C SEATING PLANE D A2 A C A1 bxN bbb A A1 A2 b c D E1 E e F H L L1 N 01 aaa bbb ccc DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX .047 1.20 .001 .006 0.025 0.15 .031 .042 0.80 1.05 .007 .012 0.19 0.30 .003 .007 0.09 0.20 .251 .255 .259 6.40 6.50 6.60 .169 .173 .177 4.30 4.40 4.50 .252 BSC 6.40 BSC .026 BSC 0.65 BSC .144 .150 .154 3.66 3.81 3.91 .112 .118 .122 2.85 3.00 3.10 .018 .024 .030 0.45 0.60 0.75 (.039) (1.0) 20 20 0° 8° 0° 8° .004 0.10 .004 0.10 .008 0.20 C A-B D F SEE DETAIL SIDE VIEW A EXPOSED PAD H H c GAGE PLANE BOTTOM VIEW 0.25 L (L1) DETAIL NOTES: 1. 01 A CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. DATUMS -A- AND -B- TO BE DETERMINED AT DATUM PLANE -H3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. © 2009 Semtech Corporation © 2009 Semtech Corporation www.semtech.com 19 SC441A Land Pattern -TSSOP-20 EDP F DIM (C) H G Y P Z C F G H P X Y Z DIMENSIONS INCHES MILLIMETERS (.222) .157 .161 .126 .026 .016 .061 .283 (5.65) 4.00 4.10 3.20 0.65 0.40 1.55 7.20 X 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 Corporation www.semtech.com 20