issue 5 C august 2009 www.diodes.com ? diodes incorporated, 2009 AP8801EV2 user guide description the AP8801EV2, figure 1, is a pcb constructed using an fr4 base for evaluating the ap8801 led driver with internal switch in msop-8l package. the evaluation board can be used to drive an external choice of leds; the total forward voltage across the leds depending on the number and type connected. [1] the suggested operating voltage for the evaluation board ranges from 8v to 45v maximum. higher voltage means lower supply current. the nominal current for the evaluation board is set at 460ma with a 0r43 sense resistor, r1. test point ctrl provides a connection point for dc or pwm dimming and shutdown. note: the evaluation board does not have reverse polarit y protection warning: with 500ma output, the connected led will be hot and very bright figure1: AP8801EV2 evaluation board ap8801 device description the ap8801 is a step-down dc/dc converter designed to drive leds with a constant current. the device can drive up to thirteen leds, depending lar gely on the forward voltage of the leds, in series from a voltage source of 8v to 48v. series connecti on of the leds provides identical led currents resulting in uniform brightness and eliminates the need for ballast resistors. the ap8801 switches at frequencies up to 500khz. this allows the use of s mall size external components, hence minimizing the pcb area needed. maximum output current of ap8801 is set via an exte rnal resistor connected between the v in and set input pins. dimming is achieved by applying either a dc voltage or a pwm signal at the ctrl input pin. an input voltage of 0.2v or lower at ctrl shu ts down the output at sw and puts the device into a low-current standby state. the device includes the output switch and a current sense circuit, which requires an external sense resistor to set the nominal current up to 500ma.
AP8801EV2 issue 5 C august 2009 www.diodes.com ? diodes incorporated 2009 2 ap8801 device features led driving current up to 500ma input voltage from 8v to 48v high efficiency up to 92% high switching frequency up to 500khz minimum pwm/dc input for dimming control built-in soft-start function built-in output open-circuit protection sop-8l, msop-8l are available in green molding compound (no br, sb) lead free finish/rohs compliant (note 1) device applications commercial & industrial lighting small lcd panel backlighting appliance interior lighting architecture detail lighting ap8801 device packages, pin and definitions (1) sop-8l (2) msop-8l sop-8l 1 2 3 4 5 6 7 8 ( top view ) ap8801 nc sw ctrl set sw v in gnd gnd msop-8l 1 2 3 4 5 6 7 8 ( top view ) ap8801 nc sw ctrl set sw v in gnd gnd pin description name description sw switch pin. connect inductor/freewheeling diode her e. minimize trace area at this pin to reduce emi. gnd gnd pin set set nominal output current pin. configure the o utput current of the device. ctrl dimming and on/off control input. input voltage of 0.2v or lower forces the device into low current standby mode and shut off the output. a pwm signal allows the output current to be adjusted above or below the level set by the resistor connected to set input pin. the input impedance is about 50k, and if the pin is left open v ctrl = v ref v in input supply pin. must be locally bypassed with a capacitor. nc no connection ordering information evalboard order number AP8801EV2 please note: evaluation boards are subject to availability and qualified leads. device order number device package tape & reel s (sop8l) -13 ap8801 m8 (msop-8l) -13
AP8801EV2 issue 5 C august 2009 www.diodes.com ? diodes incorporated 2009 3 AP8801EV2 evaluation board reference design the AP8801EV2 is an evaluation board configured to be used with the ap8801 in msop-8l package. the target application is a driver for one or more series-connected leds for luminaires in both commercial and automotive applications. the suggested maximum operating voltage for the eva luation board is 45v , and a 0r43 sense resistor r1 sets the nominal current at 460ma. an accurate way of determining the current, avoidin g the need to insert an ammeter in the current path, is to measure the voltage on the sense resist or. a 10k resistor and a 1uf capacitor can be used to form a low pass filter and the voltage across th e capacitor represents a more stable dc reading of current. using this method, 200mv represents 0.46 a mp when using a 0r43 sense resistor. the ctrl pin connects to a low pass filter within t he ap8801 chip to provide some decoupling, but the external capacitor c2 (100nf) is used to prov ide additional decoupling to reduce any high frequency noise as well as providing soft start fun ction. both dc and pwm dimming can be achieved by driving the ctrl pin. for dc dimming, the ctrl pin may be driven between 0.3v and 1.25v. driving the c trl pin below 0.2v will shut down the output current. for pwm dimming, an external open-collecto r npn transistor or open-drain n-channel mosfet can be used to drive the ctrl pin. the pwm f requency can be around 100hz to 1khz. for better resolution, pwm capacitor c2 should be r emoved on the evaluation board, to give a more accurate duty cycle . shorting r2 will connect the test pin ctrl to devic e pin ctrl if needed. the external capacitor c2 on the ctrl pin sets the soft start time. the amoun t of soft start time achievable is approximately 0.2ms/nf. for other reference designs or further applications information, please refer to the ap8801 datasheet.
AP8801EV2 issue 5 C august 2009 www.diodes.com ? diodes incorporated 2009 4 ap8801 operation in normal operation, when voltage is applied at v in , the ap8801s internal ndmos switch is turned on. current starts to flow through sense resistor r 1, inductor l1, and the led(s). the current ramps up linearly. the ramp rate is determined by the inp ut voltage v in and inductor l1. this rising current produces a voltage ramp across r1. the internal cir cuit of the ap8801 senses this voltage and applies a proportional voltage to the input of the internal comparator. when this voltage reaches an internally set upper threshold, the ndmos switch is turned off. the inductor current continues to flow through r1, l1, led(s), schottky diode d1, and back to the supply rail. the current decays, with the rate of decay determined by the forward voltage dro p of the leds and the schottky diode. this decaying current produces a falling voltage at r1 w hich is sensed by the ap8801. a voltage proportional to the sense voltage across r1 is appl ied at the input of the internal comparator. when this voltage falls to the internally set lower thre shold, the ndmos switch is turned on again. this switch-on-and-off cycle continues to provide an ave rage current (set by the sense resistor r1) to the leds. please refer to the datasheet for the thresho ld limits, ap8801 internal circuits, electrical characteristics and parameters . AP8801EV2 evaluation board - bill of materials ref value package part number manufacturer notes u1 ap8801 msop-8l ap8801m8g diodes inc dc-dc converter d1 100v, 2a smb b2100 diodes inc schottky diode r1 0r43 1206 sr732bttdr430f generic koa +/-1%, +/-100ppm r2 0r0 0805 generic - +/-1% c1 4.7uf 50v 1210 c1210c475k5rac grm32er71h475k k emet murata +/-10% c2 - - not fitted - c3 100nf, 100v 0805 c0805c104k1rac nmc0805x7r104k100 kemet nic +/-10% c4 100nf, 100v 1206 c1206c104k1rac grm319r72a104k nmc1206x7r104k100 kemet murata nic +/-10% l1 68uh - mss1038-683ml npis104f680mtrf npis24h680mtrf coilcraft nic nic 68uh the fr4 pcb design, with adequate copper top and bo ttom and plated through vias for thermal coupling, guarantees a good thermal dissipation for the ap8801 device. other sources of heat are the schottky diode, the inductor and the sense resistor , therefore care must be taken in their placement. warning: at 45v operation with 500ma output, the bo ard temperature rises by around 50c from ambient after 30 minutes of operation.
AP8801EV2 issue 5 C august 2009 www.diodes.com ? diodes incorporated 2009 5 figure 3: component layout and circuit board view AP8801EV2 connection point definition name description vin positive supply voltage. gnd supply ground (0v). ctrl internal voltage ref. pin (1.25v). this pin ca n be used to achieve dimming and soft-start, and for switching the output current of f. leave floating for normal operation. see 'circuit features' section to achieve dimming, soft-start and for switching the output current off. led a led a connects to the external led anode led k led k connects to the external led cathode AP8801EV2 basic operation at full voltage 1. connect external leds across test pins led a ( anode) and led k (cathode). the number of external leds that can be connected depends on t heir operating power and forward voltage drop, but typically 12 x 3.2v leds can be c onnected using a 45v rail. for an external load other than leds, the positive terminal of the load should be connected to the anode and the negative to the cathode. 2. connect vin and gnd. warning: the board does not have reverse battery/su pply protection. 3. set the psu to the desired input voltage (usuall y between 12v and 45v) 4. turn on the psu. the external leds will illumina te and the current should be approximately 460ma 5. the switching waveform at sw can be acquired usi ng the test point j1 warning: do not stare at the leds directly. c4
AP8801EV2 issue 5 C august 2009 www.diodes.com ? diodes incorporated 2009 6 circuit features n.b. remove power whilst changing components! soft-start 1. the ap8802 has a in-built soft start function. a ca pacitor, c2 may be fitted to the evaluation board to increase the soft start time by slowing th e rise time of the adjust pin at start-up at the rate of 0.2ms/nf. the board is supplied with a zero -ohm resistor in position r2. please see the data sheet for further details. pwm 1. remove the soft start capacitor c2 2. refer to the datasheet for instructions on how t o perform pwm switching off the output current 3. shorting the ctrl pin to gnd will cause the led current to go to zero. releasing this pin will switch on the system (creating a soft-start power u p sequence if the c2 capacitor is used). changing the led current 1. refer to the datasheet for the derating curve an d the power dissipation capability of the package 2. remove r1 3. calculate and fit a new sense resistor, r1, the value of which is based on the required led current without dimming. r1 can be calculated using following equation : r1= 0.2v / i led where i led = the led current. r1 = the sense resistors value in ohms. 0.2v is the nominal sense voltage with ctrl open cir cuit or set to 1.25v. performance the system efficiency depends on the sense resistor , supply voltage, switching inductor, and the number of leds. with a 45v supply and 12 leds, the switching freque ncy is typically 320 khz and efficiency levels >90% are achievable. visit our website www.diodes.com to find useful tools for circuit design and simula tion. reference [1] ap8801 datasheet C www.diodes.com
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