ALT80802KEJJTR

ALT80802 Wide Input Voltage, Adjustable Frequency, Buck-Boost or Buck 2 Amp LED Driver FEATURES AND BENEFITS • Automotive AEC-Q100 qualified • Supports buck-boost or buck mode operation • Supply voltage from 3.8 to 50 V ▫▫ Handles automotive load dump and cold crank ▫▫ Can be run in buck mode from a pre-boost supply • 150 mΩ integrated MOSFET switch • Supports up to 16 V output in buck-boost mode for 4 WLEDs • Programmable switching frequency up to 2.5 MHz for small solution size and operation above AM band • Designed for low EMC with frequency dithering • Integrated level shifting allows ground-referenced enable and fault flag in buck-boost mode • PWM dimming via direct logic input or power supply voltage • Robust protection against: ▫▫ Adjacent pin-to-pin short ▫▫ Pin-to-VSS (IC ground) short ▫▫ Component open/short faults • Turn/stop lights • Map light • Dimmable interior lights PACKAGE: 10-Pin DFN with Exposed Thermal Pad and Wettable Flank (suffix EJ) Not to scale The ALT80802 is a high-frequency switching regulator that provides constant output current to drive high-power LEDs. It integrates a power MOSFET for step-down or inverting buckboost conversion. With current-mode control and simple external compensation, the ALT80802 can achieve fast transient response. The wide input range of 3.8 to 50 V makes the ALT80802 suitable for a wide range of lighting applications, including those in an automotive input environment. The device rating also enables a simple solution for driving 3 to 4 WLEDs in buckboost configuration—a very common application requirement for automotive lighting applications. The ALT80802 is designed to aid in EMC/EMI design by frequency dithering, soft freewheel diode turn-off, and wellcontrolled switch node slew rates. A programmable oscillator allows the ALT80802 to switch outside EMI-sensitive frequency bands such as the AM band. With current-mode control and simple external compensation, the ALT80802 can achieve fast transient response. The control loop of the ALT80802 is designed for PWM dimming operation to achieve low dimming on-time and low turn-on overshoot. In buck-boost operation, the ALT80802 reduces the current overshoot normally caused by right half plane zero effect during a PWM dimming turn-off transient. Extensive protection features of the ALT80802 include pulse-bypulse current limit, hiccup mode short-circuit protection, open/ short freewheeling diode protection, BOOT open/short voltage protection, VIN undervoltage lockout, and thermal shutdown. Also, it includes internal clamp to prevent output voltage runaway if output LED string is opened in buck-boost operation. The ALT80802 is available in industry-standard 10 pin DFNpackage with thermal pad and wettable flank. CBST CBST LO BST LO LED+ SW GND CIN RFREQ COUT VSS COMP FREQ + VIN – CZ RZ D RSENSE Downloaded from Arrow.com. CS EN FFn CIN CIC RFREQ COUT COMP GND LED– Figure 1: ALT80802 Buck Simplified Schematic ALT80802-DS MCO-0000502 + VIN – VIN FREQ CS EN FFn LED+ SW BST VIN VSS APPLICATIONS Automotive lighting • Daytime running lights • Front and rear fog lights DESCRIPTION CZ RZ D LED– RSENSE Figure 2: ALT80802 Buck-Boost Simplified Schematic September 10, 2018 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 SPECIFICATIONS SELECTION GUIDE Part Number Package Packing [1] ALT80802KEJJTR 10-pin DFN with thermal pad and wettable flank 1500 pieces per 7-inch reel [1] Contact Allegro for additional packing options. ABSOLUTE MAXIMUM RATINGS [2] Characteristic Symbol Input Voltage VIN Switch Node Voltage VSW Notes t < 250 ns t < 50 ns Bootstrap Pin to Switch Node VBST-SW VSS to GND VGND-VSS Limits output to –20 V Rating Unit −0.3 to 55 V −0.3 to VIN + 0.3 V −1.5 V VIN + 3 V −0.3 to 6 V −0.3 to 20 V EN, FREQ, CS, FFn With respect to VSS pin −0.3 to VIN + 0.3 V All other pins With respect to VSS pin −0.3 to 6 V Junction Temperature TJ −40 to 150 °C Storage Temperature Range Tstg −40 to 150 °C Stresses beyond those listed in this table may cause permanent damage to the device. The absolute maximum ratings are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the Electrical Characteristics table is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability. [2] RECOMMENDED OPERATING CONDITIONS Characteristic Symbol Test Conditions Value Unit DC Input voltage VIN GND connected to VSS 6 to 36 V Transient Input Voltage VIN GND connected to VSS 3.8 to 50 V Junction Temperature TJ −40 to 150 °C THERMAL CHARACTERISTICS: May require derating at maximum conditions; see application information Characteristic Symbol Junction-to-Ambient Thermal Resistance RθJA [3] Additional Test Conditions [3] Value Unit 45 °C/W DFN-10 (EJ) package on 4-layer PCB based on JEDEC standard thermal information available on the Allegro website. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 2 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Table of Contents Frequency Dithering..........................................................11 Pulse-by-Pulse Current Limit..............................................11 Switch Overcurrent Protection and Hiccup Mode..................11 Secondary Switch Overcurrent Protection............................11 BOOT Capacitor Protection. ..............................................11 Freewheeling Diode Protection...........................................11 Output Overcurrent Protection........................................... 12 Output Overvoltage Protection. ......................................... 12 Thermal Shutdown. ......................................................... 12 Applications Information. ..................................................... 13 Setting the Switching Frequency. ...................................... 13 Setting the Output Voltage................................................ 13 Inductor.......................................................................... 13 Freewheeling Diode......................................................... 14 Input Capacitor................................................................ 14 Output Capacitor............................................................. 15 Compensation Components.............................................. 15 Design Example.............................................................. 18 Typical Application Schematics............................................. 21 PCB Component Placement and Routing. ............................. 22 Buck LED Driver.............................................................. 22 Buck-Boost LED Driver. ................................................... 24 Application Circuit Examples................................................ 26 Package Outline Drawing. ................................................... 33 Features and Benefits. .......................................................... 1 Description........................................................................... 1 Applications. ........................................................................ 1 Package.............................................................................. 1 Simplified Schematics. .......................................................... 1 Specifications....................................................................... 2 Selection Guide................................................................. 2 Absolute Maximum Ratings. ............................................... 2 Recommended Operating Conditions................................... 2 Thermal Characteristics...................................................... 2 Functional Block Diagram...................................................... 3 Pinout Diagram and Terminal List. .......................................... 5 Electrical Characteristics........................................................ 6 Fault Table........................................................................... 8 Functional Description........................................................... 9 Overview.......................................................................... 9 PWM Control. ................................................................... 9 Error Amplifier................................................................... 9 Slope Compensation........................................................ 10 Internal Regulator............................................................ 10 Enable and PWM Dimming............................................... 10 Undervoltage Lockout (UVLO). ......................................... 10 Startup and Shutdown...................................................... 10 MOSFET Driver and Bootstrap Capacitor. .......................... 10 Boot Charge UVLO BST SW VIN EN Level Shi Off Delay ON LDO Fault Detect FFn VREG BST Level Shi ON FREQ Osc PWM Generator CS - Dither Generator GND + 18 V + 0.2 V COMP VSS Figure 3: Functional Block Diagram Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 3 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 PINOUT DIAGRAM AND TERMINAL LIST VIN 1 EN 2 FFn 3 FREQ GND 10 BST 9 SW 8 CS 4 7 COMP 5 6 VSS PAD Package EJ Pinouts Terminal List Table Pin Name Pin Number Description VIN 1 Power input for the control circuits and the drain of the internal high-side N-channel MOSFET. Connect this pin to a power source. A high quality ceramic capacitor should be placed very close to this pin and GND. EN 2 Input for Enable and PWM dimming; rated up to VIN and logic-level compatible. FFn 3 Open-drain fault flag output which is pulled low in case of fault. Connect through an external pull-up resistor to the desired level. This pin should be left open if not used. FREQ 4 Frequency setting pin. A resistor, RFREQ, from this pin to VSS sets the PWM switching frequency. See Table 2 to determine the value of RFREQ. GND 5 Enable and fault flag ground reference. Connect to input supply ground. VSS 6 ALT80802 return. Connect to lowest circuit potential. This is input ground when configured as a buck converter and should be connected to the GND pin. It is the negative output when configured as a buck-boost converter. See typical application schematics for more detail. COMP 7 Output of the error amplifier and compensation node for the current-mode control loop. Connect a series RC network from this pin to VSS for loop compensation. See the Applications section of this datasheet for further details. CS 8 Feedback (negative) input to the error amplifier. Connect a resistor from this pin to VSS to program the output load current. SW 9 The source of the internal MOSFET. The output inductor (LO) and cathode of the free-wheeling diode (D) should be connected to this pin. LO and D should be placed as close as possible to this pin and connected with relatively wide traces. BST 10 Bootstrap capacitor connection. A 0.22 µF or higher capacitor is recommended between this pin and SW pin. The voltage on this capacitor drives the internal MOSFET via the high side gate driver. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 4 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 ELECTRICAL CHARACTERISTICS [1]: Valid for VIN = 12 V, VEN = 2.5 V, VCOMP = 1.4 V, VSS = GND, –40°C ≤ TJ ≤ 125°C, typical values at TJ = 25°C, unless otherwise specified Characteristics Symbol Test Conditions Min. Typ. Max. Unit VEN ≥ 2.5 V, VIN with respect to VSS 3.8 12 50 V GENERAL SPECIFICATIONS Operating Input Voltage VIN VIN UVLO Start VIN(START) VIN rising, with respect to VSS 3.0 3.3 3.6 V VIN UVLO Stop VIN(STOP) VIN falling, with respect to VSS 2.7 3.0 3.3 V IQ(SLEEP) VEN = 0 V – 11 20 µA Supply Quiescent Current [1] PWM SWITCHING FREQUENCY Switching Frequency fSW RFSET = 8.06 kΩ 1.8 2.0 2.2 MHz RFSET = 41.2 kΩ 360 400 440 kHz Dither Frequency Sweep ∆fSW – ±5 – % Dither Modulation Frequency fMOD – 12 – kHz THERMAL PROTECTION Thermal Shutdown Threshold [2] TTSD – 170 – °C Thermal Shutdown Hysteresis [2] THYS – 20 – °C Minimum On-Time tON(MIN) – 80 100 ns Minimum Off-Time tOFF(MIN) – 100 – ns – 150 – mΩ 0.192 0.200 0.208 V TJ rising PULSE-WIDTH MODULATION (PWM) INTERNAL MOSFET MOSFET On Resistance RDS(on) VBOOT-SW = 5 V, TJ = 25°C [2] ERROR AMPLIFIER Current Sense Voltage VCS Current Sense Pin Bias Current Error Amplifier Voltage Gain Error Amplifier Transconductance Error Amplifier Min. Source Current [3] Error Amplifier Min. Sink Current [3] 3.8 V ≤ VIN ≤ 50 V, –40°C ≤ TJ ≤ 150°C ICS – – 100 nA AVOL – 1000 – V/V gm ICOMP = ±3 µA – 120 – µA/V IEA(SOURCE) VCS = 0.1 V – –13.6 – µA IEA(SINK) VCS = 0.3 V – 13.6 – µA For input and output current specifications, negative current is defined as coming out of the node or pin (sourcing), positive current is defined as going into the node or pin (sinking). [2] Ensured by design and characterization; not production tested. [3] Minimum source and sink current is the minimum current ensured to be provided when COMP demands maximum sink/source current. [1] Continued on next page... Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 5 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 ELECTRICAL CHARACTERISTICS [1] (continued): Valid for VIN = 12 V, VEN = 2.5 V, VCOMP = 1.4 V, VSS = GND, –40°C ≤ TJ ≤ 125°C, typical values at TJ = 25°C, unless otherwise specified Characteristics Symbol Test Conditions Min. Typ. Max. Unit CURRENT PROTECTION Pulse-by-Pulse Switch Current Limit ILIM Duty cycle 0 to 85% 3.5 5.5 6.5 A Secondary Current Limit ILIM(SEC) Hiccup after 2 counts – 7.1 – A COMP to Current Sense Transconductance [2] GCS – 9 – A/V Slope Compensation Output Overcurrent SE(2MHz) Measured at fSW = 2 MHz – 3.1 – A/µs VOCP With respect to nominal VCS voltage – 400 – % VOVP GND – VSS, when in buck-boost topology 16 18 20 V VEN(H) VEN with respect to GND 1.8 – – V VEN(L) VEN with respect to GND – – 0.4 V – 100 – mV OVERVOLTAGE PROTECTION Maximum Output Voltage LOGIC ENABLE EN Logic High Voltage EN Logic Low Voltage EN Hysteresis EN Pin Pull-Down Resistance VEN(HYS) RENPN VEN = 5 V – 80 – kΩ tPWML Measured while EN = low, during dimming control, and internal references are powered-on (exceeding tPWML results in shutdown) 12 20 – ms Fault Pull-Down Voltage VFFn(PD) Fault condition asserted, pull-up current = 1 mA – – 0.4 V Fault Pin Leakage Current IFFn(LKG) Fault condition cleared, pull-up to 12 V – – 1 µA Maximum PWM Dimming Off Time FAULT PIN (FFn) Cooldown Timer for Fault Retry tRETRY – 6 – ms Delay Timer for Reporting Open LED Fault tOPEN – 50 – µs For input and output current specifications, negative current is defined as coming out of the node or pin (sourcing), positive current is defined as going into the node or pin (sinking). [2] Ensured by design and characterization, not production tested. [1] Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 6 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Table 1: Fault Table Failure Mode Symptom Observed Fault Flag Asserted? Protection Mode ALT80802 Response Inductor shorted Dim light from LED Yes Hiccup Internal MOSFET switch is shorted. Current spike trips secondary current limit after 2 counts. IC enters hiccup mode with 6 ms retry timer. In buck topology, IC continues to switch at maximum tON (since this fault cannot be distinguished from VIN too low for LED forward drop). Output voltage, VOUT, increases until it reaches input voltage, VIN. Fault flag will be asserted if current sense pin voltage, VCS, drops below 150 mV for more than 50 µs. In buck-boost topology, IC continues to switch at maximum tON. Output voltage VOUT keeps increasing until it is clamped to VOVP. Fault flag will be asserted if current sense pin voltage, VCS, drops below 150 mV for more than 50 µs. LED string open No light from LED Depends* Clamp LED string shorted No light from LED No No VOUT will be regulated to current sense voltage VCS (200 mV typical), no fault is detected. LED string partially shorted Some LEDs are not on No No Normal operation, no fault is detected. Diode open Dim light from LED Yes Hiccup Detects missing diode fault and shuts off switching. IC enters hiccup mode with 6 ms retry timer. Diode shorted No light from LED Yes Hiccup Current spike trips SW secondary current limit. IC enters hiccup mode. IC enters hiccup mode with 6 ms retry timer. Output capacitor shorted No light from LED Yes Hiccup IC unable to regulate LED current at VOUT = 0 V. Switch current increases until it trips current limit protection. IC enters hiccup mode with 6 ms retry timer. Output capacitor open LED may flicker Depends Depends Sense resistor open No light from LED Yes Hiccup Output overcurrent protection is triggered. IC enters hiccup mode with 6 ms retry timer. Sense resistor shorted Dim light from LED Yes* Hiccup SW current increases, which eventually trips pulse-by-pulse SW current limit. IC enters hiccup mode with 6 ms retry timer. FSET resistor open Dim light from LED Yes No Operates at 772 kHz switching frequency. May hit thermal limit. FSET resistor shorted Dim light from LED Yes No Operates at 772 kHz switching frequency. May hit thermal limit. Boot capacitor open Dim light from LED Yes Hiccup IC triggers missing Boot protection. IC enters hiccup mode with 6 ms retry timer. Boot capacitor shorted No light from LED Yes Hiccup IC triggers Boot shorted protection. IC enters hiccup mode with 6 ms retry timer. LED current ripple increases. Note (*) • In case of LED current not in regulation, fault flag is asserted after approximately 50 μs timeout delay. In buck-boost topology, if binning resistors are used, fault flag may not be asserted during an open LED fault. • If sense resistor is shorted with high resistance wire, protection may not be triggered. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 7 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 FUNCTIONAL DESCRIPTION Overview The ALT80802 is a buck or buck-boost regulator that incorporates all the control and protection circuitry necessary to satisfy a wide range of LED driver applications. The device employs current-mode control to provide fast transient response, simple compensation, and excellent stability. The ALT80802 is designed to satisfy the most demanding automotive applications. Extensive protection features prevent the device and the external components from most of the common fault conditions. Care was taken when defining the device pinout to optimize protection against adjacent pin-to-pin short circuits and pin-to-ground (VSS) short circuits. PWM Control A high-speed PWM comparator, with minimum on-time less than 100 ns, is included in the ALT80802. The inverting input of the comparator is connected to the output of the error amplifier. The non-inverting input is connected to the current sense signal. At the beginning of each PWM cycle, the clock signal sets the PWM flip-flop and the internal power MOSFET is turned on. When the current sense signal rises above the error amplifier voltage (COMP pin voltage), the comparator resets the PWM flip-flop and the high-side MOSFET is turned off. If current sense signal is still higher than the error amplifier voltage before the next clock on signal, the PWM flip-flop will not be set and the next PWM cycle is skipped to prevent output overcharged. This pulse-skipping mode of operation usually happens at high input voltage and low output voltage when extremely small duty cycle is required. Note that in pulse-skipping mode, output ripple will be much higher. In buck topology, the device will start to pulse skip when: Equation 1: 1 In buck-boost topology, the device will start to pulse skip when: Equation 2: 1 where fSW is the switching frequency and tON(MIN) is the minimum on-time. If the current sense signal is lower than the error amplifier voltage for the entire PWM cycle, the PWM flip-flop will be reset 100 ns before the next PWM cycle. This maximum on-time mode of operation means the regulator is in dropout region where output cannot be regulated up to its target value. LED cannot be turned on if output voltage cannot reach to its turn-on threshold. In buck topology, the device will be in dropout region when: Equation 3: 1 In buck-boost topology, the device will be in dropout region when: Equation 4: 1 where fSW is the switching frequency and tOFF(MAX) is the maximum on-time. It is recommended to keep VIN above dropout region to avoid LED brightness change. ALT80802 does not support dropout region operation with PWM dimming. Error Amplifier The primary function of the transconductance error amplifier is to regulate the voltage at the CS pin. By connecting a CS resistor in series with the LED, output current is regulated. The negative input of the error amplifier is connected to the CS pin, and the positive input is connected to the internal reference voltage of 200 mV. The voltage difference between the two inputs is amplified to charge or discharge the compensation network connected to the COMP pin. To stabilize the regulator, a series RC compensation network (RZCZ) must be connected from the error amplifier output (COMP pin) to VSS as shown in the typical application schematic. In most applications, an additional low-value capacitor (CP) should be connected in parallel with the RZ-CZ compensation network to roll-off the loop gain at higher frequencies. However, if the CP capacitor is too large, the phase margin of the regulator may be reduced. In most cases, a CP value of 39 pF or less is recommended. The minimum COMP voltage is clamped to 750 mV and its maximum is clamped to 1.5 V. COMP is internally pulled down to VSS during hiccup mode. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 8 ALT80802 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver Slope Compensation The ALT80802 incorporates internal slope compensation (SE) to allow PWM duty cycles above 50% for a wide range of input/ output voltages and inductor values. The slope compensation signal is added to the sum of the current sense amplifier output and the PWM ramp offset. The amount of slope compensation scales with the maximum on-time (1/fSW – tOFF(MIN)) centered around 3.1 A/μs at 2 MHz. The value of the output inductor should be chosen such that SE is between 0.5× and 2× the down slope of the inductor current (SLD). Internal Regulator An internal series-pass regulator (LDO) generates around 2.9 V for most of the internal circuits of the ALT80802. The power for this LDO is derived from VIN. The LDO is in full regulation once VIN is greater than 3.0 V. Enable and PWM Dimming The enable (EN) input allows the system to selectively turn on/ off the ALT80802 control loop. The EN pin is rated to 55 V, so the EN pin can be connected directly to VIN if there is no suitable logic signal available to wake up the regulator. be overcharged and an LED current spike will be seen. To reduce this current spike, the ALT80802 incorporates an internal bleeding circuit that will divert the extra current away from the LED during the PWM dimming turn-off period. If EN is low for more than tPWML, the IC enters shutdown mode to reduce power consumption. The next high signal on EN will initialize a full startup sequence before LED current starts to build. Note that this startup sequence is not present during PWM dimming operation. The EN signal is referenced to the GND pin of the ALT80802. This allows the user to use system-referenced signals to this pin even when the output is configured as an inverting buck-boost regulator. Undervoltage Lockout (UVLO) An undervoltage lockout (UVLO) comparator monitors the voltage at the VIN pin (with reference to VSS) and keeps the regulator disabled if the voltage is below the lockout threshold (VIN(START)). The UVLO comparator incorporates enough hysteresis (VIN(HYS)) to prevent on/off cycling of the regulator due to IR drops in the VIN path during heavy loading or during startup. Startup and Shutdown An external logic signal can be applied to the EN pin to control the on/off of LED current. Average brightness of the LED is directly proportional to the duty cycle of the control signal. This technique is commonly known as PWM dimming. If both VIN and VEN are higher than their thresholds, the IC starts up. The reference block starts first, generating stable reference voltages and currents, and then the internal regulator is enabled. The regulator provides stable supply for the remaining circuits. When the EN pin is forced from high to low, the power MOSFET and the error amplifier are turned off, but the IC remains in standby mode for tPWML (20 ms typical) before it completely shuts down. This delay allows PWM dimming frequency down to 100 Hz. In standby mode, the COMP pin is disconnected from the error amplifier and the COMP pin voltage stays at the level before EN turns low. In this way, the steady-state control signal is stored. When the IC receives another EN turn-on signal within tPWML, the system immediately recovers to steady-state operation. As a result, ALT80802 allows down to 15 µs PWM dimming on-time. Three events can shut down the IC: EN low, VIN low, and thermal shutdown. In the shutdown procedure, the power MOSFET is turned off first to avoid any fault triggering. The COMP voltage and the internal supply rail are then pulled down. In buck-boost topology, the average inductor current is the sum of the average input current and output current. When EN is forced off during PWM dimming operation, the power MOSFET is turned off, cutting the connection from inductor to input capacitor. The inductor current will dump all its energy in terms of current to the output capacitor. This current is much higher than the output current as it also contains the input current portion in buck-boost topology. As a result, the output capacitor will MOSFET Driver and Bootstrap Capacitor The position of the internal N-channel power MOSFET requires special consideration when driving it. The source of this MOSFET is connected to the SW node and its voltage can be either close to VIN or VSS. For this reason, a floating gate charge driver is required. This driver requires a voltage greater than VIN to ensure the MOSFET can be turned on. A simple charge pump—consisting of an internal charge circuit, an external capacitor (BST capacitor), and the freewheeling diode —is required to power the high-side gate driver. The internal charge circuit is power by VIN. When the SW node is sufficiently below VIN, the charge circuit will charge the BST capacitor to around 5 V with respect to the SW node. This BST voltage is used to turn the high-side MOSFET on. As the SW node rises, the Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 9 ALT80802 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver BST capacitor will maintain the BST pin at 5 V above SW, ensuring sufficient voltage to keep the MOSFET on. Also, the BST charge circuit incorporates its own UVLO of 1.8 V rising and 0.4 V hysteresis. When BST voltage (with respect to SW pin) is less than UVLO, the power MOSFET is turned off. Frequency Dithering The ALT80802 includes a dithering function, which changes the switching frequency within a certain frequency range. By shifting the switching frequency of the regulator in a triangle fashion around the programmed switching frequency, the overall system noise magnitude can be greatly reduced. The dithering sweep is internally set at ±5%. The switching frequency will ramp from a low of 0.95 times the programmed frequency to a high of 1.05 times the programmed frequency. The rate or modulation at which the frequency sweeps is governed by an internal 12 kHz triangle pattern. Pulse-by-Pulse Current Limit A high-bandwidth current sense amplifier monitors the current in the power MOSFET. The current signal is supplied to the PWM comparator and overcurrent comparator. If the MOSFET current exceeds ILIM, the MOSFET will be turned off. This protects the MOSFET from excessive current and possible damage. Switch Overcurrent Protection and Hiccup Mode A switch overcurrent (OC) counter and hiccup mode circuit protect the regulator when the output of the regulator is shorted to VSS (shorting output capacitor) or when the load current is too high (shorting CS resistor). The OC counter is enabled and begin counting every clock cycle when COMP pin voltage, VCOMP, is clamped at its maximum voltage. If VCOMP remains at its maximum voltage, the counter keeps counting pulses from the overcurrent comparator. If VCOMP decreases, the OC counter is cleared. If the OC counter reaches 120 counts, a hiccup latch is set, and the part enters hiccup mode. In hiccup mode, the COMP pin is quickly pulled down by a relatively low resistance (4 kΩ). Switching is halted for 6 ms to provide time for the device to cool down. The FFn pin is pulled low to indicate a fault condition. After the hiccup off time expires, the device begins a startup sequence. If the fault condition remains, another hiccup cycle occurs. If the fault has been removed, the device starts up normally and the output automatically recovers to target value. Secondary Switch Overcurrent Protection If the switch current continues to rise during the OC counting period, a secondary switch current limit of 7.1 A can be reached and the power MOSFET is turned off. If this secondary overcurrent is detected for more than 1 clock cycle, the hiccup latch is set immediately, and the part enters hiccup mode. This usually happens when SW is shorted to VSS. BOOT Capacitor Protection The ALT80802 monitors the voltage across the BOOT capacitor to detect if the capacitor is missing or short-circuited. If the BOOT capacitor is missing, the device enters hiccup mode after 7 clock cycles. If the BOOT capacitor is shorted, the device enters hiccup mode after 120 clock cycles. If BOOT capacitor voltage is overcharged to more than 6.3 V, BOOT overvoltage protection is triggered, and the IC enters hiccup mode after 7 PWM cycles. Freewheeling Diode Protection If the freewheeling diode is missing or damaged (open), the SW pin is subjected to unusually high negative voltages. This negative voltage may cause the device to malfunction and could lead to damage. The ALT80802 includes protection circuitry to detect when the freewheeling diode is missing. If the SW pin is below −1.25 V for more than 50 ns, the device enters hiccup mode after detecting one missing diode fault. Also, if the freewheeling diode is shorted, the device experiences extremely high currents through the high-side MOSFET. If this occurs, the device triggers a secondary switch current limit and enters hiccup mode. During a diode short-circuit fault in buck-boost topology, VIN is directly connected to VSS pin when the power MOSFET turns on. This might cause a voltage spike from VSS to GND. Note that the maximum rating for GND is –0.3 V with respect to VSS. If the VSS voltage spike is higher than GND, it may cause a logic error in the IC. As a result, for buck-boost topology, a Schottky diode must be connected between VSS to GND to clamp the voltage spike during this fault. Note that the reverse breakdown voltage of the diode must be higher than the maximum output voltage (18 V) and the current rating should be higher than 500 mA. ALT80802 GND VSS Figure 4: VSS to GND Positive Clamp in Buck-Boost Applications Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 10 ALT80802 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver Output Overcurrent Protection The ALT80802 provides an always-on output overcurrent protection that monitors CS pin voltage to protect against extremely high LED current. If CS pin voltage, VCS, rises to 800 mV, the device enters hiccup mode immediately. Output Overvoltage Protection in Buck-Boost In buck-boost topology, during an open LED fault, output current drops to zero and the control loop will try to compensate the loss of current by demanding higher inductor current. Output voltage across the capacitor is charged up immediately. In the ALT80802, an 18 V Zener diode is placed between the positive output (GND) to the negative input of the error amplifier. When output voltage rises to over 18 V, the negative input of the error amplifier is charged up, forcing the inductor current to drop. In this way, output voltage can be clamped to 18 V. However, if the part starts up with an open LED fault, it may take much longer time for the error amplifier to discharge the COMP pin voltage. This delay time may cause the output voltage to rise beyond 20 V, which is higher than the maximum rating for the IC. If inductor current happens to be at a high level, a large current may flow into the IC via the GND pin and the IC may be damaged. To prevent any damage to the IC, it is suggested to use an external circuit, as shown in Figure 5, to stop the switching event before high current flows into the GND pin. 10 kΩ EN or PWM Dimming 100 Ω LED+ Input Ground 40.2 Ω COUT IC Ground EN NPN ALT80802 GND VSS Figure 5: VSS to GND Positive Clamp in Buck-Boost Applications During an open LED fault, the CS pin voltage drops to zero and the FFn pin will be pulled low if the CS pin voltage stays below 150 mV for more than 50 µs. Note that this undervoltage timer is halted during the PWM dimming off period and will resume when the next dimming cycle starts. Thermal Shutdown The ALT80802 protects itself from overheating by means of an internal thermal monitoring circuit. If the junction temperature exceeds the thermal shutdown threshold (TTSD, 170°C typical), the COMP pin will be pulled to VSS and the power MOSFET will be turned off. The ALT80802 will automatically restart when the junction temperature decreases more than the thermal shutdown hysteresis (THYS, 20°C typical). Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 11 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 APPLICATIONS INFORMATION Setting the Switching Frequency The switching frequency (fSW) of a regulator using the ALT80802 can be set by connecting a resistor from the FREQ pin (RFREQ) to VSS. The recommended RFREQ value for various switching frequencies can be obtained from Table 2: Table 2: RFREQ vs. fSW fSW (MHz) RFREQ (kΩ) 2.5 6.34 2.0 8.06 1.8 8.87 1.5 10.7 1.0 16.2 0.8 20.5 0.5 33.2 0.4 41.2 0.3 56.2 0.2 84.5 0.2 619 RFREQ resistor can also be calculated with following equation: mal foldback of the LED current or changing current based on binning resistors. Figure 6 shows the application schematic for adjusting LED current based on binning resistors. In this schematic, R1 is in parallel with R3 and RBIN. These 3 resistors combining with R2 form a resistor divider that raises the voltage across the sense resistor. 1 2 3 4 9 BST VIN ALT80802 LED+ LED+ BIN BIN U1 SW CS EN FFn COMP FREQ GND VSS 5 6 10 R3 8 LED1 LED2 RBIN R1 7 LED- R2 LED- LED3 Rsense Figure 6: Application Circuit Example for Binning Resistors The regulated voltage across RSENSE can be calculated with the following equation: Equation 7: Equation 5: Output current can be calculated with the following equation: where RFREQ is in kΩ and fSW is in MHz. While the ALT80802 can switch at frequencies up to 2.5 MHz, care must be taken when operating at higher frequencies. The minimum controllable on-time for the ALT80802 is around 80 ns. This means that at higher frequencies, high input voltages, and low output voltages, pulse skipping may be seen. Setting the Output Voltage A resistor (RSENSE) from the CS pin to VSS sets the output current. The output current can be calculated with following equation: Equation 6: The bias current of the CS is sufficiently low that is allows for a series resistor between RSENSE and CS pin. This resistor allows the user to perform analog dimming. This can be useful for ther- Equation 8: In this way, the regulated output current can be tuned by changing RBIN. Note that the purpose of R3 is to filter potential high frequency noise coming from the long LED string cable. Inductor To ensure that the inductor operates in continuous mode, the value of the inductor should be set such that half of the peak-topeak inductor current is not greater than the average inductor current. In buck topology, the average inductor current is the average output current. In buck-boost topology, the average inductor current is the sum of average input current and output current. As a result, for buck regulators, the following must be guaranteed: Equation 9: Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 12 ALT80802 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver For buck-boost regulators, the following must be guaranteed: For buck regulators, the peak inductor current can be calculated by: Equation 10: Equation 16: where Dmin is the minimum duty cycle at maximum input voltage. For buck-boost regulators, the peak inductor current can be calculated by: To avoid subharmonic oscillation in the current-mode controlled regulators when duty cycle is greater than 50%, the inductor value should be set to match the slope compensation value at the designed frequency. Equation 17: Slope compensation (SE) will vary with switching frequency. SE can be calculated with the following equation: The saturation current of the inductor should be higher than the pulse-by-pulse current limit of the IC (5.5 A typical). Equation 11: Freewheeling Diode The freewheeling diode allows the current in the inductor to flow to the load when the high-side switch is off. To reduce losses due to the diode forward voltage and recovery times, use a Schottky diode. where SE is in A/µs and fSW is in MHz. The typical value of SE(2MHz) is 3.1 A/µs. For a stable system, the following is recommended: Equation 12: where SLD is the down slope of the inductor. For buck or buckboost regulators: Equation 13: where L is the inductor value in µH. As a result, the following must be guaranteed: Equation 14: The recommended inductor value based on SE can be calculated using the following equation: Equation 15: where Dmax is the maximum duty cycle at minimum input voltage. The current rating of the inductor should be higher than the peak current during operation. In buck topology, the voltage rating of the diode must be higher than the maximum input voltage. The average current rating of the diode must be higher than maximum output current. In buckboost topology, the voltage rating of the diode must be higher than the maximum sum of input voltage and output voltage. The average current rating of the diode must be higher than maximum sum of output current and input current. Note that the peak current of the diode is the peak inductor current. If the application requires PWM dimming, it is recommended to choose a diode with low reverse current IR. During PWM dimming off period, output capacitor voltage is discharged mostly by the reverse current of the diode, especially at high temperature. A smaller IR helps to reduce voltage drop of the output capacitor. Input Capacitor Three factors should be considered when choosing the input capacitors. First, they must be chosen to support the maximum expected input voltage with adequate design margin. Second, their RMS current rating must be higher than the expected RMS input current to the regulator. For simplification, choose the input capacitor with an RMS current rating greater than half of the load current. Generally, a MLCC capacitor can provide enough RMS current with low heat generation. Third, they must have enough capacitance and a low enough ESR to limit the input voltage dv/dt to much less than the hysteresis of the VIN pin UVLO circuitry (350 mV (typ)) at maximum loading and minimum input voltage. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 13 ALT80802 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver The input capacitor(s) must limit the voltage deviations at the VIN pin to something significantly less than the ALT80802 VIN pin UVLO hysteresis during maximum load and minimum input voltage. For buck regulators, the minimum input capacitance can be calculated as: Equation 18: For buck-boost regulators, the minimum input capacitance can be calculated as: Equation 21: where ∆IL is the peak-to-peak inductor current, ILPK is the peak inductor current. To reduce the overall output ripple, it is recommended to use ceramic output capacitors, especially for buck-boost regulators. The ESR and ESL of the ceramic capacitors are virtually zero. If ceramic output capacitors are used, for buck regulators, calculate: Equation 22: Equation 19: For buck-boost regulators, calculate: where ΔVIN is the output capacitor voltage deviation, η is the estimated efficiency of the regulator. ΔVIN should be chosen to be much less than the hysteresis of the VIN pin, UVLO comparator (ΔVIN ≤ 100 mV is recommended). Note that the DC bias on the capacitor can derate the capacitance value. For example, a 50 V, 4.7 µF rated ceramic capacitor can be less than 3 µF when 30 V DC bias is applied. Capacitance value can also change due to temperature. X7R capacitors are recommended for low capacitance variation over temperature. In general, for 2 MHz applications, a 4.7 µF ceramic capacitor with X7R dielectric is sufficient. Output Capacitor The output capacitors filter the output voltage to provide an acceptable level of ripple voltage, and they store energy to help maintain voltage regulation during a transient event. The voltage rating of the output capacitors must support the output voltage with sufficient design margin. The output voltage ripple (ΔVOUT) is a function of the output capacitor parameters: COUT, ESR, and ESL. For buck regulators, the output voltage ripple can be calculated by: Equation 20: For buck-boost regulators, the output voltage ripple can be calculated by: Equation 23: In general, for 2 MHz applications, a 1 µF ceramic output capacitor with X7R dielectric is sufficient. Compensation Components The ALT80802 employs current-mode control for easy compensation and fast transient response. The system stability and transient response are controlled through the COMP pin. The COMP pin is the output of the internal transconductance error amplifier. A series capacitor-resistor combination sets a pole-zero pair to control the characteristics of the control system. Power Stage Vc GCS COUT Error Amplifier Stage COMP CP I OUT RZ CZ RO CS gm RSENSE 0.2 V Figure 7: Basic Current-Mode Control Schematic Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 14 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 The objective of the selection of compensation components is to ensure a high DC gain and wide bandwidth for optimal smallsignal transient response, and adequate margin to avoid instability. As an LED driver or current regulator, output current is the controlled target. The small-signal loop can be modeled as shown in Figure 7, where the loop is broken into two blocks: power stage and error amplifier stage. 1200 1000 IF (mA) 800 di The power stage includes an inner current loop of the currentmode controller, COUT and LED load. Although the peak inductor current is being controlled, to a first approximation for simplifying the equations, it is acceptable to use the output current IOUT. The error amplifier stage includes a current sense resistor RSENSE, an error amplifier, and compensation components. Compensation Design for Buck Regulators dv 1500 600 400 200 2.4 2.6 2.8 3.0 VF (V) 3.2 3.4 3.6 Figure 8: Typical I-V Curve of a White LED Equation 24: There is also a zero in the power stage formed by the ESR of the output capacitor. However, if ceramic capacitors are used, this zero can be ignored. where GCS is the current sense gain of the current amplifier. The typical value of GCS is 9 A/V. For the error amplifier stage, the DC gain of the amplifier is 1000 V/V, and the transconductance gm value is 120 µA/V. The effective output impedance of the error amplifier RO can be given as: The power stage DC gain can be calculated as: The output capacitor integrates the ripple current through the inductor, effectively forming a single pole with the output load. The pole fP(ps) can be found at: Equation 27: Equation 25: The DC gain of the error amplifier is high enough to ensure good output current regulation. The gain is rolled off with a single pole formed by the output impedance of the amplifier RO and the capacitor CZ connected to the COMP pin. The position of this pole is: 1 where RLED is the effective resistance for the LED when conducting target output current IOUT. The small signal LED resistance can be calculated as: 1000 Equation 28: 1 Equation 26: Note that this dv and di can be found by the I-V curve of the LED. For example, if the target output current is 700 mA, dV and dI are set around that level as shown in Figure 8. MΩ A zero is positioned at a higher frequency to cancel the effects of the power stage pole. This zero can be found at: Equation 29: 1 Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 15 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 A second pole is needed to suppressed high-frequency noise. It should be placed far away from the crossover frequency to have minimal effect on the control. This pole can be found at: Equation 32: Equation 30: where D is the duty cycle, and GCS is 9 A/V. 1 The power stage pole can be calculated as: The current sense resistor introduces a DC gain for the control loop, which can be calculated as: where RLED is the effective resistance for the LED. Equation 31: Overall loop response is the combination of the power stage and error amplifier stage. This feedback loop should be designed to have a suitable crossover frequency and phase margin. Gain dB Equation 33: The power stage also includes a right half plane zero, which frequency can be calculated as: Equation 34: where L is the inductor in the power stage. Power Stage Recommended Control Loop Design Strategy Gain dB f P(ps) Gain dB Overall Loop –20 dB/decade fC Frequency f P1(ea) f Z(ea) f P2(ea) Frequency Error Amplifier Stage Figure 9: Basic Current-Mode Control Schematic It is recommended to achieve a –20 dB/decade roll-off for the overall loop, which means that the error amplifier zero should be placed at the same frequency of the power stage pole. Figure 9 shows recommended gain plot of the power stage, the error amplifier stage, and the combined overall loop response. Compensation Design for Buck-Boost Regulators The compensation design for buck-boost regulators follows the same idea as the buck. The error amplifier stage of the buckboost regulators is the same as the buck. The only difference is the power stage response. 1. Choose a crossover frequency fC to be 1/10 of the switching frequency fSW. However, the maximum fC should be set below 75 kHz to have good noise suppression. For buck-boost regulators, cross-over frequency should be less than 1/5 of the right half plane zero frequency. 2. Calculate DC gain of the overall loop in dB, which is: GLOOP(dB) = GPS(dB) + gm(dB) + GFB(dB) 3. The estimated –20 dB/decade roll-off slew rate from the first amplifier pole to the crossover frequency will set the position of the pole fP1(ea). Calculate the CZ value. 4. Calculate the position of the power stage pole fP(ps). 5. Set the error amplifier zero fZ(ea) to be at the same frequency of the power stage pole. Calculate the RZ value. If the power stage pole fP(ps) is significantly higher than the crossover frequency (more than 5×), RZ can be removed. However, RZ is helpful in instant transient response. 6. Set the high frequency error amplifier pole to be higher than the error amplifier zero and calculate the CP value. Typically, choose a CP value between 22 pF and 39 pF. 7. If possible, test the overall loop bode plot of the system. Adjust the RZ and CZ to fine-tune the control loop crossover frequency and phase margin. Typically, phase margin should be more than 45 degrees to guarantee stability. The power stage DC gain of buck-boost regulators can be calculated as: Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 16 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Design Example can be calculated as: Equation 39: Buck LED Driver This example application is a buck LED driver using the ALT80802. The operating voltage range is 9 to 18 V, nominal input voltage is 12 V, and the target switching frequency is 2 MHz; the output load is 2 white LEDs (LUW CQAR) with 3 V forward voltage; target output current is 700 mA. The power stage DC gain can be calculated as: Equation 40: To set the output current to 700 mA, current sense resistor RSENSE is: The current sense DC gain is: Equation 35: Equation 41: Note that 280 mΩ is the common resistor value with 1% accuracy. As a result, 280 mΩ is chosen. The DC gain of the error amplifier is 1000 V/V. The nominal duty cycle can be calculated as: Equation 42: Equation 36: 6 12 To guarantee CCM operation over all input range, inductor L must satisfy: The overall loop gain can be calculated as: with estimated –20 dB/decade roll-off slew, the position of the first amplifier pole can be calculated as: Equation 43: 40 kHz Equation 37: 10 The CZ value can be calculated as: A 3.3 µH inductor can be selected. The down slope of the inductor can be calculated as: Equation 44: 1 Equation 38: 3.1 1.82 ⁄ The slope compensation to inductor down slope ratio is within the range of 0.5 to 2. As a result, the slope compensation should have little influence on the overall loop response. Input capacitors and output capacitors are selected to the standard values of 4.7 µF and 1 µF. For a 2 MHz design, the maximum crossover frequency should be set below 75 kHz. The crossover frequency can be set to 40 kHz in this application. From LUW CQAR datasheet, the small signal LED resistance 1 To match the standard capacitor value, a 1.5 nF CZ can be selected. The power stage pole can be calculated as: Equation 45: 1 1 This power stage pole is at much higher frequency than the designed crossover frequency. As a result, RZ is not needed. However, to improve the instant response, a 2.49 kΩ resistor is selected. For CP, a typical value of 22 pF can be chosen to suppress high frequency noise. If PWM binning applications, the binning circuit resistors and binning resistor selected by the design tools. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 17 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Equation 50: Buck-Boost LED Driver This example application is a buck-boost LED driver using the ALT80802. The operating voltage range is 6 to 18 V, nominal input voltage is 12 V, and the target switching frequency is 2 MHz; the output load is 4 white LEDs (LUW CQAR) with 3 V forward voltage; target output current is 350 mA. To set the output current to 700 mA, current sense resistor RSENSE is: Equation 46: The right half plane zero of buck-boost regulators can be calculated as: Equation 51: As a result, the crossover frequency can be set at 20 kHz. The power stage DC gain can be calculated as: Note that 560 mΩ is the common resistor value with 1% accuracy. As a result, 560 mΩ is chosen. The nominal output current with 560 mΩ is 357 mA. Equation 52: The nominal duty cycle can be calculated as: The current sense DC gain is: Equation 47: Equation 53: To guarantee CCM operation over all input range, inductor L must satisfy: The DC gain of the error amplifier is 1000 V/V. Equation 48: 0.5 1.5 The overall loop gain can be calculated as: Equation 54: To reduce the output ripple, a 4.7 µH inductor can be selected. The down slope of the inductor can be calculated as: With estimated –20 dB/decade roll-off slew, the position of the first amplifier pole can be calculated as: Equation 49: Equation 55: 3.1 2.55 ⁄ The slope compensation to inductor down slope ratio is within the range of 0.5 to 2. As a result, the slope compensation should have little influence on the overall loop response. Input capacitors and output capacitors are selected to be the standard values of 4.7 µF and 1 µF. As a buck-boost regulator, crossover frequency should be less than 1/5 of the right half plane zero. 20 kHz 10 The CZ value can be calculated as: Equation 56: 1 1 To match the standard capacitor value, a 1.5 nF CZ can be selected. The power stage pole can be calculated as: Equation 57: From the LUW CQAR datasheet, the small signal LED resistance can be calculated as: Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 18 ALT80802 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver This power stage pole is at much higher frequency than the designed crossover frequency. As a result, RZ is not needed. However, to improve the instant response, a 2.49 kΩ resistor is selected. For CP, a typical value of 22 pF can be chosen to suppress high frequency noise. For PWM binning applications, the binning circuit resistors and binning resistor can be selected by the design tools. To improve the EMI/EMC performance, a 100 nF capacitor should be placed between VIN and VSS to supply the Boot capacitor during boot charging transients. Note that this capacitor cannot be too large, or it will affect the output stability during VIN transients. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 19 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 TYPICAL APPLICATION SCHEMATICS C4 220 nF U1 BST VIN EN/PWM FFn C1 4.7 µF C3 100 nF VIN SW EN CS ALT80802 FFn R1 8.06 kΩ EXT (5 V Supply) LED+ D1 R4 0.28 Ω C5 1.5 nF VSS GND LED+ LED– COMP FREQ C2 R2 100 nF 10 kΩ L1 3.3 µH R3 2.49 kΩ LED1 C8 100 nF C7 1.0 µF LED2 LED– C6 22 pF LED MODULE 2 White LEDs (Vf = 3.05 V) GND DC Input Voltage Range 9-18 V PWM Dimming Range (200 Hz) 1%-100% Figure 10: 2 MHz, 700 mA, 2 LEDs Buck LED Driver with Fault Flag and PWM Dimming C1 VIN GND C2 4.7 µF C4 EXT (5 V Supply ) D1 U1 BST VIN FFn R5 10 kΩ R1 8.06 kΩ Suggested Circuit for Open LED Fault during Startup Event GND LED 2 COMP EN VSS D2 LED 3 C5 1.5 nF R2 2.49 kΩ NPN R7 100 Ω LED + LED 1 LED – CS FREQ EN/PWM C8 100 nF C7 0.47 µF SW ALT80802 FFn LED + 4.7 µH 220 nF R3 10 kΩ C3 100 nF L1 100 nF C6 22 pF R4 0.56 Ω LED 4 LED – LED MODULE 2 White LEDs (Vf = 3.05 V) R6 40.2 Ω DC Input Voltage Range 6-18 V PWM Dimming Range (200 Hz) 1%-100% Figure 11: 2 MHz, 350 mA, 4 LEDs Inverting Buck-Boost LED Driver with Fault Flag and PWM Dimming Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 20 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 PCB COMPONENT PLACEMENT AND ROUTING Buck LED Driver A good PCB layout is critical for the ALT80802 to provide clean, stable output voltages. Figure 12 shows a typical ALT80802-based buck LED driver schematic with the critical power paths/loops. Figure 13 shows an example PCB component placement and routing with the same critical power paths/loops as shown in the schematic. Follow these guidelines to ensure a good PCB layout. 1. The high di/dt pulsating current loop for a buck regulator is formed by the ceramic input capacitor (C1 and C2), power MOSFET inside of the IC, and freewheeling diode (D1). These components must be closely placed with wide traces and the loop area must be minimized. Ideally, these components are all connected using only the top metal layer. 2. Another pulsating current loop is the boot charging path which includes the input capacitor (C1 and C2), boot charge capacitor (C4), and freewheeling diode. The current of this 3. 4. 5. 6. loop should be less than 300 mA, and the trace width should be set accordingly. A 100 nF capacitor, C3, from VIN to GND provides a solid ground reference for the input of the internal LDO. This capacitor should be placed close to VIN pin and VSS pin of the IC. VSS and GND pins should be tied together with a single solid ground plane. Note that to ensure the lowest junction temperature, multiple vias are recommended to connect the thermal pad to the bottom layer ground plane. Compensation components, FSET resistor, and current sense resistors should be connected close to the IC with clean ground reference. SW node is a high dv/dt node. This high dv/dt copper area should be minimized to reduce any voltage coupling to the other layers. C4 220 nF U1 EN/PWM FFn C3 C2 C1 4.7 µF 100 nF 100 nF R2 10 kΩ EXT (5 V Supply) GND 2 BST VIN L1 VIN SW EN CS ALT80802 FFn GND R1 8.06 kΩ 1 LED– LED+ D1 COMP FREQ VSS LED+ 3.3 µH R4 0.28 Ω C5 1.5nF R3 2.49 kΩ C6 22 pF C7 1.0 µF C8 100 nF LED1 LED2 LEDLED MODULE 2 White LEDs (Vf = 3.05 V) Figure 12: Typical Buck LED Driver Application with Critical Loops Shown Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 21 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 LOOP 1 (RED) LOOP 2 (BLUE) This loop contains the main switching frequency pulsating current during operation. The loop area should be minimized to reduce the loop inductance and noise antenna size. This loop contains pulsating current when the Boot capacitor is charged. The frequency of this pulsating current can also be as high as the switching frequency. The loop area should be minimized. The turn-on and turn-off of the power MOSFET will generate high di/dt transients. Parasitic inductance within this loop will cause oscillation during these transients. Also, the peak current in this loop can be as high as 5.5 A. It is recommended to use short and wide traces to reduce the parasitic inductance and resistance. 1 2 Figure 13: Example PCB Layout for Buck LED Driver Application Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 22 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Buck-Boost LED Driver Figure 14 shows a typical ALT80802-based buck-boost LED driver schematic with the critical power paths/loops. Figure 15 shows an example PCB component placement and routing with the same critical power paths/loops as shown in the schematic. Follow the following guidelines to ensure a good PCB layout. ceramic capacitor should be connected as close as possible to the IC. The solid ground reference for the IC is VSS instead of GND. In buck topology, these two pins should be tied together with a single solid ground plane. In buck-boost topology, these two pins are completely separated. The ground plane on the PCB should be tied to VSS pin and thermal pad of the IC. Note that to ensure the lowest junction temperature, multiple vias are recommended to connect the thermal pad to the bottom layer ground plane. Compensation components, FSET resistor, and current sense resistors should be connected close to the IC with clean ground reference. The clamping diode from VSS to GND should be connected close to these two pins. SW node is a high dv/dt node. This high dv/dt copper area should be minimized to reduce any voltage coupling to the other layers. 3. 1. The high di/dt pulsating current loop for a buck-boost regulator is formed by the ceramic input capacitor (C1 and C2), power MOSFET inside of the IC, freewheeling diode (D1), and the ceramic output capacitor (C7 and C8). These components need to be placed closely with wide traces and the loop area needs to be minimized. Ideally, these components are all connected using only the top metal layer. 2. Another pulsating current loop is the boot charging path which includes the VIN to VSS ceramic capacitor (C3), boot charge capacitor, and freewheeling diode. The current of this loop should be less than 300 mA, and the trace width should be set accordingly. The boot capacitor and the VIN to VSS 4. 5. 6. C1 VIN 4.7 µF C4 1 GND C2 100 nF L1 LED+ 4.7 µH 220 nF C8 100 nF D1 U1 BST VIN SW EN FFn C3 100 nF 2 R3 10 kΩ EXT (5 V Supply) LED2 COMP FREQ VSS GND R1 8.06 kΩ LED– CS FFn D2 LED+ LED1 ALT80802 EN/PWM C7 0.47 µF LED3 C5 1.5 nF R2 2.49 kΩ C6 22 pF R4 0.56 Ω LED4 LED– LED MODULE 4 White LEDs (Vf = 3.05 V) Figure 14: Typical Buck-Boost LED Driver Application with Critical Loops Shown Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 23 ALT80802 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver LOOP 1 (RED) LOOP 2 (BLUE) This loop contains the main switching frequency pulsating current during operation. The loop area should be minimized to reduce the loop inductance and noise antenna size. This loop contains pulsating current when the Boot capacitor is charged. The frequency of this pulsating current can also be as high as the switching frequency. The loop area should be minimized. The turn-on and turn-off of the power MOSFET will generate high di/dt transients. Parasitic inductance within this loop will cause oscillation during these transients. Also, the peak current in this loop can be as high as 5.5 A. It is recommended to use short and wide traces to reduce the parasitic inductance and resistance. 1 2 c Figure 15: Example PCB Layout for Buck-Boost LED Driver Application Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 24 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 APPLICATION CIRCUIT EXAMPLES Application 1: CHMSL with 10 Red LEDs C1 VIN 4.7 µF C4 EXT (5 V Supply) SW FFn CS VSS GND D2 LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED9 LED10 C8 100 nF LED– LED MODULE 10 Red LEDs Vf = 2.32 V C5 1.5 nF R3 2.49 kΩ Q1 Suggested Circuit R7 for Open LED 100 Ω Fault during Startup Event C7 1 µF COMP EN R5 10 kΩ LED+ 4.7 µH ALT80802 EN/PWM LED+ L1 BST FREQ R1 8.06 kΩ 100 nF D1 VIN FFn C2 100 nF C3 220 nF U1 R2 10 kΩ GND R4 0.4 Ω C6 22 pF R6 40.2 Ω LED– DC Input Voltage Range 6-18 V PWM Dimming Range (200 Hz) 1%-100% Figure 16: 2 MHz, 250 mA, 10 Red LEDs Inverting Buck-Boost LED Driver with Fault Flag Application 1: Recommended Bill of Materials Reference Description C1 4.7 µF, ceramic capacitor, X7R, 50 V, 1210 C2, C3, C8 100 nF, ceramic capacitor, X7R, 50 V, 0603 C4 220 nF, ceramic capacitor, X7R, 16 V, 0402 or 0603 C5 1.5 nF, ceramic capacitor, X7R, 16 V, 0603 C6 22 pF, ceramic capacitor, X7R, 16 V, 0603 C7 1 µF, ceramic capacitor, X7R, 50 V, 0805 R1 8.06 kΩ resistor, 1/10 W, 1% R2 10 kΩ resistor, 1/10 W, 1% R3 2.49 kΩ resistor, 1/10 W, 1% Manufacturer/Part Number R4 400 mΩ resistor, 1/2 W, 1% R6 40.2 Ω resistor, 1/10 W, 1% R7 100 Ω resistor, 1/10 W, 1% D1 Diode, Schottky, 60 V, 5 A, 670 mV @ 5 A Diodes Incorporated, PDS560-13 D2 Diode, Schottky, 40 V, 1 A, 410 mV @ 1 A Diodes Incorporated, 1N5819HW-7-F Q1 Transistor, NPN, 65 V, 0.1 A, SOT23 On Semiconductor, BC846ALT1G L1 Inductor, 4.7 µH, 9.8 A(sat), 15.32 mΩ (max) Vishay, IHLP4040DZER4R7M8A Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 25 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Application 1: Performance System Efficiency with Full Brightness LED Current Line Regula�on Switching Waveform SW 10V/div ILED 200mA/div Time: 500ns/div Startup Waveform 10% LED Dimming Waveform 6-18 V Fast VIN Transient VIN 5V/div VIN 5V/div EN 5V/div VIN 5V/div ILED 200mA/div ILED 200mA/div Time: 5ms/div 200mA/div Time: 500µs/div Time: 1ms/div Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 26 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Application 2: Buck-Boost LED Driver with Binning Resistor C1 VIN C2 4.7 µF 100 nF C4 220 nF BST VIN SW FFn CS R4 ALT80802 FREQ R1 8.06 kΩ EN VSS GND D2 Suggested Circuit for Open LED Fault during Startup Event R2 2.49 kΩ Q1 R9 100 Ω LED+ R5 1 kΩ R3 2 kΩ C5 1.5 nF C6 22 pF C8 100 nF BIN 120 Ω COMP EN/PWM R7 10 kΩ C7 0.47 µF D1 U1 C3 100 nF LED+ L1 4.7 µH LED1 BIN LED– LED2 RBIN R6 0.82 Ω LED– R8 40.2 Ω LED3 LED4 LED MODULE 4 White LEDs (Vf = 3.05 V) Input Range 6-18 V PWM Dimming Range (200 Hz) 1%-100% Binning resistor values for LED current reduction: LED Current 100% 90% 80% 70% RBIN Open 2.21 kΩ 549 Ω Short Figure 17: 2 MHz, 350 mA Inverting Buck-Boost LED Driver for 1-4 LEDs with Binning Resistor on LED Module Application 2: Recommended Bill of Materials Reference Description C1 4.7 µF, ceramic capacitor, X7R, 50 V, 1210 C2, C3, C8 100 nF, ceramic capacitor, X7R, 50 V, 0603 C4 220 nF, ceramic capacitor, X7R, 16 V, 0402 or 0603 C5 1.5 nF, ceramic capacitor, X7R, 16 V, 0603 C6 22 pF, ceramic capacitor, X7R, 16 V, 0603 C7 1 µF, ceramic capacitor, X7R, 50 V, 0805 R1 8.06 kΩ resistor, 1/10 W, 1% R2 2.49 kΩ resistor, 1/10 W, 1% R3 2 kΩ resistor, 1/10 W, 1% Manufacturer/Part Number Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 27 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Application 2: Recommended Bill of Materials (continued) Reference Description R4 120 Ω resistor, 1/10 W, 1% R5 120 Ω resistor, 1/10 W, 1% R6 820 mΩ resistor, 1/2 W, 1% R7 10 kΩ resistor, 1/10 W, 1% Manufacturer/Part Number R8 40.2 Ω resistor, 1/10 W, 1% D1 Diode, Schottky, 60 V, 5 A, 670 mV @ 5 A Diodes Incorporated, PDS560-13 D2 Diode, Schottky, 40 V, 1 A, 410 mV @ 1 A Diodes Incorporated, 1N5819HW-7-F Q1 Transistor, NPN, 65 V, 0.1 A, SOT23 On Semiconductor, BC846ALT1G L1 Inductor, 4.7 µH, 9.8 A(sat), 15.32 mΩ (max) Vishay, IHLP4040DZER4R7M8A Application 2: Performance System Efficiency with Full Brightness LED Current Line Regula�on Switching Waveform SW 10V/div ILED 200mA/div Time: 500ns/div Startup Waveform 10% LED Dimming Waveform 6-18 V Fast VIN Transient VIN 5V/div VIN 5V/div EN 5V/div VIN 5V/div ILED 200mA/div 200mA/div ILED 200mA/div Time: 5ms/div Time: 500µs/div Time: 1ms/div Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 28 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Application 3: High Input Voltage Buck with 8 White LEDs C5 220 nF U1 BST VIN VIN EN/PWM FFn C1 4.7 µF C2 4.7 µF C3 100 nF L1 SW ALT80802 EN FFn GND D1 COMP EXT (5 V Supply) R4 0.56 Ω C6 4.7 nF VSS R1 41.2 kΩ LED+ LED1 LED- CS FREQ C4 R2 100 nF 10 kΩ LED+ 47 µH R3 3.24 kΩ C8 4.7 µF LED MODULE 8 White LEDs (Vf = 3.05 V) GND PWM Dimming Range (200 Hz) LED8 LED- C7 22 pF Input Range C9 100 nF 28-36 V 5%-100% Figure 18: 32 VIN, 400 kHz, 350 mA, 8 White LEDs Buck LED Driver with Fault Flag Application 3: Recommended Bill of Materials Reference Description C1, C2, C8 4.7 µF, ceramic capacitor, X7R, 50 V, 1210 C3, C4, C9 100 nF, ceramic capacitor, X7R, 50 V, 0603 C5 220 nF, ceramic capacitor, X7R, 16 V, 0402 or 0603 C6 4.7 nF, ceramic capacitor, X7R, 16 V, 0603 C7 22 pF, ceramic capacitor, X7R, 16 V, 0603 R1 41.2 kΩ resistor, 1/10 W, 1% R2 10 kΩ resistor, 1/10 W, 1% R3 3.24 kΩ resistor, 1/10 W, 1% R4 560 mΩ resistor, 1/4 W, 1% D1 Diode, Schottky, 60 V, 5 A, 670 mV @ 5 A L1 Inductor, 47 µH, >5 A(sat) Manufacturer/Part Number Diodes Incorporated, PDS560-13 Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 29 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Application 3: Performance System Efficiency with Full Brightness LED Current Line Regula�on Switching Waveform SW 10V/div ILED 200mA/div Time: 2µs/div Startup Waveform 20% LED Dimming Waveform 6-18 V Fast VIN Transient VIN 10V/div EN 5V/div VIN 5V/div EN 2V/div ILED 200mA/div 200mA/div ILED 200mA/div Time: 500µs/div Time: 500µs/div Time: 2ms/div Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 30 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Current Sense Block with NTC and Binning Resistor Example 1 2 3 4 9 BST U1 VIN SW EN CS ALT80802 FFn COMP 10 R3 30.1 Ω LED+ LED+ BIN BIN CS CS 8 7 R1 1.58 kΩ 5 RBIN NTC FREQ GND LED1 LED2 LED3 LED– LED– VSS 6 R2 1.0 kΩ Rsense 1.47 Ω NTC: 47 kΩ B57352V5473H060 Binning resistor values for LED current reduction BIN1 BIN2 BIN3 LED Current 100% 90% 80% RBIN Open 8.06 kΩ 3.24 kΩ Figure 19: 350 mA Inverting Buck-Boost LED Driver with Binning Resistor on LED Module Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 31 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 PACKAGE OUTLINE DRAWING For Reference Only – Not for Tooling Use (Reference JEDEC MO-229) Dimensions in millimeters – NOT TO SCALE Exact case and lead configuration at supplier discretion within limits shown 0.30 3.00 ±0.05 0.50 10 10 0.85 3.00 ±0.05 1.64 3.10 A 1 2 DETAIL A 10X D 0.05 1 0.75 ±0.05 C 0.25 ±0.05 SEATING PLANE C PCB Layout Reference View 0.05 0.00 0.5 BSC 1 2.38 C 0.40 ±0.10 0.08 REF 2 0.203 REF 0.40 ±0.10 0.05 REF Detail A 1.65 ±0.10 B 0.05 REF A Terminal #1 mark area B Exposed thermal pad (reference only, terminal #1 identifier appearance at supplier discretion) 10 2.38 ±0.10 C Reference land pattern layout (reference IPC7351 SON50P300X300X80-11WEED3M); all pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary to meet application process requirements and PCB layout tolerances; when mounting on a multilayer PCB, thermal vias at the exposed thermal pad land can improve thermal dissipation (reference EIA/JEDEC Standard JESD51-5) D Coplanarity includes exposed thermal pad and terminals Figure 20: Package EJ, 10-Pin DFN with Exposed Thermal Pad and Wettable Flank Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 32 Wide Input Voltage, Adjustable Frequency, Buck or Buck-Boost 2 Amp LED Driver ALT80802 Revision History Number Date Description – September 10, 2018 Initial release Copyright ©2018, Allegro MicroSystems, LLC Allegro MicroSystems, LLC reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the information being relied upon is current. Allegro’s products are not to be used in any devices or systems, including but not limited to life support devices or systems, in which a failure of Allegro’s product can reasonably be expected to cause bodily harm. The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its use; nor for any infringement of patents or other rights of third parties which may result from its use. Copies of this document are considered uncontrolled documents. For the latest version of this document, visit our website: www.allegromicro.com Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Downloaded from Arrow.com. 33