date: dec 5-06 rev e constant current led driver ? 2006 sipex corporation sp7618 solved by tm the sp76 8 is a linear, low-side constant-current driver designed to drive high power leds from an input voltage rail. the driver acts as a current source, ensuring constant led current for a range of input voltages. the sp76 8 allows implementing the lowest cost led driver for a variety of applications. internal circuitry maintains the preset constant current output for a wide voltage range at the led input. the led current can be adjusted up to 033ma with an external clock applied to the ctrl pin. the built-in thermal protection automatically stops led current to prevent overheating. a timeout function serves as a failsafe, shutting down the output for currents greater than 266ma after 4 seconds of continuous operation if the enable input is not cycled. the part can be shut down by using the en pin, or sent into sleep mode holding the ctrl pin low for more than 00 s. features very low dropout voltage ( 00mv @ a) accurate current regulation down to dropout voltage no external components built-in current dac output current adjustable with 33.3 ma/steps power-saving shutdown mode of a fast turn-on (<50 s) timeout function to protect the led thermal shutdown protection a current capability rohs compliant, lead free packaging: space saving 2mmx3mm 8pin dfn constant current led driver applications next generation mobile phones pda, dsc, mp3 players handheld computers lcd display modules keyboard backlight sp7618 8 pin dfn 8 7 6 5 1 2 3 4 gndp gndp gnd ctrl led en led v cc typical application circuit general description
2 date: dec 5-06 rev e constant current led driver ? 2006 sipex corporation supply voltage (vcc) ...6.0v input voltage at any input ..-0.6v to vcc +0.5v 8-pin dfn package thermal resistance................................ 59 0 c/w junction temperature .......... -40 0 c to +70 0 c * storage temperature .....-65 c to +50 c soldering temperature ...+240 c** *) internally limited **) duration 0s maximum ambient temperature t a = 25c, 3.0v < vcc < 5.5v, .0v < vled < 3.0v, 00ma < iled <033a. bold denotes specifcations which apply over the full operating temperature range, -40c to +85c. parameter min typ max units conditions vcc voltage operating range 2.5 5.5 v quiescent current 350 420 a i led = 35ma supply current shutdown mode a en = gnd, ctrl = dc sleep mode 350 en = vcc, ctrl = gnd after time duration 2 350 en = vcc, ctrl = dc led output current after 0 pulses on ctrl 3 25 na v led = .0v, vcc = 5.5v pulse on ctrl 4 35 ma 3 pulses on ctrl 4 02 3 pulses on ctrl 033 led current accuracy 5 -8 8 % 35ma < i led < 033 ma led current line regulation 6 0. 0.5 %/v 3.0 < vcc < 5.5v,v led =v led current load regulation 7 .5 6 % < v led < 2.5v, vcc = 5.5v led current thermal regulation 0.0 %/c i led = 00 ma v led dropout voltage 8 50 00 mv i led = 00ma @ 25c 90 85 i led = 700ma @ 25c 00 20 i led = 033ma @ 25c thermal shutdown die tempera - ture 60 c i led = 35ma thermal shutdown hysteresis 25 c setup time after last count 9 50 s ctrl input data rate 4 mhz wake-up time from shutdown 50 s en = low to high, ctrl = dc turn-off time into shutdown 30 s en = high to low, ctrl = dc turn-off time into sleep mode 25 450 s en = high, ctrl = high to low time duration before sleep mode 2 4 7 s i led > 266 ma, en = high, ctrl = high en and ctrl pin logic voltage low 0.4 v driver in shutdown mode high 1.4 vcc 10 2.5v vcc 5.5v power-on-reset 20 ms see notes on next page these are stress ratings only and functional operation of the device at these ratings or any other above those indicated in the operation sections of the specifcations below is not implied. exposure to absolute maximum rating conditions for extended periods of time may affect reliability. absolute maximum ratings electrical characteristics
3 date: dec 5-06 rev e constant current led driver ? 2006 sipex corporation block diagram vcc must be always higher than led pin voltage (v led ) after 4s at current higher than 266 ma power mosfet leakage current three initial i led current steps are higher than other 28 i led variations from specifed i led value i led variations per volt vcc change i led variations at v led change from to 2.5v 5% change of i led compared with iled value at v led = v i led = 00 ma settles 50 s later after 3rd pulse logic high level should never exceed vcc voltage delay between applying vcc voltage and operating condition pin assignments pin # pin name pin function - 2 led led current sink. connect led cathode to this pin. voltage at this pin should be above 00 mv to maintain regulation. 3 vcc power supply input. place f decoupling capacitor next to this pin. 4 en enable/shutdown pin used to enable/disable driver and reset dac: logic high C enable, logic low C disable/reset. 5 ctrl dac serial input pin is used to control led current. positive pulse edges sequentially increase led current with 33.33ma steps up to 033 ma (3 steps). when this pin is held low for longer than 00s the part enters sleep mode. 6 gnd ground pin for control and bias blocks. 7 - 8 gndp ground pin. led current fows through this pin to ground. . 2. 3. 4. 5. 6. 7. 8. 9. 0. . notes (continued from previous page)
4 date: dec 5-06 rev e constant current led driver ? 2006 sipex corporation 0 200 400 600 800 000 200 0 0.5 .5 2 2.5 3 3.5 vled, v iled, ma 3 pulses 7 pulses 5 pulses 3 pulses load regulation at vcc = 5.5v @ 25c 0 200 400 600 800 000 200 0 0.2 0.4 0.6 0.8 vled, v iled. ma 3 pulses 7 pulses 5 pulses 3 pulses load regulation at vcc = 5.5v @ 25c 700 750 800 850 900 950 000 050 00 0 00 200 300 400 500 600 700 25c 85c load regulation vs. temperature at i led = a and vcc = 5.5v application information
5 date: dec 5-06 rev e constant current led driver ? 2006 sipex corporation 0 200 400 600 800 000 200 0 5 0 5 20 25 30 35 number of pulses iled, ma dac linearity at v led = 500mv -0.5 -0.4 -0.3 -0.2 -0. 0 0. 0.2 0.3 2.5 3 3.5 4 4.5 5 5.5 6 vcc, v line regulation, % line regulation at v led = 00ma setup time at i led = 033a application information
6 date: dec 5-06 rev e constant current led driver ? 2006 sipex corporation introduction the sp76 8 is a linear low-side, constant- current driver designed to drive high power leds from an input voltage rail. the design consists of a regulator reference voltage source, dac, voltage-to-current converter, thermal shutdown, timeout shutdown, and output driver. the precision reference voltage ensures good performance over voltage and temperature. the built-in thermal protection automatically stops led current to prevent overheating. a timeout function serves as a failsafe, shutting down the output for cur - rents greater than 266ma after 4 seconds of continuous operation if the enable input is not cycled. current setting dac the led output current is defned by a cur - rent control dac. the input to the dac is the ctrl pin. the dac outputs a unit current of 33.33ma with each applied pulse at the ctrl pin. the dac has 5 bits, so the total number of steps that can be achieved is 2 5 or 32. the initial step of the dac corresponds to zero, so the maximum current output is 3 times the unit current. any multiple of the 33.33ma output current can be achieved from to 3 sequentially. the dac ctrl input is positive-edge trig - gered and may be either at high or low logic state when the part is powered up. the en - able pin logic high level may be applied with vcc voltage. the ctrl input allows an increase in led current only. if a lower i led current value is required, the device should be sent into shutdown mode (en =high to low/low to high) and a new pulse sequence should be applied to the ctrl pin. if the pulse number exceeds 3 , all other pulses are ignored and 033 ma current is set. the sp76 8 can be placed into sleep mode by applying a logic low on the ctrl pin for longer than 00 s after any pulse sequence. in the sleep mode, the driver is disabled and i led current is zero, but blocks associated with the dac remain active, and icc sleep mode current is always higher than icc current in shutdown mode. this function is used to output the i led current for a speci - fed duration. if the i led current exceeds 266 ma (8 pulses) the maximum time duration is 4s. after that, the device is automatically sent into sleep mode. this function is intended to protect the led if, for example, the device was left on in a high current condition for a long period of time. appyling a new pulse sequence to the ctrl pin at sleep mode will wake up the device but increase the i led current at 33.3ma multiplied by the number of pulses. to return the device to the previous or lower i led current value, use shutdown (en =high to low/low to high) and apply a new pulse sequence to the ctrl pin. the current control dac accepts clock rates up to 4mhz. the code for the dac loads while the output driver takes 50 s to respond. this implies that the led output current will reach its fnal value 50 s after the fnal pulse is applied to the ctrl pin. theory of operation
7 date: dec 5-06 rev e constant current led driver ? 2006 sipex corporation output driver the sp76 8 features a low-side regulated output current driver. a large mosfet device is connected between the led and gndp pins. the mosfet drain is connected to the led pins and the mosfet source is connected to the gndp pins. there are two pins on each terminal of the mosfet in order to minimize power dissipation inside the package. even though there are two led and gndp pins, there is actually only one output intended to drive one led. the light emitting diode connects from vcc to the led pins. the i led current regulation is guaranteed at dropout at the led pins to be less than 00mv. the output current is regulated to within fve percent of nominal value even if the voltage between the led and gndp pins varies over a wide range. the nominal output is guaranteed within 8% of the defned current. thermal protection the sp76 8 has built-in thermal protection. an internal p - n junction is compared to the in - ternal temperature compensated reference. when the p - n junction reaches a temperature specifc voltage, it trips a comparator which shuts down the driver stage of the part placing the device into sleep mode while dac circuit remains active. the shutdown temperature is defned to be 160c. the comparator circuit has built-in hysteresis corresponding to 25c. the output stage will resume operation and start sinking the cur - rent that was defned prior to shutdown when the internal temperature drops by 25c. if the part remains in a high ambient temperature environment or if a high power dissipation condition exists, the output stage will cycle on and off to maintain a preset internal junction temperature to avoid catastrophic damage. in that case,the led will appear to be blinking. lower the led voltage to avoid excess power dissipation in the ic. shutdown the sp76 8 has a low current shutdown function. in shutdown mode, the part draws less than 0. a current maximum. the part can be placed into shutdown using the en pin. enable and ctrl pins cannot be tri-stated or left foating. there are no predefned in - ternal states, so leaving these pins open will cause the part to operate incorrectly. timeout a time duration before shutdown feature acts as a failsafe to protect the led from long on times. the nominal timeout is defned as 4 seconds. power-on-reset this function is used to set dac and time duration counters into their initial state after power up. no signal applied to the ctrl pin can be accepted during this time, even if the input voltage is applied simultaneously to vcc and enable pins. theory of operation
8 date: dec 5-06 rev e constant current led driver ? 2006 sipex corporation to obtain the best performance from the sp76 8, a printed circuit board with ground plane is required. high quality, low series resistance ceramic f bypass capacitors should be used at the vcc pin (pin 3). these capacitors must be located as close to pins 3 C 6 as possible. the traces connecting the pins and the bypassing capacitors must be kept short and should be made as wide as possible. pins C 2 are connected together internally but we recommend connecting the led cathode to both pins to avoid additional power losses at high current. pins 6, 7 and 8 should be connected to the ground plane. board layout should prevent high led ground current from fowing through signal ground connected to pin 6. this pin is a separate ground for the reference and logic sections of the sp76 8. the quiescent current for the part fows into the vcc pin and out of the gnd pin. this current is proportional to the i led current divided by approximately 500. timing diagram board layout and grounding
9 date: dec 5-06 rev e constant current led driver ? 2006 sipex corporation p ackage: 8 pin dfn
0 date: dec 5-06 rev e constant current led driver ? 2006 sipex corporation sipex corporation headquarters and sales offce 233 south hillview drive milpitas, ca 95035 tel: (408) 934-7500 fax: (408) 935-7600 ordering information sipex corporation reserves the right to make changes to any products described herein. sipex does not assume any liability arising out of the application or use of any product or circuit described herein; neither does it convey any license under its patent rights nor the rights of others. s o l v e d b y t m part number operating temperature range package type sp768er-l...................................-40c to +85c ............................lead free 8 pin dfn (2mm x 3mm) sp768er-l/tr ...... ......................-40c to +85c ............................lead free 8pin dfn (2mm x 3mm) /tr = tape and reel pack quantity is 3,000 for dfn.
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