AP8801 500mA LED STEP-DOWN CONVERTER Description Pin Assignments The AP8801 is a step-down DC/DC converter designed to drive LEDs with (Top View) a constant current. The device can drive up to thirteen LEDs, depending on the forward voltage of the LEDs, in series from a voltage source of 8V to SET 1 48V. Series connection of the LEDs provides identical LED currents GND 2 NC 3 resulting in uniform brightness and eliminating the need for ballast resistors. The AP8801 switches at frequency up to 700kHz. This allows the use of small size external components, hence minimizing the PCB area AP8801 VIN 4 needed. Maximum output current of AP8801 is set via an external resistor applying either a DC voltage or a PWM signal at the CTRL input pin. An puts the device into a low-current standby state. SET 1 Features GND 2 LED driving current up to 500mA Operating input voltage up to 48V • High efficiency up to 92% • High switching frequency up to 700kHz • PWM/DC input for dimming control • Built-in output open-circuit protection • SO-8 and MSOP-8 are in “Green” Molding Compound (No Br, Sb) NC 3 VIN 4 AP8801 GND SW 5 SW 8 CTRL 7 GND 6 5 SW SW MSOP-8 Applications Totally Lead-Free & Fully RoHS Compliant (Notes 1 & 2) • Commercial & industrial lighting Halogen and Antimony Free. “Green” Device (Note 3) • Small LCD panel backlight • Appliances interior lighting • Architecture Detail lighting Notes: 7 6 (Top View) input voltage of 0.2V or lower at CTRL shuts down the output at SW and • CTRL SO-8 connected between the VIN and SET input pins. Dimming is achieved by • 8 1. No purposely added lead. Fully EU Directive 2002/95/EC (RoHS) & 2011/65/EU (RoHS 2) compliant. 2. See http://www.diodes.com for more information about Diodes Incorporated’s definitions of Halogen- and Antimony-free, "Green" and Lead-free. 3. Halogen- and Antimony-free "Green” products are defined as those which contain <900ppm bromine, <900ppm chlorine (<1500ppm total Br + Cl) and <1000ppm antimony compounds. Typical Application Circuit AP8801 Document number: DS31765 Rev. 7 - 2 1 of 14 www.diodes.com June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER Pin Descriptions Pin Name SW Functions Switch Pin. Connect inductor/freewheeling diode here, minimizing track length at this pin to reduce EMI. GND GND pin SET Set Nominal Output Current Pin. Configure the output current of the device. Dual Function Dimming Control Pin. • Input voltage of 0.2V or lower forces the device into low current standby mode and shuts off the output. • A PWM signal (driven by an open-drain/collector source) allows the output current to be adjusted over a wide range up to 100%. • An analog voltage between 0.3V and 2.5V adjusts the output current between 25% and 200% of the current set by 0.2V/RS. CTRL The input impedance is about 50kΩ, and if the pin is left open VCTRL = VREF VIN Input Supply Pin. Must be locally bypassed. NC No connection Functional Block Diagram Fig. 1 Block Diagram Absolute Maximum Ratings Symbol Rating Unit VIN VIN pin voltage Parameter -0.3 to +50 V VSW SW voltage -0.3 to +50 V VCTRL CTRL Pin Input Voltage -0.3 to +6 V VSENSE SET Voltage +0.3 to -5 V TJ Junction Temperature 150 o C TLEAD Lead Temperature Soldering 300 o TST Storage Temperature Range -65 to +150 o Caution: C C Stresses greater than the 'Absolute Maximum Ratings' specified above, may cause permanent damage to the device. These are stress ratings only; functional operation of the device at these or any other conditions exceeding those indicated in this specification is not implied. Device reliability may be affected by exposure to absolute maximum rating conditions for extended periods of time. Semiconductor devices are ESD sensitive and may be damaged by exposure to ESD events. Suitable ESD precautions should be taken when handling and transporting these devices. AP8801 Document number: DS31765 Rev. 7 - 2 2 of 14 www.diodes.com June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER Recommended Operating Conditions Symbol VIN VCTRLDC VCTRLL fOSC TA Duty Cycle Notes: Parameter Min Max Unit Operating Input Voltage relative to GND 8.0 48.0 V Voltage range for 24% to 200% DC dimming relative to GND (Note 4) 0.3 2.5 V Voltage Low for PWM dimming relative to GND 0 0.2 V Maximum Switching Frequency (Note 5) — 625 kHz Ambient Temperature Range -40 +105 ºC Using Inductor ≥ 100µH (Note 6) 0.10 0.95 — 4. For 100% brightness either leave floating or connect to 1.25V relative to GND. 5. AP8801 will operate at higher frequencies but accuracy will be affected due to propagation delays. 6. For most applications the LED current will be within 8% over the duty cycle range specified. Duty cycle accuracy is also dependent on propagation delay. Smaller size inductors can be used but LED current accuracy may be greater than 8% at extremes of duty cycle. This is most noticeable at low duty cycles (less than 0.1) or when the input voltage is high and only one LED is being driven. Electrical Characteristics (TA = 25°C, VIN = 24V; unless otherwise specified.) Symbol IOUT IQ Parameter Continuous switch current Conditions (Note 7) Quiescent Current Min Typ Max Unit — — 500 mA — 78 120 μA 200 216 mV VTHD Internal Threshold Voltage 184 VSENSEHYS Sense threshold hysteresis — 15 — % VREF Internal Reference Voltage — 1.25 — V SET SET pin input current VSET = VIN -0.2 — 7 — μA On Resistance of MOSFET ISW = 0.4A — 0.70 1.15 Ω — — 8 μA RDS(ON) ISW_LEAKAGE Switch leakage current θJA Thermal Resistance Junction-to-Ambient θJC Thermal Resistance Junction-to-Case Notes: SO-8 (Note 8) — 88 — °C/W MSOP-8 (Note 8) — 128 — °C/W SO-8 (Note 8) — 58 — °C/W MSOP-8 (Note 8) — 90 — °C/W 7. Refer to figure 6 for the device derating curve. 8. Test condition for SO-8 and MSOP-8: Device mounted on FR-4 PCB, 2”x2”, 2oz copper, minimum recommended pad layout on top layer and thermal vias to bottom layer ground plane. For better thermal performance, larger copper pad for heat-sink is needed. AP8801 Document number: DS31765 Rev. 7 - 2 3 of 14 www.diodes.com June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER 0.9 1.256 0.8 1.254 0.7 CTRL VOLTAGE (V) NON-SWITCHING SUPPLY CURRENT (mA) Typical Characteristics 0.6 0.5 0.4 0.3 1.252 1.250 1.248 0.2 1.246 0.1 1.244 -40 -25 -10 5 0 0 5 10 15 20 25 30 35 40 45 50 55 60 20 35 50 65 80 95 110 125 AMBIENT TEMPERATURE (°C) Figure. 3 VCTRL vs. Ambient Temperature INPUT VOLTAGE (V) Figure. 2 Supply Current (not switching) vs. Input Voltage 500 1.2 450 1.0 LED CURRENT (mA) 400 RDS(ON) (Ω) 0.8 0.6 0.4 350 300 250 200 150 100 0.2 50 0 -40 -25 -10 5 0 0 20 35 50 65 80 95 110 125 1.5 2.0 800 RSET = 0.56Ω 500 RSET = 0.68Ω 400 300 200 RSET = 1.33Ω 100 SWITCHING FREQUENCY (kHz) 600 LED CURRENT (mA) 1.0 RSET RESISTANCE (Ω) Figure. 5 LED Current vs. RSET AMBIENT TEMPERATURE (C) Figure. 4 RDS(ON) vs Ambient Temperature 0 0 0.5 700 600 500 400 300 200 100 0 0.5 1.0 1.5 2.0 CTRL VOLTAGE (V) Figure. 6 LED Current vs. VCTRL AP8801 Document number: DS31765 Rev. 7 - 2 2.5 4 of 14 www.diodes.com 0 0.5 1.0 1.5 2.0 2.5 CTRL VOLTAGE (V) Figure. 7 Switching Frequency vs. VCTRL June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER 10 100 8 95 6 90 4 85 EFFICIENCY (%) OUTPUT CURRENT DEVIATION (%) Typical Characteristics (TA = +25°C, VIN = 60V, L =68µH unless otherwise stated.) 2 0 -2 -4 80 75 70 65 -6 60 -8 55 50 -10 6 12 18 24 30 36 42 6 48 12 18 24 30 36 42 48 SUPPLY VOLTAGE (V) Figure. 9 Efficiency vs. Input Voltage SUPPLY VOLTAGE (V) Figure. 8 LED Current vs. Input Voltage 700 100 80 500 DUTY CYCLE (%) SWITCHING FREQUENCY (kHz) 90 600 400 300 200 70 60 50 40 30 20 100 10 0 0 6 12 18 24 30 36 42 48 SUPPLY VOLTAGE (V) Figure. 10 Switching Frequency vs. Input Voltage Figure. 12 Steady State Waveforms AP8801 Document number: DS31765 Rev. 7 - 2 6 12 18 24 30 36 42 48 SUPPLY VOLTAGE (V) Figure. 11 Duty Cycle vs. Input Voltage Figure.13 Start-Up Showing LED Current Soft -Start 5 of 14 www.diodes.com June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER Application Information LED Current Control The LED current is controlled by the resistor RSET in Figure 14 connected between VIN and SET. The nominal average output current in the LED(s) is defined as: ILED = VTHD R SET where VTHD is the voltage between the VIN and SET pins and is nominally 200mV. Figure. 14 Typical Application Circuit for ILED = 0.3A Inductor Selection This section highlights how to select the inductor suitable for the application requirements in terms of switching frequency, LED current accuracy and temperature. 600 Switching Frequency @ILED = 500mA 100 47 12V - 1 LED 500 24V - 3 LEDs FREQUENCY (kHz) 68 48V - 6 LEDs 150 400 100 220 300 47 150 200 220 68 470 100 100 470 150 220 470 0 0 50 100 150 200 250 300 350 400 450 500 INDUCTOR VALUE (µH) Figure. 15 Switching Frequency vs. Inductor Value AP8801 Document number: DS31765 Rev. 7 - 2 6 of 14 www.diodes.com June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER Application Information (cont.) The inductor influences the LED current accuracy that the system is able to provide. The following section highlights how to select the inductor in relation to the device packages and the LED current, while maintaining the chip temperature below 70°C. 12 68µH 11 100µH 150µH 10 220µH 470µH 9 NUMBER of LEDs 8% Accuracy, <70°C Case Temperature 47µH 8 7 470µH 220µH 6 5 100µH 150µH 4 68µH 3 47µH 2 1 0 5 10 15 20 25 30 35 40 45 SUPPLY VOLTAGE (V) Figure. 16 Minimum Recommended Inductor with 500mA LED Current Capacitor Selection The small size of ceramic capacitors makes them ideal for AP8801 applications. X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as Z5U. A 2.2μF input capacitor is sufficient for most intended applications of AP8801. A 4.7μF input capacitor is suggested for application with an input voltage equal or higher than 40V. Diode Selection Schottky diodes, e.g. B1100, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AP8801 applications. AP8801 Document number: DS31765 Rev. 7 - 2 7 of 14 www.diodes.com June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER Application Information (cont.) LED Current Adjustment/Dimming. The LED current for the AP8801 can be adjusted by driving the CTRL with a digital signal (PWM dimming) or by driving the CTRL with a dc voltage between 0.3V and 2.5V (DC dimming). If the CTRL pin is driven by an external voltage (lower than 2.5V), the average LED current is: VCTRL VTHD × VREF R SET ILED = where VREF is nominally 1.25V 600 LED Current @ VCTRL = 1.25V LED Current @ VCTRL = 2.5V LED Current @ VCTRL = 0.625V 0.4 500 0.82 LED CURRENT (mA) 0.47 1 400 0.56 1.2 0.68 300 1.5 0.82 200 2 1 0.47 1.2 3 1.5 0.68 100 2 0.82 1 3 1.2 1.5 2 3 0 0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3 RSET VALUE (Ω) Figure. 17 LED Current vs. RSET and VCTRL Figure 17 shows that reducing the CTRL voltage by a factor of 2 also reduces the LED current by a factor of 2. The AP8801 has the ability vary the LED current by a factor of 2 above the default value set by RSET down to a factor of 0.24 of the nominal LED current. This provides an 8.33:1 dynamic range of the dc dimming. A low pass filter on the CTRL pin of the AP8801 automatically provides some soft-start function of the LED current on initial start-up (this phenomenon can be seen in figure 7); the built in soft-start period can be increased by the addition of an external capacitor onto the CTRL pin. The AP8801’s dimming range can be increased above this dc dimming factor by applying a PWM signal to the CTRL pin using this method dimming dynamic ranges above 100 can be achieved. AP8801 Document number: DS31765 Rev. 7 - 2 8 of 14 www.diodes.com June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER Application Information (cont.) PWM Dimming of LED Current When a low frequency PWM signal with voltages between 2.5V and a low level of zero is applied to the CTRL pin the output current will be switched on and off at the PWM frequency. The resultant LED current ILEDavg will be proportional to the PWM duty cycle. See figure 18. A Pulse Width Modulated (PWM) signal with a max resolution of 8-bit, can be applied to the CTRL pin to change the output current to a value above or below the nominal average value set by resistor RSET. To achieve this resolution the PWM frequency has to be lower than 500Hz. The ultimate resolution will be determined by the number of switching cycles required to get back to nominal LED current once the PWM voltage is high relative to PWM frequency. Lower switching frequencies and higher PWM frequencies will result in lower PWM dimming dynamic ranges. Figure. 18 Low Frequency PWM Operating Waveforms There are different ways of accomplishing PWM dimming of the AP8801 LED current: Directly Driving CTRL Input A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the CTRL pin to adjust the output current to a value above or below the nominal average value set by resistor RSET. When driving the CTRL with a voltage waveform care should be taken not to exceed a drive voltage of 2.5V (where extra brightness is required) or 1.25V if a maximum of 100% brightness is required. A way of avoiding over-driving the CTRL pin is use an open collector/drain driver to drive the CTRL pin. Driving the CTRL Input via Open Collector Transistor The recommended method of driving the CTRL pin and controlling the amplitude of the PWM waveform is to use a small NPN switching transistor. This uses the internal pull-up resistor between the CTRL pin and the internal voltage reference to pull-up CTRL pin when the external transistor is turned off. Driving the CTRL Input from a Microcontroller If the CTRL pin is driven by a MOSFET (either discrete or open-drain output of a micro-controller) then Schottky diode maybe be required due to high Gate / Drain capacitance, which could inject a negative spike into CTRL input of the AP8801 and cause erratic operation but the addition of a Schottky clamp diode (eg. Diodes Inc. SD103CWS) to ground and inclusion of a series resistor (3.3k) will prevent this. AP8801 Document number: DS31765 Rev. 7 - 2 9 of 14 www.diodes.com June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER Application Information (cont.) Soft-Start An external capacitor from the CTRL pin to ground will provide a soft-start delay, by increasing the time taken for the voltage on this pin to rise to the turn-on threshold and by slowing down the rate of rise of the control voltage at the input of the comparator. Adding capacitance increases this delay by approximately 200µs/nF. The graph below shows the variation of soft-start time for different values of capacitor. 16 14 SOFT-START TIME (ms) 12 10 8 6 4 2 0 -2 0 20 40 60 80 100 120 CAPACITANCE (nf) Figure. 19 Soft-Start Time vs. Capacitance from ADJ Pin to Ground Thermal Considerations The graph below in figure 20, gives details for power derating. This assumes the device to be on a FR-4 PCB, 2”x2”, 2oz copper, minimum recommended pad layout on top layer and thermal vias to bottom layer ground plane standing in still air. 1.2 SO-8 POWER DISSIPATION (W) 1.0 0.8 MSOP-8 0.6 0.4 0.2 0 -50 AP8801 Document number: DS31765 Rev. 7 - 2 -25 0 25 50 75 100 AMBIENT TEMPERATURE (°C) Figure. 20 Maximum Power Dissipation 10 of 14 www.diodes.com 125 June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER Application Information (cont.) Package Selection The device comes with a wide selection of packages. The suggested package is able to provide a case temperature below 70˚C (with an ambient temperature of +25°C) for the combination of input voltage and load requested. 12 LEDs ILED = 500mA L = 100µH 10 LEDs 8 LEDs SO-8 6 LEDs 4 LEDs MSOP-8 SO-8 2 LEDs 8 12 16 20 24 28 32 36 40 44 48 Figure. 21 Package Section Fault Condition Operation The AP8801 has by default open LED protection. If the LEDs should become open circuit the AP8801 will stop oscillating; the SET pin will rise to VIN and the SW pin will then fall to GND. No excessive voltages will be seen by the AP8801. If the LEDs should become shorted together the AP8802H will continue to switch however the duty cycle at which it will operate will change dramatically and the switching frequency will most likely decrease. The on-time of the internal power MOSFET switch will be significantly reduced because almost all of the input voltage is now developed across the inductor. The off-time will be significantly increased because the reverse voltage across the inductor is now just the Schottky diode voltage (See Figure 20) causing a much slower decay in inductor current. During this condition the inductor current will remain within its controlled levels and so no excessive heat will be generated within the AP8801. Figure. 22 Switching Characteristics (normal open to short LED chain) AP8801 Document number: DS31765 Rev. 7 - 2 11 of 14 www.diodes.com June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER Ordering Information AP8801 XX G - 13 Device Green Packing S : SO-8 M8 : MSOP-8 G : Green 13 : Tape & Reel Package Code AP8801SG-13 AP8801M8G-13 Note: Package 13” Tape and Reel Packaging (Note 9) Quantity Part Number Suffix S SO-8 2500/Tape & Reel -13 M8 MSOP-8 2500/Tape & Reel -13 9. Pad layout as shown on Diodes Inc. suggested pad layout document AP02001, which can be found on our website at http://www.diodes.com/datasheets/ap02001.pdf. Marking Information (1) SO-8 ( Top View ) 5 8 Logo Part Number G : Green YY : Year : 08, 09,10~ WW : Week : 01~52; 52 represents 52 and 53 week X : Internal Code AP8801 YY WW X X 1 4 (2) MSOP-8 ( Top View ) 8 7 Logo AP8801 1 Document number: DS31765 Rev. 7 - 2 5 YWX Part Number AP8801 6 2 3 A~Z : Green Y : Year : 0~9 W : Week : A~Z : 1~26 week; a~z : 27~52 week; z represents 52 and 53 week 4 12 of 14 www.diodes.com June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER Package Outline Dimensions 0.254 (1) SO-8 E1 E A1 L Gauge Plane Seating Plane Detail ‘A’ 7°~9° h 45° Detail ‘A’ A2 A A3 SO-8 Dim Min Max A 1.75 A1 0.10 0.20 A2 1.30 1.50 A3 0.15 0.25 b 0.3 0.5 D 4.85 4.95 E 5.90 6.10 E1 3.85 3.95 e 1.27 Typ h 0.35 L 0.62 0.82 0° 8° θ All Dimensions in mm b e D (2) MSOP-8 D 4x 10 ° 0.25 E Gauge Plane x Seating Plane a y 4x10° L Detail C 1 b E3 A3 A2 A e A1 AP8801 Document number: DS31765 Rev. 7 - 2 E1 c See Detail C 13 of 14 www.diodes.com MSOP-8 Dim Min Max Typ A 1.10 A1 0.05 0.15 0.10 A2 0.75 0.95 0.86 A3 0.29 0.49 0.39 b 0.22 0.38 0.30 c 0.08 0.23 0.15 D 2.90 3.10 3.00 E 4.70 5.10 4.90 E1 2.90 3.10 3.00 E3 2.85 3.05 2.95 e 0.65 L 0.40 0.80 0.60 a 0° 8° 4° x 0.750 y 0.750 All Dimensions in mm June 2012 © Diodes Incorporated AP8801 500mA LED STEP-DOWN CONVERTER IMPORTANT NOTICE DIODES INCORPORATED MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARDS TO THIS DOCUMENT, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION). 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