SC440A High Efficiency Integrated Driver for 6-Strings of 30mA LEDs POWER MANAGEMENT Features Description The SC440A 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 2A internal power switch. Wide input range 4.5V to 27V 42V maximum output voltage with adjustable OVP for smaller output capacitor Drives up to 72 WLEDs in 6 strings Programmable WLED current for up to 30mA per string 1% string-to-string current matching 2A integrated power switch Up to 91% efficiency 0.2% to 100% PWM dimming Analog dimming 800KHz switching frequency for small external component Open/short LED protection Short LED protection disable <1uA shut-down current Thermal protection 4mm x 4mm x1mm MLPQ-24 package (Pb-free, Halogen free, WEEE & RoHS compliant) It can drive up to 42V output voltage in 6 strings with current up to 30mA per string. The string-to-string current matching is 1% typical, 3% maximum 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 500: 1. The 800kHz 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. The SC440A also features a comprehensive open and short circuit LED protection functions. It disables the corresponding strings with LED open or LED short conditions while keeping other strings under normal operation. This feature allows LCD panels to remain viewable even under LED failure, wire disconnect, or shortcircuit conditions. The short LED protection function can be disabled. The internal thermal shutdown protects the IC from overheating at abnormal conditions. The SC440A is available in a common 4mm x 4mm x1mm MLPQ-24 package. Applications Medium-sized 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 (4.5V -27V) 1 2 SW SW VIN VOUT SCP_EN 6 Strings OVPIN EN FFLAG PWM OVPRTN 12 SC440A 6 IO1-6 COMP IOSET AGND IOGND SS EDP Jan. 12, 2010 PGND PGND www.semtech.com 1 SC440A 22 21 20 IO6 IO5 23 IO4 IO3 24 Ordering Information IOGND IO2 Pin Configuration 19 18 PWM 17 VOUT 3 16 COMP EN 4 15 SCP_EN SS 5 14 IOSET OVPRTN 6 13 VIN 7 8 9 10 11 12 PGND T N/C FFLAG TOP VIEW SW 2 SW AGND PGND 1 OVPIN IO1 Device Package SC440AMLTRT(1)(2) 4mm x 4mm x1mm MLPQ - 24 SC440AEVB Evaluation Board Notes: (1) Available in tape and reel only. A reel contains 3,000 devices. (2) Available in Pb-free and Halogen free package only. Device is WEEE and RoHS compliant. 4mm x 4mm x1mm MLPQ - 24 θJA = 29°C/W Marking Information Top View 440A yyww xxxxx xxxxx Nnnnn = Part number (Example: 440A) yyww = Date Code (Example: 0752) xxxxx = Semtech Lot No. (Example: E9010 xxxxx = 01-10) © 2010 Semtech Corp. www.semtech.com 2 SC440A Absolute Maximum Ratings Recommended Operating Conditions VIN Pin: Supply Voltage …………………………… -0.3 to 30V Supply Input Voltage …………………………… 4.5V to 27V Maximum Output Power……………………………… Maximum Output Voltage ………………………… 7.5W IOSET Voltage ……………………………………… -0.3 to 2V SW, VOUT, IO1~IO6, OVPIN, OVPRTN Voltage ………………………………… Maximum LED Current ……………………………… -0.3 to 45V 42V 30mA Thermal Information SS, COMP Voltage …………………………………… -0.3 to 3V Junction to Ambient(1) ……………………………… 29°C/W SCP_EN Voltage …………………………………… -0.3 to 3.5V Maximum Junction Temperature……………………… 150°C EN, PWM, FFLAG Voltage …………………… -0.3 to VIN +0.3V Storage Temperature ………………………… -65 to +150°C PGND to AGND and IOGND…………………………… ± 0.3V Peak IR Reflow Temperature …………………………. 260°C (2) ESD Protection Level ………………………………… 2000V 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 = 2kΩ. Parameter Symbol Conditions Under-Voltage Lockout Threshold UVLO-TH VIN rising UVLO Hysteresis UVLO-H Min Typ Max Units 4.2 4.5 V Input Supply VIN Quiescent Supply Current IIN-Q No switching VIN Supply Current in Shutdown IIN-S EN / PWM = low 250 mV 4.5 mA 1 μA 0.96 MHz Oscillator Switching Frequency(1) FS 0.64 (1) DMAX 90 Minimum Duty Cycle (1) DMIN Maximum Duty Cycle 0.8 % 0 % Minimum On-Time TON-MIN 200 ns Minimum Off-Time TON-MIN 100 ns 2.5 A Internal Power Switcher Switch Current Limit ISW 2.0 Switch Saturation Voltage VSAT ISW = 1A 200 500 mV Switch Leakage Current IS-LEAK VSW = 12V 0.01 1 μA Sourcing Current IO-H VCOMP = 0.5V 7.3 μA Sinking Current IO-L VCOMP = 2V 5.5 μA Compensation © 2010 Semtech Corp. www.semtech.com 3 SC440A Electrical Characteristics (continued) Unless otherwise noted, VIN = 12V, -40°C < TA = TJ < 85°C, RIOSET= 2kΩ. Parameter Symbol Conditions Min EN, PWM High Voltage VEN_H VIN = 4.5V to 27V 2 EN, PWM Low Voltage VEN_L VIN = 4.5V to 27V IEN_LEAK VEN = VPWM = 0V to 5.0V Typ Max Units Control Signals EN, PWM Leakage Current (1) PWM Dimming Frequency FPWM V 0.01 50 0.4 V 1 μA 50k Hz PWM Dimming Minimum Duty Cycle(1) DMIN_PWM FPWM = 200Hz 0.4 % PWM Dimming Minimum Pulse Width TPWM_MIN FPWM = 200Hz 10 μs VFFLAG IFFLAG = 2 mA, normal operation 0.25 V SS Source Current ISS_H Vss = 0V 6 SS Sink Current ISS_L Vss = 2V at OVP or OTP 1.6 SS Switching Threshold VSS_TH TJ = 25°C SS End Voltage VSS_END FFLAG Voltage μA 0.7 0.9 1.05 V 2.6 V Over-Voltage Protection OVPIN Threshold Voltage VOVPIN_TH OVPIN Leakage Current IOVPIN_L OVPRTN Saturation Voltage OVPRTN Leakage Current VOUT Internal Pull-Down in Over-Voltage Fault 1.475 1.535 1.595 V OVPIN = VIN = 20V 0.1 1 μA VOVPRTN IOVPRTN =100μA 60 IOVPRTN_L OVPRTN = VIN mV 1 IOVP μA 0.9 mA 0.1 μA IVOUT_L VOUT = 40V Backlight Current Accuracy IO1~IO6 EN, PWM = 1; TJ = 25°C Overshoot Protection Threshold IO1~IO6 0.8 V Overshoot Protection Hysteresis Any of IO1~IO6 100 mV VOUT Leakage Current Current Source (IO1 ~ IO6) 27 PWM =10% to 100%; TJ = 25°C PWM = 200Hz / 500Hz LED Current Matching (1) Maximum LED Current IO_MAX LED Short Circuit Protection Threshold VIO_SCP TJ = 25°C IO Off Leakage Current IO_LEAK SCP_EN Bias ISCP_EN 30 +/-1 32 +/-3 35 2.2 mA % mA 2.35 2.55 V EN = 0V, VIO1 ~ VIO6 = 12V 0.1 1 μA SCP_EN = 0V -40 μA TOTP 150 °C TOTP_H 30 °C Over-Temperature Protection Thermal Shutdown Temperature Thermal Shutdown Hysteresis Notes: (1) Guaranteed by design. © 2010 Semtech Corp. www.semtech.com 4 SC440A Typical Characteristics UVLO Threshold vs Temperature Switching Frequency vs Temperature Switching Frequency (kHz) 4.4 VIN UVLO (V) Rising 4.3 4.2 4.1 Falling 4 805 VIN=4.5V 795 VIN = 21V 785 775 765 -40 -20 0 20 40 60 80 -40 -20 0 Temperature (ºC) 60 80 60 80 COMP Current vs Temperature VIN=12V 700 8.5 600 COMP Current (uA) IO Pin Sense Voltage (mV) 40 Temperature (ºC) IO Pin Sense Voltage vs LED Current Vin=12V, 8 LEDs per string 500 400 300 200 8 Source 7.5 7 6.5 6 Sink 5.5 5 Ta = 25°C 100 4.5 5 10 15 20 25 30 -40 -20 0 LED Current (mA) OVPIN Threshold Voltage vs Temperature VIN = 12V 1.55 20 40 Temperature (ºC) SS Current vs Temperature VIN = 12V 6 1.54 Source SS Current (uA) OVPIN Threshold Voltage (V) 20 1.53 1.52 1.51 5 4 3 Sink 2 1.5 1 -40 -20 0 20 40 Temperature (ºC) © 2010 Semtech Corp. 60 80 -40 -20 0 20 40 60 80 Temperature (ºC) www.semtech.com 5 SC440A Typical Characteristics (continued) LED Source SCP Threshold vs Temperature 2.435 LED Current Source SCP Threshold (V) VOUT Pull Down Current (uA VOUT Pull Down Current vs Temperature 0.92 VIN=4.5V 0.9 0.88 0.86 VIN=21V 0.84 4.5V 2.43 2.425 21V 0.82 2.42 -40 -20 0 20 40 60 80 -40 -20 20 40 60 Temperature (ºC) LED Current Setting vs Temperature Efficiency vs Input Voltage 31 80 VOUT=26.5V, D1=SS14 L1=10u, IHLP-2525CZ1 / Vishay, Co=10uF 30.5 Efficiency LED Current Setting (mA) 0 Temperature (ºC) 30 RIOSET = 2kohm 29.5 -40 -20 0 20 40 60 0.91 0.90 0.89 0.88 0.87 0.86 0.85 0.84 0.83 5 80 7 9 11 13 15 Input Voltage (V) 17 19 21 Temperature (ºC) PWM Dimming Duty Cycle vs LED current PWM Dimming Duty Cycle vs LED current 35 30 25 20 30mA x 6 string load, 8 LEDs per string, L1=MOS6020-10u, C2=10uF 200 LED current (mA) LED current (mA) 5mA x 6 string load, 8 LEDs per string, L1=MOS6020-10u, C2=10uF Fpwm = 200 Hz 15 10 5 0 Fpwm = 50kHz 150 Fpwm = 200 Hz 100 50 Fpwm = 50kHz 0 0 20 40 60 Duty Cycle © 2010 Semtech Corp. 80 100 0 20 40 60 80 100 Duty Cycle www.semtech.com 6 SC440A Typical Characteristics (continued) Start up Shut down VIN 5V/DIV VIN 5V/DIV SS 2V/DIV SS 2V/DIV VOUT 20V/DIV EN 1V/DIV IO6 1V/DIV Time ( 10ms/DIV) Conditions: VIN = 5V, Load = 30mA x 6 strings, 8 LEDs per string Time ( 200ms/DIV) Conditions: VIN = 5V, Load = 30mA x 6 strings, 8 LEDs per string Main Power Switching Waveform Main Power Switching Waveform SW 10V/DIV SW 10V/DIV VOUT(AC) 50mV/DIV VOUT(AC) 50mV/DIV Time ( 0.5us/DIV) Conditions: VIN = 5V, Load = 30mA x 6 strings, 8 LEDs per string © 2010 Semtech Corp. Time ( 0.5us/DIV) Conditions: VIN = 12V, Load = 30mA x 6 strings, 8 LEDs per string www.semtech.com 7 SC440A Typical Characteristics (continued) TOP and OTP Recovery PWM dimming SS 2V/DIV PWM FLAG 2V/DIV SW VOUT 10V/DIV IO6 2V/DIV Keep Original here 20V/DIV 5V/DIV Time ( 100ms/DIV) Conditions: VIN = 5V, Load = 30mA x 6 strings, 8 LEDs per string Time ( 5ms/DIV) Conditions: VIN = 5V, Load = 30mA x 6 strings, 8 LEDs per string OVP VIN Adaptor Plug-in Case VIN 10V/DIV OVPIN 1V/DIV VOUT 1V/DIV IO6 1V/DIV SS 2V/DIV SW 20V/DIV FLAG 5V/DIV Time ( 1s/DIV) Conditions: VIN = 5V, Load = 30mA x 6 strings, 8 LEDs per string © 2010 Semtech Corp. Time ( 50us/DIV) Conditions: VIN = 6V~19V, Load = 30mA x 6 strings, 8 LEDs per string www.semtech.com 8 SC440A Pin Descriptions Pin # Pin Name Pin Function 1 IO1 2 AGND Analog ground 3 FFLAG Power failure signal output with open collector. 4 EN Enable the device including requlator and LED drivers. 5 SS Soft-start pin 6 OVPRTN Over-voltage return 7 OVPIN Over-voltage input 8, 12 PGND Power ground 9, 10 SW Collector of the internal power transistor – connect to the boost inductor and the rectifying diode. 11 N/C Left open. 13 VIN Input power supply. 14 IOSET 15 SCP_EN 16 COMP The output of the internal transconductance error amplifier – this pin is used for loop compensation. 17 VOUT Output voltage pin – internal over-voltage Connect the output capacitor and the anode of control pin for LED backlight strings. 18 PWM PWM control pin for LED backlight strings. 19 IO6 Provides constant sink current to LED string 6. 20 IO5 Provides constant sink current to LED string 5. 21 IO4 Provides constant sink current to LED string 4. 22 IOGND Constant sink current ground, tying to AGND. 23 IO3 Provides constant sink current to LED string 3. 24 IO2 Provides constant sink current to LED string 2. T Thermal Pad Provides constant sink current to LED string 1. Current source IO value set pin – by selecting the resistor connected from this pin to GND, the corresponding maximum current on all 6 strings are set. LED short-circuit protection pin – leaving this pin open enables the function, tying it to ground disables it. If any IO pin connects to VIN, SCP_EN needs to be left open to activate the protection. Thermal pad for heatsinking purposes — connect — not connected internally Note: Any unused IO pin should be pulled up to VIN with the SCP_EN left open. Table 1. EN STATUS 0 backlight disable 1 backlight enable Note: When EN = 0; the boost is turned OFF and disabled. © 2010 Semtech Corp. www.semtech.com 9 SC440A Block Diagram SW HICCUP FFLAG SW OSC IO6 IO5 S Q R IO4 FAULT-1 IO3 + LED OPEN / SHORT CIRCUIT PROTECTION ILIM - ONE IO CHANNEL SHOWN IO2 ISENSE - + CURRENT - LED CURRENT SETTING PGND + DISABLE1 IO1 SOURCE PGND COMP SS I to V IOSET IOGND PWM VOUT OVP OVPIN OVPRTN OVP Detect Fault Fault 3V3 HICCUP CONTROL LOGIC VIN HICCUP UVLO 4uA TSD SS UVLO & TSD Bandgap EN HICCUP AGND SCP_EN 5uA Figure 1. SC440A Block Diagram © 2010 Semtech Corp. www.semtech.com 10 SC440A Applications Information SC440A Detailed Description The SC440A contains a high frequency, current-mode boost regulator and six 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. 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 6μA 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 increases and clamped by the SS pin voltage. When the SS pin voltage reaches its switching threshold, the SC440A starts to switch and the output voltage increases. Operation The SC440A regulates the boost converter output voltage based on instantaneous requirement of the six 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 4.5mA (typical) when all LED strings are off. Each internal LED current source (IO1 ~ IO6) 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. 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 SC440A is a LED current programmable regulator. The LED current set point is chosen using external an 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 60 RIOSET Where, RIOSET is in kΩ. ILED is the output current of each string in mA. © 2010 Semtech Corp. If the EN pin voltage is pulled below 0.4V and VIN reaches to its UVLO, SC440A 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 SC440A runs in standby mode, drawing 4.5mA (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 SC440A, 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, the SC440A 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 SC440A Applications Information (continued) If PWM pin voltage goes low while SC440A is at normal operation, SC440A 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 SC440A is an 800kHz fixed-frequency, peak currentmode boost switching regulator with an integrated 2.5A (typical), power transistor. Referring to the Block Diagram, 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 2.5A (the typical 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) SC440A 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 the OVPIN pin. If the voltage at the OVPIN pin exceeds 1.5V, the boost converter will shut off and a 1mA pull down current source will be applied to the VOUT pin to quickly © 2010 Semtech Corp. 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 17). The relationship can be described as follows: OVP_trip OVPIN_TH u R2 R4 R4 Where OVPIN_TH is 1.535V typical. An OVP event causes a fault which disables the boost converter and enables the strong pulldown and the FFLAG pin will also go high. Meanwhile, the soft-start capacitor is discharged. When the soft-start capacitor voltage falls below 0.5V, SC440A enters a soft-start process. The OVP detection circuitry provides a disconnect feature during the shutdown state to prevent any leakage from the output. The external OVP resistor divider should be connected between VOUT and OVPRTN while its central tap is connected to OVPIN. If this disconnect function is not desired, just connect the end of the OVP resistor divider directly to GND. The OVPIN pin is sensitive to noise, a proper decoupling cap (1nF ~ 10nF) is required. LED Short-Circuit Protection If one or more LEDs are detected as shortened, that corresponding string will be latched off if SCP_EN is floating. The voltages on all internal LED current sources (IO pins) are monitored to see if any exceeds 2.3V. (The IO voltage on abnormal LED string will rise earlier than other floating LED strings). If any IO pin voltage exceeds 2.3V, that IO current source will be latched off and the FFLAG will go high. The latch can be reset if VIN falls below UVLO or recycle EN signal. Other normal LED strings remain at their normal operation. The protection will be disabled if SCP_EN is tied to GND. If all IO pin voltages reach 0.8V then the internal main switch will be off until any of the IO voltages is lower than 0.7V. Unused Strings The SC440A may be operated with less than 6 strings. In this case, all unused strings should be tied to VIN and leave the SCP_EN pin floating. www.semtech.com 12 SC440A Applications Information (continued) LED Open-Circuit Protection If any LED is detected as open circuit, that string will be latched off. Then the COMP pin 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 remaining strings to reach the shorted LED detection level and those strings are latched off. The FFLAG pin will also go high. 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. A hiccup cycle is initiated, SS is discharged slowly with a 1.6μA current source and a 1mA discharge path is 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 softstart sequence is initiated to resume normal operation. 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 1.6μA current source, and a 1mA discharge path is turned on to pull down VOUT. As temperature falls below TSD trip point, SC440A will retry when SS falls below 0.5V and VOUT falls to VIN. 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 SC440A can resolve PWM from 10% to 90% duty at its highest frequency. An additional advantage of PWM dimming comes to customers who prefer to avoid in-rush currents when filling the boost output capacitor. Simply apply the PWM signal to the device at 10% duty for a 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. Linear Dimming The linear dimming control is available for SC440A by applying an external control voltage on IOSET pin through an external resistor as shown below. External environment brightness compensation can also be achieved when the control voltage is generated by a light sensing circuit. IOSET R_EXT 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 SC440A allows dimming over two decades © 2010 Semtech Corp. V_EXT R_IOSET LED String Connection Generally, LED strings are connected to the IO1 ~ IO6 pins through a mechanical connector which cannot support an electrical connection at times. This connection might cause noise on the IO1 ~ IO6. If this function is enabled, the SC440A LED short-circuit protection may false trip when the noise level is large enough. Certain ceramic decoupling capacitors (100pF ~ 8.2nF) on pins IO1 ~ IO6 to GND can help prevent the SC440A from entering the www.semtech.com 13 SC440A Applications Information (continued) false protection, or, simply disable this feature by tying Pin 15 to GND. Parallel Operation When two or more SC440As 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. This situation can also be improved by adding more input 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 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, IIN VOUT IOUT VIN Ș IIN - Input current; IOUT – Output current; VOUT – Boost output voltage; VIN – Input voltage; η – Efficiency of the boost converter. takes the advantages of small inductance and quick transient response while avoiding the bandwidthlimiting instability of the RHP zero found in CCM boost converters. 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, IL -peak IIN VIN D 2 FS L 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 SC440A. Of course, there is a trade-off between the DCR and inductor size. Table 2 lists recommended inductors and their vendors. Then the duty ratio is, Table 2. Recommended Inductors D VOUT VIN VD VOUT VD Inductor DR74, 4.7μH ~ 15μH IHLP-2525CZ-01, 4.7μ ~ 10μH VD – Forward conduction drop of the output rectifying diode When the boost converter runs in DCM ( L < Lboundary), it © 2010 Semtech Corp. DS85LC, 6.8μH ~ 10μH Website www.cooperet.com www.vishay.com www.tokoam.com www.semtech.com 14 SC440A Applications Information (continued) Output Capacitor Selection The next task in SC440A 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 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, 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 © 2010 Semtech Corp. Output Rectifying Diode Selection Schottky diodes are the ideal choice for SC440A due to their low forward voltage drop and fast switching speed. Table 4 shows several different Schottky diodes that work well with the SC440A. 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. Layout Guidelines The SC440A contains a boost converter and the placements of the power components outside the SC440A should follow the layout guidelines of a general boost converter. The application circuit on page 17 will be used as an example. The layout illustration diagram is shown on page 19. R5 and C7 form a decoupling filter for the SC440A. 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, C2, should be placed at the bottom, as displayed on page 19, so that the loop formed by C2, D1, and the internal switch, is the smallest. The output of the boost converter is used to power up the LEDs. R6, C9 and C10 (open, not used), 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 IO6 and should return to analog ground. IOGND should also be connected to AGND. Since there is pad at the bottom of the SC440A for heat dissipation, a copper area right underneath the pad is used for better heat spreading. On the bottom layer of the www.semtech.com 15 SC440A Applications Information (continued) board another copper area, connected through vias to the top layer, is used for better thermal performance. The pad at the bottom of the SC440A should be tied to the analog ground. The analog ground should be connected to the power ground at one point for better noise immunity. © 2010 Semtech Corp. www.semtech.com 16 SC440A Case 1: Schematic for 6 strings 8 LEDs per string application L1 10uF C2 D1 6.8u 8 LEDs per channel C6 2.2u 1.5k LED LED R9 20k 17 PWM PGND OVPIN SW FFLAG VOUT AGND PWM IO6 18 COMP GND 19 20 21 22 23 R3 20k 6 CR4 LED LED LED LED LED LED LED LED 1n C8 47n 4 EN IO3 16 SC440A SCP_EN IO2 12n R6 R4 10k 5 SS IOSET IOGND 15 C9 R7 10k 7 OVPRTN IO4 14 8 VIN IO5 GND 9 SW SINK 13 R8 2k 10 PGND R5 1 2.2u C7 LED R2 221k 25 12 VIN=5V ~ 20V LED 3 2 1 IO1 24 Case 2: Schematic for 5 strings 8 LEDs per string application 0 SS14 D1 L1 10u C6 8 LEDs per channel C1 10uF Coilcraft, MOS6020 2.2u R2 7 OVPIN 8 PGND SW 9 10 SW 11 PWM 19 © 2010 Semtech Corp. NC AGND IO6 18 VOUT IO1 R4 CR4 10k 1nF 6 5 C8 47nF 4 3 2 1 IO2 17 FFLAG R3 20k 24 R9 20k COMP IO3 16 EN 23 1.5k SC440A SCP_EN IOGND 12n R6 SS U1 22 C9 IOSET IO4 15 0 0 14 R7 20k OVPRTN 21 2.2u C7 P2 VIN IO5 13 R8 3k 20 P1 PGND SINK 12 25 0 Input 12V 221k 0 www.semtech.com 17 SC440A BOM for 6 string 8-LED Application Case ITEM QUANTITY REFERENCE PART 1 1 CR4 1nF 2 1 C2 10μF, 50V, 1210 3 2 C6, C7 2.2μ, 25V, 0805 4 1 C8 47nF 5 1 C9 12nF 6 1 D1 SS14 7 1 L1 6.8μ, IHLP-2525CZ1 8 1 R4 10k 9 1 R2 221k 10 2 R3, R9 20k 11 2 R7 10k 12 1 R8 2k 13 1 U1 SC440A, Semtech 14 1 R5 1, 0805 15 48 LEDs SML-LX0603UWD 16 1 R6 1.5k Case 3: Schematic for 6 strings 12 low Vf LEDs per string application L1 C6 10u C1 4.7uF 50V D1 C2 4.7uF 50V C3 4.7uF 50V 2.2u GND 17 18 PGND OVPIN SW SC440A SCP_EN EN COMP FFLAG VOUT AGND PWM 19 © 2010 Semtech Corp. SS 20 21 22 IO1 IO3 PWM LED 23 6 5 R4 10k LED LED R3 20k C8 47n CR4 LED LED LED LED LED LED LED LED 1n 4 3 2 1 IO2 R9 20k 7 IOSET IOGND 6.04k 16 R7 10k OVPRTN IO4 12n R6 8 SW SINK 15 C9 9 VIN IO5 GND 14 10 PGND 13 R8 3k 2.2u C7 IO6 R5 1 LED R2 249k 25 12 VIN=12V 12 low Vf LEDs per string SS15 Toko, DS86C 24 www.semtech.com 18 SC440A Layout Illustration Diagrams PCB TOP COMPONENTS TOP © 2010 Semtech Corp. PCB B OTTOM COMPONENTS BOTTOM www.semtech.com 19 SC440A Outline Drawing - MLPQ-24 D A B DIM PIN 1 INDICATOR (LASER MARK) A A1 A2 b D D1 E E1 e L N aaa bbb E A2 A DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX .031 .035 .039 .000 .001 .002 - (.008) .007 .010 .012 .152 .157 .163 .100 .106 .110 .152 .157 .163 .100 .106 .110 .020 BSC .012 .016 .020 24 .004 .004 0.90 1.00 0.02 0.05 (0.20) 0.25 0.30 4.00 4.15 2.70 2.80 4.00 4.15 2.70 2.80 0.50 BSC 0.30 0.40 0.50 24 0.10 0.10 0.80 0.00 0.18 3.85 2.55 3.85 2.55 SEATING PLANE aaa C A1 C D1 LxN E/2 E1 2 1 N e bxN bbb C A B D/2 NOTES: © 2010 Semtech Corp. 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS. www.semtech.com 20 SC440A Land Pattern - MLPQ-24 K DIMENSIONS (C) G H Z DIM C G H K P X Y Z INCHES (.156) .122 .106 .106 .020 .010 .033 .189 MILLIMETERS (3.95) 3.10 2.70 2.70 0.50 0.25 0.85 4.80 X P NOTES: 1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. 2. 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 © 2010 Semtech Corp. www.semtech.com 21