A80602KESJSR

A80602 and A80602-1 High Power LED Driver with Pre-Emptive Boost for Ultra-High Dimming Ratio and Low Output Ripple FEATURES AND BENEFITS DESCRIPTION • Automotive AEC-Q100 qualified • Enhanced fault handling for ASIL B system compliance • Wide input voltage range of 4.5 to 40 V for start/stop, cold crank, and load dump requirements • Operate in Boost or SEPIC mode for flexible output • Gate driver for external NMOS to deliver higher output power • Six integrated LED current sinks, up to 140 mA each • Boost switching frequency synced externally or programmed from 200 kHz to 2.3 MHz • Programmable boost frequency dithering to reduce EMI • Advanced control allows minimum PWM on-time down to 0.3 µs, and avoids MLCC audible noises • LED contrast ratio: 15,000:1 at 200 Hz using PWM dimming alone, 150,000:1 when combining PWM and analog dimming • Excellent input voltage transient response even at lowest PWM duty cycle • Gate driver for optional PMOS input disconnect switch • Extensive fault protection features The A80602 is a multi-output LED driver for automotive applications such as exterior lighting, heads-up display, and midsize LCD backlighting. It implements a current-mode boost/ SEPIC converter with gate driver for external N-MOSFET. This allows greater output power even at minimum supply voltage. The A80602 provides six integrated current sinks driving up to 140 mA per string. Multiple sinks can be paralleled together to achieve higher LED currents up to 840 mA. The IC operates from single power supply from 4.5 to 40 V; once started, it can continue to operate down to 4 V. This allows it to withstand stop/start, cold crank, and load dump conditions encountered in automotive systems. The A80602 can control LED brightness through external PWM signal. By using the patented Pre-Emptive Boost control, an LED brightness contrast ratio of 15,000:1 can be achieved using PWM dimming at 200 Hz. A higher ratio of 150,000:1 is possible when using a combination of PWM and analog dimming. Continued on next page... PACKAGE: APPLICATIONS 24-Pin 4 mm × 4 mm QFN with Wettable Flank • Automotive infotainment backlighting • Automotive heads-up display • Automotive interior and exterior lighting Not to scale VIN L1 RSENSE Cin VOUT *optional D1 Q1 RADJ ROVP CDRV RGDRV RCS COUT GATE Vsense Vc CVDD RPU VDRV GDRV VIN LED1 A80602 Up to 11 WLEDs in series Up to 140 mA/channel Combine to drive up to 840 mA total LED2 EN/PWM LED3 ... PWM tON ≥ 0.3 µs PGND OVP VDD FAULT 100% CS ADIM LED6 AGND COMP PEB iLED ADIM APWM 100 kHz 0-90% ISET FSET DITH CP PEB = Pre-Emptive Boost (RD controls the delay time) DITH = Dithering Control (Modulation frequency and range) RISET RFSET RM CM RD RZ CZ Figure 1: A80602 in Boost configuration where VOUT is higher than VIN A80602-DS MCO-0000620 March 4, 2019 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple DESCRIPTION (continued) Switching frequency can be externally synchronized or programmed between 200 kHz and 2.3 MHz. This allows operation either above or below the AM band. A programmable dithering feature further reduces EMI. The A80602 provides protection against output short, overvoltage, open- or shorted-LED pin, and overtemperature. A cycle-by-cycle current limit protects the external boost switch against high current overloads. An external P-MOSFET can optionally be used to disconnect input supply in case of output to ground short fault. The A80602-1 is similar to A80602 except it adopts ‘One-Out-All-Out’ fault handling (See Fault Table section for details). SELECTION GUIDE [1] Part Number Fault Handling A80602KESJSR One-Out-Continue A80602KESJSR-1 One-Out-All-Out [1] Contact Allegro LED Driver Package Packing 6 × 140 mA 24-pin 4 × 4 mm wettable flank QFN with exposed thermal pad and sidewall plating 6000 pieces per reel for additional packing options. ABSOLUTE MAXIMUM RATINGS [2] Characteristic Symbol LEDx Pin VLEDx OVP pin VOVP VIN VSENSE, GATE VDRV, GDRV CS Notes Rating –0.3 to 40 V –0.3 to 40 V VIN –0.3 to 40 V VSENSE, VGATE Higher of –0.3 and (VIN – 7.4) to VIN +0.4 V VDRV, VGDRV –1.0 to 7.5 V VCS –0.3 to 7 V External input signals must not be higher than VIN + 0.4 V –0.3 to 5.5 V Range K –40 to 125 °C EN/PWM, FAULT, ADIM, COMP, DITH, PEB, FSET, ISET, VDD x = 1..6 Unit Operating Ambient Temperature TA Maximum Junction Temperature TJ(max) 150 °C Tstg –55 to 150 °C Storage Temperature [2] 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. THERMAL CHARACTERISTICS: May require derating at maximum conditions; see application information Characteristic Package Thermal Resistance [4] Additional Symbol RθJA Test Conditions [4] Value Unit 37 °C/W ES package measured 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 2 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Table of Contents PWM Dimming................................................................ 18 Pre-Emptive Boost (PEB).................................................. 19 Analog Dimming.............................................................. 20 ADIM Mode................................................................. 20 APWM Mode............................................................... 21 Extending LED Dimming Ratio.......................................... 22 Analog Dimming with External Voltage............................... 23 VDD............................................................................... 24 VDRV............................................................................. 24 Shutdown....................................................................... 24 Fault Detection and Protection.............................................. 25 FAULT Status.................................................................. 25 LED String Partial-Short Detect......................................... 27 Overvoltage Protection..................................................... 28 Boost Switch Overcurrent Protection.................................. 29 Input Overcurrent Protection and Disconnect Switch............ 30 Setting the Input Current Sense Resistor............................ 31 Input UVLO..................................................................... 31 Fault Protection During Operation...................................... 31 Package Outline Drawing..................................................... 34 Appendix: External MOSFET Selection Guide........................ 35 Features and Benefits............................................................ 1 Description........................................................................... 1 Applications.......................................................................... 1 Package.............................................................................. 1 Selection Guide.................................................................... 2 Absolute Maximum Ratings.................................................... 2 Thermal Characteristics......................................................... 2 Typical Application – SEPIC................................................... 3 Functional Block Diagram...................................................... 4 Pinout Diagram and Terminal List............................................ 5 Electrical Characteristics........................................................ 6 Functional Description..........................................................11 Enabling the IC.................................................................11 Powering Up: LED Detection Phase................................... 12 Powering Up: Boost Output Undervoltage........................... 14 Soft Start Function........................................................... 14 Frequency Selection......................................................... 15 Synchronization............................................................... 15 Loss of External Sync Signal............................................. 16 Switching Frequency Dithering.......................................... 17 LED Current Setting......................................................... 18 L2 Breakdown voltage of Q1 and D1 must be > VIN + VOUT VIN D1 L1 CC CIN Q1 ROVP COUT1 CDRV VC RPU GATE Vsense CVDD VDRV GDRV CS VIN COUT2 PGND OVP VDD A80602 FAULT LED1 Up to 11 WLEDs in series Up to 140 mA/ch. Combine to drive up to 840 mA total LED2 LED3 EN/PWM ... PWM t on ≥ 0.3 µs VOUT ≤ 40 V LED6 ADIM/ APWM 100% iLED ADIM APWM 100kHz 0-90% AGND ISET FSET DITH COMP PEB CP RISET RFSET RDITH CDITH RPEB RZ CZ Figure 2: A80602 in SEPIC configuration where VOUT can be either higher or lower than VIN Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 3 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple External SYNC RFSET VSENSE CDITH RDITH Oscillator CDRV Boost Enable COMP Comparator RCS PGND 1 MHz 4 MHz OCP2 System oscillator TSD VOUT FSET or ISET pin Open/Short Internal VDD (4.25 V) CVDD Regulator UVLO Block ROVP 1.235 V REF OVP sense Vref AGND Open/Short LED Detect Input current sense amp Chopping freq = 4 MHz VIN On/Off GATE GATE OFF Boost Enable PMOS Driver Current level LED1 LED Driver Block LED2 LED6 AGND Enable ... iADJ ... + VSENSE Clock detector Vref Int VDD EN/PWM 50 kΩ 1 MHz Keep-Alive Timer ISET Block LED Enable start PEB Pre-Emptive Boost External FAULT Internal FAULT delay A80602 OVP Fault Block Enable RPEB CS ÷4 LED1 . . LED4 VDD External PWM 100 Hz – 20 KHz GDRV Current sense Soft Start Ramp (8 ms) VLED Multi-input Error Amp RSENSE NMOS Gate Drive PGND ref VIN VDRV Enable fSW CCOMP GATE Gate Driver LDO (6.5 V) VIN Frequency dithering COMP VOUT L1 DITH FSET/SYNC SW 50 kΩ ADIM/ APWM ISET RISET VDD RPU FAULT AGND Figure 3: Functional Block Diagram Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 4 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple 20 CS 19 VDRV 21 GDRV 22 GATE 23 VSENSE 24 VIN PINOUT DIAGRAM AND TERMINAL LIST FAULT 1 18 PGND VDD 2 17 OVP AGND 3 16 LED1 PAD 15 LED2 LED6 11 LED5 12 EN/PWM 10 13 LED4 9 6 ADIM PEB 8 14 LED3 7 5 DITH 4 ISET FSET COMP Package ES, 24-Pin QFN Pinouts Terminal List Table Number 1 Name FAULT Function This pin is an open drain type configuration that will be pulled low when a fault occurs. Connect a pull-up resistor between this pin and desired logic level voltage. Output of internal LDO (bias regulator). Connect a 1 µF decoupling capacitor between this pin and AGND. VDD is regulated at ~4.25 V. 2 VDD 3 AGND LED current Ground. Also serves as ‘quiet’ ground for analog signals. 4 COMP Output of the error amplifier and compensation node. Connect a series RZ-CZ network from this pin to AGND for control loop compensation. 5 ISET Connect RISET resistor between this pin and AGND to set the 100% LED current. 6 PEB Pre-Emptive Boost control: Connect resistor from PEB pin to AGND to fine-tune the delay between boost switch and LED current sinks. Leave pin open for minimum PEB delay of 1 µs. 7 DITH Dithering control: connect a capacitor to AGND to set the dithering modulation frequency (1 to 22 kHz). Connect a resistor between DITH and FSET pins to set the dithering range (such as ±5% of fSW). 8 FSET Frequency/synchronization pin. A resistor RFSET from this pin to AGND sets the switching frequency fSW (with dithering superimposed) between 200 kHz and 2.3 MHz. It can also be used to synchronize fSW to an external frequency between 260 kHz and 2.3 MHz (frequency dithering is disabled in this case). 9 ADIM Analog dimming. Apply a PWM clock (40 to 1000 kHz) to pin and the duty cycle of this clock determines the LED current. Alternatively, apply DC level between 0.2 and 2 V to vary LED current between 10% and 100%. If unused, pull pin above 2 V for 100%. 10 EN/PWM Enables the IC when this pin is pulled high. It also controls the on/off state of LED current sinks to reduce the light intensity by using pulse width modulation. Typical PWM dimming frequency is in the range of 200 Hz to 2 kHz. When this pin is pulled low, the IC remains in standby mode for up to 16 ms, then shuts down completely. 11-16 LED6..LED1 LED current sinks #6 to #1. Connect the cathode of each LED string to pin. Unused LED pin must be terminated to AGND through a resistor (4.75 kΩ for LED1/3/4/6, 2.37 kΩ for LED2/5). 17 OVP 18 PGND Power Ground for internal Gate Driver. Connect pin to external power GND with shortest path. 19 VDRV Gate driver supply voltage (~6.5 V). Connect a 2.2 µF MLCC to PGND for buffer. 20 CS 21 GDRV 22 GATE Overvoltage Protection. Connect external resistor from VOUT to this pin to adjust the over voltage protection threshold. Current Sense for peak current control of power switch. Connect to sense resistor at the Source terminal of external power MOSFET. Gate driver for power switch. Connect to Gate of external power MOSFET. (External FET must be fully enhanced at VGS = 5 V). Output gate driver pin for external P-channel MOSFET (input disconnect switch). Connect this pin to the negative sense side of the input current sense resistor Rsc. The threshold voltage is measured as VIN – VSENSE. There is also fixed iADJ current sink to allow for trip threshold adjustment. 23 VSENSE 24 VIN Input power to the IC as well as the positive side of input current sense resistor. – PAD Exposed pad of the package providing enhanced thermal dissipation. Must be connected to the ground plane(s) of the PCB with at least 8 vias, directly in the pad. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 5 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple ELECTRICAL CHARACTERISTICS [1]: Unless otherwise noted, specifications are valid at VIN = 12 V, TJ = 25°C, • indicates specifications guaranteed over the full operating temperature range with TJ = –40°C to 125°C, typical specifications are at TJ = 25°C Characteristics Symbol Test Conditions Min. Typ. Max. Unit INPUT VOLTAGE SPECIFICATIONS ● 4.5 − 40 V VUVLO(rise) VIN VIN rising ● − – 4.45 V VIN UVLO Stop Threshold VUVLO(fall) VIN falling ● UVLO Hysteresis [2] VUVLO_HYS Operating Input Voltage Range [3] VIN UVLO Start Threshold − – 4.05 V 300 400 500 mV INPUT CURRENTS VIN Pin Operating Current VIN Pin Sleep Current IOP ISLEEP EN and PWM = H, CGATE = 1 nF from GDRV to PGND, fSW = 2 MHz ● − 22 32 mA VIN = 16 V, VEN / VPWM = VSYNC = 0 V ● − 1 5 µA INPUT LOGIC LEVELS (EN/PWM, ADIM) Input Logic Level-Low VIL ● − − 0.4 V Input Logic Level-High VIH ● 1.5 − − V 30 50 70 kΩ Input Pull-Down Resistor REN/PWM, RADIM Input = 5 V ANALOG DIMMING (ADIM) iADIM50 DC 1.0 V applied to ADIM pin − 50 − % iADIM25 DC 0.5 V applied to ADIM pin 23 25 27 % APWM Frequency Range [2] fAPWM Clock signal applied to ADIM pin ● 40 − 1000 kHz APWM Duty Cycle Range [2] DAPWM Clock signal applied to ADIM pin ● 0 − 90 % 4.05 4.25 4.45 V Analog Dimming Current Level (shown as % of full-scale current) VDD REGULATOR Regulator Output Voltage VDD VIN > 6 V, iLOAD < 1 mA VDD UVLO Start Threshold VDDUVLOrise VDD rising, no external load − 3.2 − V VDD UVLO Stop Threshold VDDUVLOfall VDD falling, no external load − 2.65 − V ERROR AMPLIFIER Amplifier Gain [2] gm VCOMP = 1.5 V − 900 − μA/V VCOMP = 1.5 V, A80602 (symm COMP) − –500 − μA Source Current IEA(SRC) VCOMP = 1.5 V, A80602-1 (asymm COMP) − –700 − μA Sink Current IEA(SINK) VCOMP = 1.5 V − +500 − μA COMP Pin Pull Down Resistance RCOMP FAULT = 0, VCOMP = 1.5 V − 1.4 − kΩ Continued on the next page… Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 6 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple ELECTRICAL CHARACTERISTICS [1] (continued): Unless otherwise noted, specifications are valid at VIN = 12 V, TJ = 25°C, • indicates specifications guaranteed over the full operating temperature range with TJ = –40°C to 125°C, typical specifications are at TJ = 25°C Characteristics Symbol Test Conditions Min. Typ. Max. Unit OVERVOLTAGE PROTECTION OVP Pin Voltage Threshold VOVP(th) OVP pin connected to VOUT OVP Pin Sense Current Threshold iOVP(th) OVP Sense Current Temperature Coefficient [2] ∆iOVP Current into OVP pin IOVPLKG VOUT = 16 V, EN = L OVP Pin Leakage Current OVP Variation at Output Undervoltage Detection Threshold ΔOVP VUVP(th) ● Current into OVP pin at 125°C Measured over temperature Measured at VOUT when ROVP = 188 kΩ ● ● ● 2.2 2.5 2.8 V 140 146.5 153 µA 140 150 160 µA − −36 − nA/°C − 0.1 1 µA − − 4 % − − 7 % Measured at VOUT when ROVP = 188 kΩ [2] 2.4 2.55 2.7 V Measured at VOUT when ROVP = 0 Ω 0.13 0.20 0.25 V Measured at VIN > 7.5 V − 6.5 − V Measured at iGDRV = 100 mA − 2.5 − Ω EN = L, VIN = 0 V − 100 − kΩ BOOST SWITCH GATE DRIVER Gate Driver Supply Voltage Gate Driver Pull-Up and Pull-Down Gate Pull-Down When Disabled Peak Sink Current [2] VDRV RGDRV RGDRV_OFF iSINK Peak Source Current [2] iSOURCE Gate Rise / Fall Time [2] tRISE, tFALL Measured at VGDRV = VDRV − 2 − A Measured at VGDRV = 0 V − 2 − A Measured with CLOAD = 1.5 nF; VGDRV between 10% and 90% of VDRV − 7 − ns Minimum Gate Driver On-Time tSW(ON) ● − − 100 ns Minimum Gate Driver Off-Time tSW(OFF) ● − − 100 ns ● 175 210 245 mV BOOST SWITCH CURRENT SENSE Primary Current Sense Limit iCS(LIM1) Exceeding iCS(LIM1) causes gate driver to truncate existing cycle, but does not shut down Secondary Current Sense Limit [2] iCS(LIM2) Exceeding iCS(LIM2) causes gate driver to shut down and latch off − 300 − mV Secondary Current Sense Limit Propagation Delay tCSDELAY Overdrive CS threshold by 10%, excluding leading edge blanking − 32 − ns 1.95 2.15 2.35 MHz OSCILLATOR FREQUENCY Oscillator Frequency FSET Pin Voltage fSW RFSET = 10 kΩ ● RFSET = 110 kΩ − 200 − kHz VFSET RFSET = 10 kΩ − 1.00 − V VSYNCL FSET/SYNC pin logic Low ● − − 0.4 V VSYNCH FSET/SYNC pin logic High ● 1.5 − − V SYNCHRONIZATION Sync Input Logic Level Synchronized PWM Frequency fSWSYNC ● 260 − 2300 kHz Synchronization Input Min Off-Time tPWSYNCOFF ● 150 − − ns Synchronization Input Min On-Time tPWSYNCON ● 150 − − ns Continued on the next page… Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 7 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple ELECTRICAL CHARACTERISTICS [1] (continued): Unless otherwise noted, specifications are valid at VIN = 12 V, TJ = 25°C, • indicates specifications guaranteed over the full operating temperature range with TJ = –40°C to 125°C, typical specifications are at TJ = 25°C Characteristics Symbol Test Conditions LEDx Accuracy [4] ErrLED iISET = 120 µA (RISET = 8.33 kΩ) LEDx Matching ΔLEDx iISET = 120 µA LEDx Regulation Voltage VLED Measured individually with all other LED pins tied to ≥1 V, iISET = 120 µA, VADIM > 2.1 V IISET to ILEDx Current Gain AISET iISET = 120 µA ISET Pin Voltage VISET VADIM > 2.1 V Allowable ISET Current iISET VADIM > 2.1 V Min. Typ. Max. Unit ● − 0.7 3 % ● − 0.8 2 % A80602 ● 650 750 850 mV A80602-1 ● 760 860 960 mV LED CURRENT SINKS LED String Partial-Short Detect VLEDSD Sensed from each LED pin to GND while its current sink is in regulation; all other LED pins tied to 1 V LED Pin Shorted-to-GND Test Duration [2] tLEDSTG Wait time before proceeding with Soft-Start (if no LED pin is shorted to GND) Soft-Start Ramp-Up Time [2] tSSRU Enable Pin Shut Down Delay tEN(OFF) Minimum PWM On-Time ● 955 978 1000 A/A 0.955 0.985 1.015 V ● 20 − 144 µA ● 4.9 5.5 6.1 V − 1.5 − ms 6.6 8.2 9.8 ms Maximum time duration before all LED channels come into regulation, or OVP is tripped, whichever comes first EN goes from High to Low; exceeding tEN(OFF) results in IC shutdown ● 10 16 22 ms tPWMH First and subsequent PWM pulses ● − 0.3 0.4 µs IGSINK INPUT DISCONNECT GATE PIN Gate Pin Sink Current VGS = VIN, no input OCP fault − −113 − µA Gate Pin Source Current IGSOURCE VGS = VIN – 6 V, input OCP fault tripped − 6 − mA Gate Shutdown Delay When OverCurrent Fault Is Tripped [2] tGATEFAULT VIN – VSENSE = 200 mV; monitored at FAULT pin − − 3 µs VGS PMOS Gate to source voltage measured when gate is on − −6.7 − V Gate Voltage VSENSE PIN VSENSE Pin Sink Current VSENSE Trip Point iADJ ● 16 20 24 µA Measured between VIN and VSENSE, RADJ = 0 Ω ● 88 98 108 mV VFAULT iFAULT = 1 mA − − 0.5 V IFAULT-LKG VFAULT = 5 V VSENSETRIP FAULT PIN FAULT Output Pull-Down Voltage FAULT Pin Leakage Current − − 1 µA External FAULT Input Low VFIL No internal faults; FAULT pin externally pulled down ● − − 0.8 V External FAULT Input High VFIH No internal faults ● 1.5 − − V tFIL No internal faults; delay (in fSW cycles) from FAULT pin externally pulled L to LED off; ignored if FAULT returns to H before that − 8 − cycles Thermal Shutdown Threshold [2] TSD Temperature rising 155 170 − °C Thermal Shutdown Hysteresis [2] TSDHYS − 20 − °C External FAULT Deglitch Timer THERMAL PROTECTION (TSD) 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 V = 4.5 V is only required at startup. After startup is completed, IC can continue to operate down to V = 4 V. IN IN [4] LED current is trimmed to cancel variations in both Gain and ISET voltage. [1] Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 8 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple APPLICATION EXAMPLES VIN *optional VOUT > 40 V D1 L1 RSENSE CIN Q2 Q1 ROVP COUT CVDRV GATE VSENSE VC RPU CVDD VDRV GDRV CS OVP VIN A80602 LED1 LED2 EN/ PWM APWM 100 kHz 0-90% AGND ... Q3 .. Q8 LED6 iLED ADIM Cascode Transistors LED3 ADIM 100% VDRV VT VDD FAULT PWM ton ≥ 0.3 µs PGND ISET RISET FSET RFSET DITH CP RM CM Max. LEDx pin voltage limited to VDRV - VT COMP PEB RD RZ CZ Figure 4: Cascode transistors can be used in case VOUT needs to be higher than 40 V To drive an output load with more than 11 white LEDs in series, the above application circuit can be used. The cascode transistors Q3..Q8 limits LED1-6 pin voltages to VDRV – VT, even in case of LED string direct short. (VT = threshold voltage of transistor, typically under 3 V). The cascode transistors can be any small-signal N-channel MOSFET, rated for the maximum output voltage and LED current per channel. As an example: consider T2N7002BK in SOT23 package. It is rated for 60 V breakdown and 400 mA continuous current. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 9 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple VIN D1 VOUT ≤ 40 V L1 Q1 4.7 µF COUT ROVP1 ROVP2 CVDRV VIN VSENSE VC OVP VIN VDD A80602-1 (MASTER) FAULT CVDD LED1 LED6 ADIM OVP A80602-1 (SLAVE) FAULT LED3 PGND VIN VDD LED2 EN/ PWM GDRV CS VDRV GATE VSENSE LED1 LED2 LED3 EN/ PWM ... 10 kΩ CVDD PGND ... GDRV CS VDRV GATE LED6 Up to 140 mA × 6 ADIM COMP AGND ISET FSET DITH COMP PEB CP RISET RFSET RM CM RPEB1 Up to 140 mA × 6 RZ CZ AGND RISET ISET FSET RFSET DITH PEB RPEB2 Figure 5: Two A80602-1 connected in Master/Slave Configuration to drive 140 mA × 12 LED strings Remarks on Master-Slave configuration: • Only one Slave to a Master. • Master-Slave operation requires asymmetrical COMP (for example: source = –700 µA and sink = 500 µA). This is available in A80602-1 only. • Also requires bidirectional FAULT pin of A80602-1, so that the slave can halt the switching of master. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 10 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple FUNCTIONAL DESCRIPTION The A80602 is a multi-string LED regulator with six precision current sinks and a gate driver for external boost MOSFET switch. It incorporates a patented Pre-Emptive Boost (PEB) control algorithm to achieve PWM dimming ratio over 15,000:1 at 200 Hz. PEB control also minimizes output ripple to avoid audible noise from output ceramic capacitors. Only if no faults were detected, then the IC can proceed to start switching. During operation, the EN/PWM pin can be toggled to control the brightness of LEDs channels by using PWM dimming. If EN/ PWM is pulled Low for longer than 16ms, the IC shuts off. The switching frequency can be either synchronized to an external clock or generated internally. Spread-spectrum technique (with user-programmable dithering range and modulation frequency) is provided to reduce EMI. A clock-out signal (CLKOUT) allows other converters to be synchronized to the switching frequency of A80602. Enabling the IC The A80602 wakes up when EN/PWM pin is pulled above logic high level, provided that VIN pin voltage is over the VIN_UVLO threshold. The boost stage and LED channels are enabled separately by PWM = H signal after the IC powers up. The IC performs a series of safety checks at power up, to determine if there are possible fault conditions that might prevent the system from functioning correctly. Power-up checks include: • VOUT shorted to GND Figure 6: Startup showing (from top to bottom) EN/PWM, VDD, VDRV, and ISET. • LED pin shorted to GND • FSET pin open/shorted • ISET pin open/shorted to GND, etc. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 11 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Powering Up: LED Detection Phase The VIN pin has an undervoltage lockout (UVLO) function that prevents the A80602 from powering up until the UVLO threshold is reached. Once the VIN pin goes above UVLO and a high signal is present on the EN pin, the IC proceeds to power up. At this point, the A80602 is going to enable the disconnect switch and will try to check if any LED pins are shorted to GND and/or are not used. The LED detection phase starts when the GATE voltage of the input disconnect PMOS switch is pulled down to 3.3 V below VIN. Unused LED pins should be terminated with a resistor to GND. Value of this termination resistor is 4.75 kΩ for channel 1, 3, 4, 6; or 2.37 kΩ for channel 2, 5. At the end of LED detection phase, any channel with pull-down resistor is then disabled and will not contribute to the boost regulation loop. VOUT LED Ch1 to 4 are enabled LED1 LED2 LED3 LED4 LED5 GND LED6 LED Ch5 & 6 are disabled 4.75 kΩ 2.37 kΩ Figure 8: How to signal an unused LED channel during startup LED detection phase Table 1: LED Detection phase voltage threshold levels Figure 7: Startup showing EN/PWM, GATE, LED1, and ISET. Note that LED Detection Phase starts as soon as GATE pin is pulled down to 3.3 V below VIN. Once the voltage threshold on VLED pins exceeds ~120 mV, a delay of approximately 1.5 ms is used to determine the status of the pins. LED Pin Voltage Measured Interpretation Outcome < 120 mV LED pin shorted to GND fault Cannot proceed with soft-start unless fault is removed ~230 mV LED channel not in use LED channel is removed from operation > 340 mV LED channel in use Proceed with soft-start Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 12 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple If an LED pin is shorted to ground, the A80602 will not proceed with soft start until the short is removed from the LED pin. This prevents the A80602 from ramping up the output voltage and putting an uncontrolled amount of current through the LEDs. The FAULT pin is pulled low in case of LED pin shorted-to-GND fault, but the IC continues to retry. Once the fault is removed, the soft-start process will continue. The same applies in case of FSET or ISET pin is shorted to GND. Figure 9: Normal startup showing all channels passed LED Detection phase (only LED1 and LED2 pin voltages are shown). Output Voltage = 26.5 V (8× LED). Total LED current = 100 mA × 6. Figure 11: LED1 is shorted-to-GND initially, then released. After the fault is removed, the IC auto-recovers and proceeds with soft-start. FAULT is released at the end of LED detection phase. Figure 10: Normal startup showing LED1 channel is disabled with a 4.75 kΩ resistor to GND. Total LED current = 100 mA × 5. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 13 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Power Up: Boost Output Undervoltage During startup, after the input disconnect switch has been enabled, the output voltage is checked through the OVP (overvoltage protection) pin. If the sensed voltage does not rise above VUVP(th), the output is assumed to be at fault and the IC will not proceed with soft start. Output UVP level is linked to the OVP level programmed according to the equation: VUVP = VOVP / 12 Undervoltage protection may be caused by one of the following faults: • Output capacitor shorted to GND • Boost inductor or diode open • OVP sense resistor open After an UVP (undervoltage protection) fault, the A80602 is immediately shutdown and latched off. To enable the IC again, the latched fault must be cleared. This can be achieved by powering-cycling the IC, which means either: • VIN falls below falling UVLO threshold, or • EN = L for >16 ms. This is illustrated by the following startup timing diagram (not to scale): EN PWM VIN 3.3 V GATE 6.7 V 0 1V LEDx 0 LED detection phase 93% OVP 1.5 ms OVP VOUT VIN 0 tSSRU i LED 0 A B C D Soft-Start E Regulation Figure 12: Complete startup process of A80602 Alternatively, latched fault can be cleared by keeping EN = H but pulling PWM = L for >16 ms. This method has the advantage that it does not interrupt the CLKOUT signal. Explanation of Events: Soft Start Function B: When GATE is pulled down to 3.3 V below VIN, ISET becomes enabled. IC is now waiting for PWM = H to startup. During startup, the A80602 ramps up its boost output voltage following a fixed slope, as determined by OVP set point and SoftStart Timer. This technique limits the input inrush current, and ensures consistent startup time regardless of the PWM dimming duty cycle. The soft-start process is completed when any one of the following conditions is met: • All enabled LED channels have reached their regulation current, • Output voltage has reached 93% of its OVP threshold, or • Soft-start ramp time (tSS) has expired. A: EN = H wakes up the IC. VDD ramps up and CLKOUT becomes available. IC starts to pull down GATE slowly. C: Once PWM = H, the IC checks each LEDx pins to determine if it is in use, disabled, or shorted to GND. D: Soft-Start begins at the completion of LED pin short-detect phase of ~1.5 ms. VOUT ramps up following a fixed slope set by OVP and soft-start timer of ~8 ms. E: Soft-start terminates when all LED currents reached regulation, VOUT reached 93% OVP, or soft-start timer expired. To summarize, the complete startup process of A80602 consists of: • • • • Power-up error checking Enabling input disconnect switch LED pin open/short detection Soft-start ramp Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 14 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Frequency Selection t PWSYNCON The switching frequency of the boost regulator is programmed by a resistor connected to FSET pin. The switching frequency can be selected anywhere from 200 kHz to 2.3 MHz. The chart below shows the typical switching frequency verses FSET resistor value. 154 ns 150 ns 150 ns t PWSYNCOFF t = 454 ns Figure 14: Pulse width requirements for an External Sync clock at 2.2 MHz Based on the above, any clock with a duty cycle between 33% and 66% at 2.2 MHz can be used. The table below summarizes the allowable duty cycle range at various synchronization frequencies. Table 2: Acceptable Duty Cycle range for External Sync clock at various frequencies Sync. Pulse Frequency Duty Cycle Range 2.2 MHz 33% to 66% 2 MHz 30% to 70% 1 MHz 15% to 85% Figure 13: Switching Frequency as a function of FSET Resistance Alternatively, the following empirical formula can be used: Equation 1: fSW = 21.5 / (RFSET + 0.2) where fSW is in MHz and RFSET is in kΩ. If a fault occurs during operation that will increase the switching frequency, the internal oscillator frequency is clamped to a maximum of 3.5 MHz. If the FSET pin is shorted to GND, the part will shut down. For more details, refer to the Fault Mode Table section. Synchronization The A80602 can also be synchronized using an external clock. At power up, if the FSET pin is held low, the IC will not start. Only when the FSET pin is tristated to allow for the pin to rise to about 1 V, or when a sync clock is detected, the A80602 will then try to power up. The basic requirement of the external sync signal is 150 ns minimum on-time and 150 ns minimum off time. The diagram below shows the timing restrictions for a synchronization clock at 2.2 MHz. 600 kHz 9% to 91% 300 kHz 4.5% to 95.5% If it is necessary to switch over between internal oscillator and external sync during operation, ensure the transition takes place at least 500 ns after the previous PWM = H rising edge. Alternatively, execute the switchover during PWM = L only. This restriction does not apply if PWM dimming is not being used. PWM 500 ns Ext_Sync / FSET 1 V Internal Clock Internal oscillator External Sync Figure 15: Avoid switching over between Internal Oscillator and External Sync in highlighted region Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 15 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Loss of External Sync Signal Suppose the A80602 started up with a valid external SYNC signal, but the SYNC signal is lost during normal operation. In that case, one of the following happens: • If the external SYNC signal is high impedance (open), the IC continues normal operation after approximately 5 μs, at the switching frequency set by RFSET. No FAULT flag is generated. • If the external SYNC signal is stuck low (shorted to ground), the IC will detect an FSET-shorted-to-GND fault. FAULT pin is pulled low after approximately 10 μs, and switching is disabled. Once the FSET pin is released or SYNC signal is detected again, the IC will proceed to soft-start. To prevent generating a fault when the external SYNC signal is stuck at low, the circuit shown below can be used. When the external SYNC signal goes low, the IC will continue to operate normally at the switching frequency set by the RFSET. No FAULT flag is generated. External Sychronization 220 pF Signal Schottky Barrier Diode FSET/SYNC RFSET 10 kΩ Figure 16: Countermeasure for External Sync Stuck-at-Low Fault It is important to use a small capacitance for the AC-coupling capacitor (220 pF in the above example). If the capacitance is too large, the IC may incorrectly declare a FSET-shorted-to-GND fault and restart. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 16 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Switching Frequency Dithering To minimize the peak EMI spikes at switching frequency harmonics, the A80602 offers the option of frequency dithering, or spread-spectrum clocking. This feature simplifies the input filters needed to meet the automotive CISPR 25 conducted and radiated emission limits. For maximum flexibility, the A80602 allows both dithering range and modulation frequency to be independently programmable using two external components. The Dithering Modulation Frequency is given by the approximate equation: Equation 2: fDM (kHz) = 25 / CDITH (nF) where CDITH is the value of capacitor connected from DITH pin to GND. The dithering Range is given by the approximate equation: Equation 3: Range (±%) = 20 × RFSET / RDITH where RFSET is the resistor from FSET pin to GND, RDITH is the resistor between DITH and FSET pins. As an example, by using RFSET = 10 kΩ, RDITH = 40.1 kΩ, and CDITH = 22 nF, the resulted switching frequency is fSW = 2.15 MHz ±5% modulated at 1.1 kHz. This is illustrated by the following diagram. FSET RFSET 10 kΩ iFSET = 100 µA ±5 µA RDITH 40.1 kΩ DITH VDITH 1.2 V iDITH = ±20 µA VFSET 1.0 V 0.8 V CDITH 22 nF Dithering Range = ±5% iDITH 20 µA 0 Modulation frequency = 1.1 kHz –20 µA Per iod = 0.8 × C / i (0.88 ms when C = 22 nF) fSW (MHz) 2.25 2.15 2.05 Time (ms) 0 0.88 Figure 17: How to Program Switching Frequency Dithering Range and Modulation Frequency There are no hard limits on dithering range and modulation frequency. As a general guideline, pick a dithering range between ±5% and 10%, with the modulation frequency between 1 kHz and 3 kHz. In practice, using a larger dithering range and/or higher modulation frequency do not generate any noticeable benefits. If dithering function is not desired, it can be disabled by disconnecting the RDITH between DITH and FSET pins. Connect DITH pin to VDD if CDITH is not populated. Dithering is always disabled when fSW is controlled by external sync. RDITH and CDITH have no effects in this case even if they were populated. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 17 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple LED Current Setting The maximum LED current can be up to 140 mA per channel, and is set through the ISET pin. Connect a resistor RISET between this pin and GND. The relation between ILED and RISET is given below: Equation 4: ILED = ISET × AISET ISET = VISET / RISET Therefore RISET = (VISET × AISET ) / ILED = 963 / ILED where ILED current is in mA and RISET is in kΩ. This sets the maximum current through the LEDs, referred to as the ‘100% current’. The average LED current can be reduced from the 100% current level by using either PWM dimming or analog dimming. Table 3: ISET resistor values vs. LED current. Resistances are rounded to the nearest E-96 (1%) resistor value. Standard Closest RISET Resistor Value LED current per channel 6.81 kΩ 140 mA 9.53 kΩ 100 mA 12 kΩ 80 mA 16 kΩ 60 mA 24 kΩ 40 mA Figure 18: PWM dimming operation at 20% 1 kHz. CH1 = PWM (5 V/ div), CH2 = SW (20 V/div), CH3 = VOUT, CH4 = iLED (200 mA/div). By using the patented Pre-Emptive Boost (PEB) control algorithm, the A80602 is able to achieve minimum PWM dimming on-time down to 300 ns. This translates to PWM dimming ratio up to 15,000:1 at the PWM dimming frequency of 200 Hz. Technical details on PEB will be explained in the next section. PWM Dimming When the EN/PWM pin is pulled high, the A80602 turns on all enabled LED current sinks. When it is pulled low, all LED current sinks are turned off. By changing the duty cycle of PWM signal, the average LED current (and hence brightness) can be accurately controlled. During PWM = L, the IC floats the compensation (COMP) pin and waits for the next PWM rising edge. But if PWM stays Low longer than tEN(OFF) duration (16 ms typical), the IC shuts off completely. Figure 19: Zoom in view for PWM on-time = 10 µs. Notice that the LED current is shifted with respect to PWM signal. Ripple at VOUT is ~0.2 V when using 2 × 4.7 µF MLCC as output capacitors. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 18 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple CH1 (Yellow) = PWM (5 V/div); CH2 (Red) = Inductor current (500 mA/div); CH3 (Blue) = VOUT (1 V/div); CH4 (Green) = LED current (200 mA/div); time scale = 2 µs/div. Figure 20: Zoom-in view showing A80602 is able to regulate LED current at PWM on-time down to 300 ns. The typical PWM dimming frequencies fall between 200 Hz and 1 kHz. There is no hard limit on the highest PWM dimming frequency that can be used. However at higher PWM frequency, the maximum PWM dimming ratio will be reduced. This is shown in the following table: Table 4: Maximum PWM Dimming Ratio that can be achieved when operating at different PWM Dimming Frequency PWM Frequency PWM Period Maximum PWM Dimming Ratio 200 Hz 5 ms 15,000:1 1 kHz 1 ms 3,000:1 3.3 kHz 300 µs 1,000:1 20 kHz 50 µs 150:1 Figure 21: Traditional PWM Dimming operation where boost switch and LED current are enabled at the same time. Note that VOUT shows overall ripple of ~0.5 V When PWM signal goes high, a conventional LED driver turns on its boost switching at the time with LED current sinks. The problem is that the inductor current takes several switching cycles to ramp up to its steady-state value before it can deliver full power to the output load. During the first few cycles, energy to the LED load is mainly supplied by the output capacitor, which results in noticeable dip in output voltage. Pre-Emptive Boost The basic principle of pre-emptive boost (PEB) can be best explained by the following two waveforms. The first one shows how a conventional LED driver operates during PWM dimming operation. The second one shows that of the A80602. Common test conditions for both cases: PWM = 1% at 1 kHz (on-time = 10 µs), fSW = 2.15 MHz, L = 10 µH, VIN = 12 V, LED load = 8 series (VOUT = ~25 V) at 100 mA × 4. COUT = 2 × 4.7 µF 50 V 1210 MLCC. COMP: RZ = 280 Ω, CZ = 68 nF. Common scope settings: Figure 22: A80602 PWM dimming operation with PEB delay set to 3 µs. Note that VOUT ripple is reduced to ~0.2 V. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 19 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple In the A80602, the boost switch is also enabled when PWM goes high. However, the LED current is not turned on until after a short delay of tPEB. This allows the inductor current to build up before it starts to deliver the full power to LED load. During the pre-boost period, VOUT actually bumps up very slightly, while the following dip is essentially eliminated. When PWM goes low, both boost switching and LED remains active for the same delay of tPEB. Therefore the PWM on-time is preserved in LED current. Analog Dimming PEB delay can be programmed using an external resistor, RPEB, from PEB pin to GND. Their relationship is shown in the following chart: 2. In APWM mode: apply a clock signal with duty cycle between 90% and 0% at the pin. PEB Delay (µs) vs. PEB Resistor value (kΩ) 10 The peak (100%) level of LED current is set by the RISET resistor. The actual peak LED current may also be adjusted continuously from approximately 10% up to 100%, by using the ADIM pin. There are two methods to do so: 1. In ADIM mode: apply a DC voltage between 0.2 V and 2 V at the pin. ADIM MODE An analog voltage is applied at the ADIM/APMW pin. This DC voltage linearly controls the peak LED current, as illustrated by the chart below: 9 8 Normalized LED Current vs. ADIM Voltage 6 100% 5 90% 4 80% 3 70% 2 60% 1 0 LED Current tPEB (µs) 7 50% 6 8 10 12 14 16 RPEB (kΩ) 18 20 22 24 Figure 23: How PEB delay time varies with value of PEB pin resistor to GND. Ideally, tPEB is equal to the inductor current ramp up time. But the latter is affected by many external parameters, such as switching frequency, inductance, VIN and VOUT ratio, etc. Therefore, some experimentation is required to optimize the PEB delay time. In general for switching frequency at 2 MHz, tPEB = 2.5 to 4 µs is a good starting point. The advantage of PEB is that even a non-optimized delay time can significantly reduce the output ripple voltage compared to a conventional LED driver. 40% ADIM Decreasing 30% ADIM Increasing 20% 10% 0% 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 ADIM Voltage (V) 2 2.2 Figure 24: In analog dimming mode, the LED current is linearly proportional to ADIM voltage between 0.2 V and 2 V approximately There is an internal pull-down resistor (50 kΩ typical) from ADIM pin to GND. When this pin is left floating, LED current is actually being dimmed down to ~10%. Therefore, if analog dimming is not required, the ADIM pin should be pulled to over 2 V (but below VDD) to ensure 100% LED current. One simple technique is to pull up ADIM to VDD through a 30 kΩ resistor. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 20 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple APWM MODE When a clock signal is detected at ADIM pin, the A80602 goes into APWM mode. The typical APWM signal frequency is between 40 kHz and 1 MHz. The duty cycle of this signal is inversely proportional to the percentage of current delivered to the LED. The relationship is shown below: Normalized LED Current vs. APWM Duty Cycle 100% 80% LED Current Measured 60% Theore�cal Figure 26: PWM = H. Total LED current drops from 400 mA (4 × 100 mA/ch) to 300 mA when APWM of 25% duty cycle is applied. Note that LED current takes ~1 ms to settle after change in APWM. 40% 20% 0% 0% 20% 40% 60% APWM Duty Cycle 80% 100% Figure 25: Showing LED current is inversely proportional to the APWM duty cycle. Test conditions: VIN = 12 V, VOUT = 25 V (8 × WLED), total LED current = 100 mA × 4, APWM frequency = 100 kHz As an example, a system that delivers a full LED current of 100 mA per channel would deliver 75 mA when an APWM signal with a duty-cycle of 25% is applied (because analog dimming level is 100% – 25% = 75%). This is demonstrated by the following waveforms (only LED channels 1 to 4 are enabled). Figure 27: PWM = 10% at 1 kHz. Peak LED current drops from 400 mA (4 × 100 mA/ch) to 300 mA when APWM of 25% duty cycle is applied Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 21 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple To use ADIM pin as a trim function, the user should first set the 100% current based on efficacy of LED from the lowest bin. When using LED with higher efficacy, the required current is then trimmed down to the appropriate level using APWM duty cycle. As an example, assume that: • LED from lowest bin has an efficacy of 80 lm/W • LED highest bin has an efficacy of 120 lm/W Suppose the maximum LED current was set at 100 mA based on LEDs from lowest bin. When using LEDs from highest bin, the current should then be reduces to 67% (80/120). This can be achieved by sending APWM clock with 33% duty cycle. Extending LED Dimming Ratio The dynamic range of LED brightness can be further extended, by using a combination of PWM duty cycle, APWM duty cycle, and analog dimming method. For example, the following approach can be used to achieve a 100,000:1 dimming ratio at 200 Hz: • Vary PWM duty cycle from 100% down to 0.01% to give 10,000:1 dimming. This requires PWM dimming on-time be reduced down to 0.5 µs. • With PWM dimming on-time fixed at 0.5 µs, reduce peak LED current from 100% down to 10%. This can be achieved by either: The net result of using both PWM and APWM is 100,000:1 dimming ratio, as shown in the chart below: 100 Normalized LED Current (%) One popular application of analog dimming is for LED brightness calibration, commonly known as ‘LED Binning’. LEDs from the same manufacturer and series are often grouped into different ‘bins’ according to their light efficacy (lumens per watt). It is therefore necessary to calibrate the ‘100% current’ for each LED bin, in order to achieve uniform luminosity. Average LED Current vs. PWM Dimming Duty Cycle 10 1 0.1 PWM Dimming 0.01 APWM + PWM Ideal 0.001 0.001 0.01 0.1 1 10 PWM Dimming Duty Cycle (%) 100 Figure 28: How to achieve 100,000:1 dimming ratio by using both PWM and APWM. Test conditions: VIN = 12 V, VOUT = 25 V (8 × WLED), total LED current = 400 mA, PWM frequency = 200 Hz, APWM frequency = 100 kHz. Note that the A80602 is capable of providing analog dimming range greater than 10:1. By applying APWM with 96% duty cycle, for example, an analog dimming range of 25:1 can be achieved. However, this requires the external APWM signal source to have very fine pulse-width resolution. At 200 kHz APWM frequency, a resolution of 50 ns is required to adjust its duty cycle by 1%. □□ Apply a clock signal at APWM pin, and vary its duty cycle from 0% to 90%, or □□ Apply a DC voltage at APWM pin, and vary its level from 2 V down to 0.2 V. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 22 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Analog Dimming with External Voltage Besides using ADIM pin, the LED current can also be reduced by using an external voltage source applied through a resistor to the ISET pin. The dynamic range of this type of dimming is dependent on the ISET pin current. The recommended iSET range is from 20 µA to 144 µA for the A80602. Note that the IC will continue to work at iSET below 20 µA, but the relative error in LED current becomes larger at lower dimming level. Below is a typical application circuit using a DAC (digital-analog converter) to control the LED current. The ISET current (which directly controls the LED current) is normally set as VISET/RISET. The DAC voltage can be higher or lower than VISET, thus adjusting the LED current to a lower or higher value. A80602 R2 VDAC In the following application example, the thermistor used is NTCS0805E3684JXT (680 kΩ @ 25°C). R1 = 336 kΩ, R2 = 20 kΩ, and R3 = 8.45 kΩ. The LED current per channel is reduced from 97 mA at 25°C to 34 mA at 125°C. VDD (4.25 V) NTC R2 R1 A80602 ISET (1.0 V) R3 GND Figure 30: Thermal foldback of LED current using NTC thermistor ISET RISET GND Figure 29: Adjusting LED current with an external voltage source Equation 5: iISET = VISET VDAC − VISET  −  R2 RISET   where VISET is the ISET pin voltage (typically 1.0 V), and VDAC is the DAC output voltage. When VDAC is higher than 1.00 V, the LED current is reduced. When VDAC is lower than 1.00 V, the LED current is increased. Figure 31: LED current varies with temperature when using thermistor NTCS0805E3684JXT for thermal foldback Some common applications for the above scheme include: • LED binning • Thermal fold-back using external NTC (negative temperature coefficient) thermistor Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 23 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple VDD Shutdown The VDD pin provides regulated bias supply for internal circuits. Connect a CVDD capacitor with a value of 1 μF or greater to this pin. The internal LDO can deliver up to 2 mA of current with a typical VDD voltage of about 4.25 V. This allows it to serve as the pull up voltage for FAULT pin. If the EN/PWM pin is pulled low for longer than tEN(OFF) (~16 ms), the A80602 enters shutdown (sleep mode). The next time the EN/PWM pin goes high, all internal fault registers are cleared. The IC needs to go through a complete soft start process after PWM goes high. VDRV The VDRV pin provides a regulated gate driver supply for external boost power MOSFET. Connect a CVDRV capacitor with a typical value of 2.2 μF to this pin. The gate driver can deliver up to 2 A of peak sink and source current, with a typical VDRV voltage of 6.5 V. However, its average output current is limited to approximately 36 mA. Note that average gate driver current is: Equation 6: iVDRV = fSW × QG where fSW is the switching frequency and QG is the total gate charge of the power MOSFET for VGS = 0 to 6.5 V. At higher switching frequency, it is important to select a power MOSFET with low QG to limit the average gate driver current. Refer to the appendix section for details on MOSFET selection. Figure 32: After EN/PWM (Yellow) goes Low for ~16 ms, the IC completely shuts down so both VDD (Blue) and COMP (Red) decay. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 24 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple FAULT DETECTION AND PROTECTION FAULT Status The FAULT pin is an open-drain output that will be pulled low when a fault occurs. A pull-up resistor (typically around 10 kΩ) is required between this pin and desired logic level voltage (typically 3.3 to 5 V). Multiple devices with open-drain FAULT pins can be connected in parallel to form a wired-AND configuration. This way, when any device reports a fault, the system FAULT signal is pulled low. The A80602-1 (One-Out-All-Out option) has a bidirectional FAULT pin. This means the same pin also serves as an input to monitor the status of system FAULT signal. When the FAULT pin is pulled low externally for >8 fSW cycles by another device, the A80602-1 disables its own boost switch and all LED current sinks in response. This feature is required in Master/Slave configuration, for example. The following two simplified flow charts demonstrate the difference between A80602 (unidirectional FAULT pin) and A80602-1 (bidirectional FAULT pin). IC Off EN/PWM=H & VIN>UVLO Power up EN/PWM=L (VDD, BG ready; GATE pulled L; Fault checking) FAULT State (FAULT pulled L ) Any Fault detected? Yes No IC Ready EN/PWM=L (FAULT pulled L ) EN/PWM=H Pin shorted to GND fault  FAULT =L LED Pin Check (In Use, Disabled, or Shorted to GND) Time-out without faults  FAULT released Soft Start (enable boost SW and LED current sinks) Any Fault detected? PWM Dimming Yes No LED=on Clear 16ms timer EN/PWM = L EN/PWM = H LED=off Start 16 ms timer Timer expired Figure 33: Simplified A80602 Startup Flowchart Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 25 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple IC Off EN/PWM=H & VIN>UVLO EN/PWM=L Power up (VDD, Bgready; Fault checking) FAULT State (FAULT pulled L ) Any Internal Fault detected? Yes No EN/PWM=L IC Ready (FAULT pulled L ) EN/PWM=H Pin shorted to GND fault  FAULT =L LED Pin Check (In Use, Disabled, or Shorted to GND) Time-out without faults  FAULT released Any External Fault detected? Yes (FAULT pulled L externally) No (FAULT =H) Soft Start (enable boost SW and LED current sinks) Any Internal Fault detected? Yes No Any External Fault detected ? Yes (FAULT pulled L externally) No PWM Dimming LED=on Clear 16 ms timer EN/PWM = L EN/PWM = H LED=off Start 16 ms timer Timer expired Figure 34: Simplified startup flow chart for A80602-1, showing responses to both Internal and External FAULT signals Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 26 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple LED String Partial-Short Detect All LED current sink pins (LED1 to LED6) are designed to withstand the maximum output voltage, as specified in the Absolute Maximum Ratings table. This prevents the IC from being damaged if VOUT is directly applied to an LED pin due to an output connector short. In case of direct-short or partial-shorted fault in any LED string during operation, the LED pin with voltage exceeding VLEDSD will be removed from regulation. This prevents the IC from dissipating too much power due to large voltage drop across the LED current sink. Figure 35: A80602 Normal startup sequence showing voltage at LED1 and LED2 pins. VIN = 6 V, output = 8 × WLED in series, current = 6 × 100 mA For A80602, the FAULT pin is pulled low in case any LED string is directly or partially shorted. The suspect LED string is disabled, while the rest of the LED strings continue to operate. FAULT pin is latched at low until it is reset by either EN = L or PWM = L for >16 ms For A80602-1, all LED strings are turned off in case any LED string has detected a partial short. FAULT pin is latched at low until the IC is reset. Figure 37: A80602-1 startup sequence when LED string#2 has a partial-short fault (6 × WLED instead of 8). As soon as LED2 pin rises above VLEDSD (~5 V), the channel is disabled but FAULT remains High. At least one LED pin must be at regulation voltage (below ~1.2 V) for the LED string partial-short detection to activate. In case all of the LED pins are above regulation voltage (this could happen when the input voltage rises too high for the LED strings), they will continue to operate normally. Figure 36: A80602 startup sequence when LED string#2 has a partialshort fault (6 × WLED instead of 8). As soon as LED2 pin rises above VLEDSC (~5 V), the channel is disabled and FAULT = Low. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 27 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Overvoltage Protection The A80602 offers a programmable output overvoltage protection (OVP). The OVP pin has a threshold level of 2.5 V typical. Overvoltage protection is tripped when current into this pin exceeds ~150 µA. A resistor can be used to set the OVP threshold up to 40 V approximately. This is sufficient for driving 11 white LEDs in series. The formula for calculating the OVP resistor is shown below: Equation 7: ROVP = (VOVP – VOVP(th)) / iOVP(th) where VOVP is the desired OVP threshold, VOVP(th) = 2.5 V typical, iOVP(th) = 150 µA typical. To determine the desired OVP threshold, take the maximum LED string voltage at cold and add ~10% margin on top of it. The OVP event is not a latched fault and, by itself, does not pull the FAULT pin to low. If the OVP condition occurs during a load dump, for example, the IC will stop switching but not shut down. OVP condition is typically caused by an open LED fault, or disconnected output connector. It may be detected either at startup or during normal operation. This is explained separately below. Figure 38: A80602 startup with LED2 string open. VOUT hits OVP at ~30 V and LED2 is removed from regulation. FAULT pin goes Low but remaining LED strings continue to operate. For A80602-1, all LED strings are disabled in case any string is not in regulation when VOUT hits OVP. FAULT pin is pulled low and switching is stopped. The IC remains in latched off state until it is reset. CASE 1: OVP AT STARTUP During soft start period, the A80602 tries to boost VOUT until it becomes high enough for all LED string to come into regulation. But if any LED string is open, VOUT will eventually hit OVP. At this point, the A80602 will disable any LED string that is still not in regulation. The FAULT pin is pulled low and boost switching is stopped to allow VOUT to fall. Once VOUT falls below ~97% OVP, switching resumes to power the remaining LED strings. Figure 39: A80602-1 startup with LED2 string open. VOUT hits OVP and all LED string are disabled. FAULT pin goes Low and IC remains latched off until reset. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 28 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple CASE 2: OVP DURING NORMAL OPERATION Boost Switch Overcurrent Protection When one LED string becomes open during operation, current through its LED driver drops to zero. The A80602 responds by boosting the output voltage higher. When output reaches OVP threshold, the LED string without current is removed from regulation. The rest of LED strings continue to draw current and drain down VOUT. Once VOUT falls below ~97% OVP, boost will resume switching to power the remaining LED strings. The external boost switch is protected with a cycle-by-cycle primary current limit. When the voltage sensed at CS pin exceeds VCS(LIM1) (typically 210 mV), the existing switching cycle is truncated. That means the peak switching current is limited to: Equation 8: iSW(LIM1) = VCS(LIM1) / RCS where RCS is the sense resistor connected from source of boost MOSFET to power ground. As an example, if RCS = 39 mΩ, then iSW(LIM1) = 5.4 A approximately. The waveform below shows normal switching at VIN = 6 V, VOUT = ~26 V and total LED current 800 mA. Average input current is around 4.5 A. Figure 40: An open-LED string faults causes VOUT to ramp up and trip OVP. The A80602 then disables the open LED string and continues with remaining strings. The A80602-1, in contrast, will disable all LED strings in case any LED string becomes open. The IC remains in latched off state until it is reset. Figure 42: Normal 400 kHz switching waveform at VIN = 6 V. Red trace is the SW node voltage at 10 V/div. Green trace is the inductor current at 1 A/div. When the input voltage is reduced further to 5.6 V, input current increases and peak switch current reaches 5.4 A. Overcurrent protection is tripped to limit the peak SW current. Figure 41: An open-LED string faults causes VOUT to ramp up and trip OVP. The A80602-1 then disables all LED string and remains in latched off state until reset. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 29 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple If the input current level goes above the preset threshold, the part will be shut down in less than 3 µs. The FAULT pin is pulled Low and the IC remains in latched off state until it is reset. This feature prevents catastrophic failure in the system when there is a direct short from VOUT to GND (caused by a shorted output connector or cable, for example). The waveform below illustrates the input overcurrent fault condition during startup. As soon as input OCP limit is reached, the part disables the gate of the disconnect switch Q1 and latches off. Figure 43: When peak current through the inductor reaches ~5.4 A, overcurrent protection kicks in to truncate the present switching cycle. There is also a secondary current sense limit VCS(LIM2), set at about 40% higher than the cycle-by-cycle current limit. It is to protect the external MOSFET from destructive current spikes in case the boost inductor or boost diode is shorted. Once this limit is tripped, the A80602 will immediately shut down and latch off. Input Overcurrent Protection and Disconnect Switch VIN iSENSE RSENSE iADJ To L1 RADJ VSENSE VIN CG Q1 (PMOS) GATE A80602 Figure 45: Startup into an output shorted-to-GND fault. Input OCP is tripped when current (Green trace) exceeds ~6.5 A. PMOS Gate (Red) is turned off immediately and IC latches off. During startup when Q1 first turns on, an inrush current flows through Q1 into the output capacitance. If Q1 turns on too fast (due to its low gate capacitance), the inrush current may trip input OCP limit. In this case, an external gate capacitance CG is added to slow down the turn-on transition. Typical value for CG is around 4.7 to 22 nF. Do not make CG too large, since it also slows down the turn-off transient during a real input OCP fault. VIN – VSENSE = RSENSE × iSENSE + RADJ × iADJ Figure 44: Optional input disconnect switch using a PMOSFET The primary function of the input disconnect switch is to protect the system and the device from excessive input currents during a fault condition. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 30 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Setting the Input Current Sense Resistor Fault Protection During Operation The input disconnect switch threshold is typically 98 mV, measured between VIN and VSENSE pins when RADJ is 0 Ω. This threshold can be trimmed slightly using the RADJ resistor. The A80602 constantly monitors the state of the system to determine if any fault conditions occur during normal operation. The response to a triggered fault condition is summarized in the table below. It is important to note that there are several points at which the A80602 monitors for faults during operation. The locations are input current, switch current, output voltage, switch voltage, and LED pins. Some of the protection features might not be active during startup to prevent false triggering of fault conditions. To avoid false tripping, the input disconnect switch overcurrent limit should be set higher than the boost switch cycle-by-cycle current limit. For example, the boost switch OCP is set at 5.4 A, so the input disconnect switch OCP may be set 25% higher at 6.75 A. The input current sense resistor is then calculated as below. When RADJ is not used: VIN – VSENSE = RSENSE × iSENSE = 98 mV The desired sense resistor is RSENSE = 98 mV / 6.75 A = 14.5 mΩ. But this is not a standard E-24 resistor value. Pick the closest lower value which is 13 mΩ. When RADJ is used: VIN – VSENSE = RSENSE × iSENSE + RADJ × iADJ Therefore RADJ = [(VIN – VSENSE) – (RSENSE × iSENSE)] / iADJ = [98 mV – 88 mV] / 20 µA = 500 Ω Pick the closest E-96 resistor value of 499 Ω. The possible fault conditions that the part can detect include: • • • • • • • • • • Open LED Pin or open LED string Shorted or partially shorted LED string LED pin shorted to GND Open or shorted boost diode Open or shorted boost inductor VOUT short to GND SW shorted to GND ISET shorted to GND FSET shorted to GND Input disconnect switch drain shorted to GND Note that some of these faults will not be protected if the input disconnect switch is not being used. An example of this is VOUT short to GND fault. Input UVLO When VIN rises above VUVLOrise threshold, the A80602 is enabled. The IC is disabled when VIN falls below VUVLOfall threshold for more than 50 μs. This small delay is used to avoid shutting down because of momentary glitches in the input power supply. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 31 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Table 5: A80602 Fault Mode Table Fault Name Type Active Fault Flag Set Description Boost Switch Disconnect Switch LED Sink drivers Primary Switch Overcurrent Protection (Cycle-By-Cycle Current Limit) Auto-restart Always NO This fault condition is triggered when the SW current exceeds the cycle-by-cycle current limit, ISW(LIM).The present SW on-time is truncated immediately to limit the current. Next switching cycle starts normally. Off for a single cycle ON ON Secondary Switch Current Limit Latched Off Always YES When current through boost switch exceeds secondary SW current limit (iSW(LIM2)) the device immediately shuts down the disconnect switch, LED drivers and boost. The Fault flag is set. To reset the fault the EN/PWM pin needs to be pulled low for ~16 ms. OFF OFF OFF Input Disconnect Current Limit Latched Off Always YES The device is immediately shut off if the voltage across the input sense resistor is above the VSENSEtrip threshold. To reset the fault the EN/PWM pin must be pulled low for ~16 ms. OFF OFF OFF LEDx Pin Shorted to GND Auto-restart Startup YES If any of the LED pins is determined to be shorted to GND when PWM first goes high, soft-start process is halted. Only when the short is removed, then soft-start is allowed to proceed. OFF ON OFF YES If an LED string is not getting enough current, the device will first respond by increasing the output voltage until OVP is reached. Any LED string that is still not in regulation will be disabled. The device will then go back to normal operation by reducing the output voltage to the appropriate voltage level. ON ON OFF for open pins. ON for all others. OFF OFF OFF LEDx Pin Open (One-Out-Continue option) Auto-restart Normal operation LEDx Pin Open (One-Out-All-Out option) Latched Normal operation YES If an LED string is not getting enough current, the device will first respond by increasing the output voltage until OVP is reached. If any LED string is still not in regulation, all LED strings will be disabled and the device latched off. To reset the fault the EN/PWM pin must be pulled low for ~16 ms. ISET Short Protection Auto-restart Always YES Fault occurs when the ISET current goes above 150% of max current. The boost will stop switching and the IC will disable the LED sinks until the fault is removed. When the fault is removed, the IC will try to regulate to the preset LED current. OFF ON OFF FSET/SYNC Short Protection Auto-restart Always YES Fault occurs when the FSET current goes above 150% of max current. The boost will stop switching, Disconnect switch will turn off and the IC will disable the LED sinks until the fault is removed. When the fault is removed, the IC will try to restart with soft-start. OFF ON OFF STOP during OVP event. ON ON Overvoltage Protection Auto-restart Always NO Fault occurs when current into OVP pin exceeds iOVP(th) (typically 150 µA). The IC will immediately stop switching but keep the LED drivers active, to drain down the output voltage. Once the output voltage decreases to ~94% OVP level, the IC will restart switching to regulate the output current. Undervoltage Protection Auto-restart Always YES Device immediately shuts off boost and current sinks if the voltage at VOUT is below VUVP(th). This may happen if VOUT is shorted to GND, or boost diode is open before startup. It will auto-restart once the fault is removed. OFF ON OFF ON ON OFF for shorted string, ON for all others. LED String Partial Short Detection (One-OutContinue option) Auto-restart Always YES Fault occurs if an LED pin voltage exceeds VLEDSC with its current sink in regulation, while at least one other LED pin is below ~1.2 V. This may happen when two or more LEDs are shorted within a string. The LED string exceeding the threshold will be disabled and removed from operation. Device will re-enable the LED string when its pin voltage falls below threshold, or at the next PWM = H. LED String Partial Short Detection (One-Out-All-Out option) Latched Always YES If two or more LEDs are shorted within a string, all LED strings will be disabled and the device latched off. To reset the fault, EN or PWM pin must be pulled low for ~16 ms. OFF OFF OFF Overtemperature Protection Auto-restart Always YES Fault occurs when the die temperature exceeds the over-temperature threshold, typically 170°C. IC will restart after temperatures drops lower by TSDHYS OFF OFF OFF VIN UVLO Auto-restart Always NO Fault occurs when VIN drops below VUVLO(fall). This fault resets all latched faults. OFF OFF OFF Continued on next page... Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 32 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Table 5: A80602 Fault Mode Table (continued) Fault Name Type Active Fault Flag Set Description Boost Switch Disconnect Switch LED Sink drivers FAULT pin pulled Low Externally (One-OutContinue option) Always ignored Always ignored No change In One-Out-Continue mode (with unidirectional FAULT pin), external status of FAULT pin does not affect the operation of the IC in any way. No change No change No change No change In One-Out-All-Out mode (with bidirectional FAULT pin), if FAULT pin is externally pulled Low, the IC immediately shuts off its boost and LED current sinks. IC can only restart when external fault status is cleared AND there is no internal fault status pending. That means local latching faults cannot be cleared by externally forcing FAULT pin to High. OFF ON OFF FAULT pin pulled Low Externally (One-Out-All-Out option) Auto-restart Always LED String Partial-Short Detection in One-Out-Continue mode (A80602) No Faults Fault Removed LED2 String Partial-Short Fault asserted PWM LED2 LED1, LED2 LED1 VLEDSD 0 200 mA i_LED 100 mA 0 FAULT A B CD E F G H Explanation of events : A: PWM goes High and all LED drivers operate normally. (For simplicity , assume only LED 1 and LED 2 are in use , each sinking 100 mA.) B: A partial -short fault is asserted to LED 2 string. Nothing happens yet since PWM = L. C: At the next PWM = H, LED2 pin voltage stays above VLEDSD while LED 1 pin is at regulation voltage. D: After partial -short detection time (~2 µs), LED2 string is disabled and FAULT pin pulled Low. LED1 string continues to operate. E: At subsequent PWM = H, IC retries LED 1 but shuts it off again since the fault is still present. FAULT flag remains Low . F: Partial Short fault is removed from LED 2 string. Nothing happens yet since PWM = L. G: At the next PWM = H, IC retries LED 1 and it passes . But FAULT flag is not cleared H: FAULT flag is cleared at the second PWM = H after Partial Short fault was removed . Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 33 Wide Input Voltage Range A80602 and High Power LED Driver with Pre-Emptive Boost ALT80600 High Efficiency Fault Tolerant LED Driver A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple PACKAGE OUTLINE DRAWING For Reference Only – Not for Tooling Use Reference Allegro DWG-2871 (Rev. A) or JEDEC MO-220WGGD. Dimensions in millimeters – NOT TO SCALE. Exact case and lead configuration at supplier discretion within limits shown. 0.50 0.30 4.00 ±0.10 24 24 0.95 1 1 2 A 2 2.80 4.10 4.00 ±0.10 DETAIL A 24× 2.80 D 0.08 C 0.75 ±0.05 C +0.05 0.25 –0.07 4.10 SEATING PLANE C PCB Layout Reference View 0.0-0.05 0.50 BSC 0.14 REF 0.20 REF 0.40 ±0.10 0.10 REF 0.05 REF 0.203 REF 0.05 REF 0.40 ±0.10 B 2.70 Detail A +0.10 –0.15 2 1 0.20 REF 24 2.70 +0.10 –0.15 A Terminal #1 mark area B Exposed thermal pad (reference only, terminal #1 identifier appearance at supplier discretion) C Reference land pattern layout (reference IPC7351 QFN50P400X400X80-25W6M); 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 0.10 REF Package ES, 24-Contact QFN with Exposed Pad and Wettable Flank Figure 46: Package ES, 24-Pin 4 mm × 4 mm QFN with Exposed Thermal Pad and Wettable Flank Allegro MicroSystems, LLC 115 Northeast Cutoff Allegro MicroSystems, LLC Worcester, Massachusetts 01615-0036 U.S.A. 955 Perimeter Road 1.508.853.5000; www.allegromicro.com Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 34 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple APPENDIX: EXTERNAL MOSFET SELECTION GUIDE The A80602 drives an external MOSFET for the boost stage. This solution provides maximum flexibility in delivering a wide range of output voltage and current for different LED panels, compared to controllers with built-in boost switches. On the other hand, care must be taken in selection of external MOSFET, to ensure optimal tradeoff between component size, efficiency, and cost. Primary Parameters to consider include the following. ON-RESISTANCE Device with lower RDSON can directly reduce the conduction loss of the boost converter. This is especially important when the output power is high and input supply voltage is low. Note that most datasheets typically highlight this parameter at VGS = 10 V and TJ = 25°C. It is important to examine how RDSON varies with gate voltage and temperature, as shown in the following charts: BREAKDOWN VOLTAGE Pick the device with “Drain to Source Breakdown Voltage” at least 20% higher than the maximum possible SW voltage. • For boost configuration, VSW = VOUT + VF; where VF = boost diode forward drop. The A80602 has a maximum VOUT of 40 V. Therefore, the MOSFET should be rated 50 V or higher. • For SEPIC configuration, VSW = VIN + VOUT + VF. Note that VIN can be as high as 40 V during load-dump conditions. The breakdown voltage needs to be increased accordingly. GATE THRESHOLD VOLTAGE The device must be fully enhanced by the time VGS = 5 V. Note that this is not the same as “Gate to Source Threshold Voltage” in most MOSFET datasheets, which is typically specified at very small current such as 250 µA. A more reliable way is to consult the “Gate Charge Characteristics” chart of the device, and make sure that the ‘plateau’ occurs well before VGS reaches 5 V. See example from datasheet of one potential candidate: Figure 47: Gate Charge vs. Gate-Source Voltage for an example MOSFET. Note plateau at VGS = 4.2 V approximately. Figure 48: Chart showing On-Resistance varies with Darin Current and Gate Voltage. Figure 49: Nominalized On-Resistance vs. Junction Temperature at VGS = 10 V. Note that resistance increases by 100% when temperature rises from 25°C to 150°C. Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 35 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple THERMAL RATING The thermal resistance (RθJA) is primarily determined by the device’s physical size. If the thermal resistance of the device is too high, or if there is insufficient heat dissipation on the PCB, the device may enter thermal run-away situation and burn itself out. For most medium-power (10-30 W) applications, a DPAK device is generally sufficient. For high-power (>50 W) applications, a D2PAK device may be required. Depending on power loss, additional heat sink can be mounted to improve the heat dissipation from the PCB. On the other hand, selecting a device with very low QG may cause excessive voltage spikes at SW node due to high dV/dt. In this case, a snubber circuit can be added to dampen the ringing. The switching speed can be slowed down by adding a series gate resistance (such as 1-5 ohm) between the driver and the device. The downside of doing this is higher switching losses. GATE CHARGE As mentioned earlier, lower RDSON is desired to reduce conduction loss. But devices with lower RDSON typically also have higher gate charge (QG), which can lead to higher switching loss. This is especially important when switching at high frequency (such as 2 MHz) and with high output voltage. Higher gate charge also results in higher gate driver current and hence higher power loss for the controller IC. The A80602 uses an LDO to supply the driver voltage (VDRV), which has a current limit of 36 mA typical. Average gate driver current is: iVDRV = fSW × QG If the MOSFET selected has QG = 27 nC, for example, then the highest switching frequency is limited to 1.33 MHz. See the following chart for relation between maximum switching frequency and MOSFET gate charge: Figure 51: Gate Charge vs. Gate-Source Voltage chart for a suitable MOSFET (NVD5867NL). Note that its plateau is at ~3.5 V, and its total gate charge is about 10 nC as VGS ramps up from 0 to 6.5 V. Max. Switching Frequency vs. Gate Charge 3.0 fsw (MHz) 2.5 2.0 1.5 1.0 0.5 0.0 10 15 20 25 30 Gate Charge (nC) 35 40 Figure 50: Maximum Switching Frequency vs. Gate Charge (to keep average VDRV current under 36 mA). Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com 36 A80602 and High Power LED Driver with Pre-Emptive Boost A80602-1 for Ultra-High Dimming Ratio and Low Output Ripple Revision History Number Date Description – March 4, 2019 Initial release Copyright 2019, 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 37