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general description the max6978 serial-interfaced led driver provides eight open-drain, constant-current sinking led driver outputs rated at 5.5v. the max6978 operates from a 3v to 5.5v supply. the max6978 supply and the leds supply can power up in any order. the constant-current outputs are programmed together to up to 55ma using a single external resistor. the max6978 operates with a 25mb, industry-standard, 4-wire serial interface. the max6978 includes circuitry that automatically detects open-circuit leds. fault status is loaded into the serial-interface shift register when le goes high and is automatically shifted out on dout when the next data transmission is shifted in. the max6978 includes a fail-safe feature for safety- related applications, which blanks the display if the ser- ial interface fails. if the serial interface becomes inactive for more than 1s, all driver output latches are automatically cleared. this turns off all leds connected to the outputs. the shift register data is not disturbed. the outputs remain off until the driver output latches are updated with data turning them on, so recovery is auto- matic if the transmission failure is temporary. the watchdog function requires no software change to the application driving the max6978. the max6978 uses the industry-standard shift-register- plus-latch-type serial interface. the driver accepts data shifted into an 8-bit shift register using data input din and clock input clk. input data appears at the output dout eight clock cycles later to allow cascading of multiple max6978s. the latch-enable input le loads the 8 bits of shift-register data into an 8-bit output latch to set which leds are on and which are off. the output- enable input oe gates all eight outputs on and off, and is fast enough to be used as a pwm input for led intensity control. the max6978 is one of a family of 12 shift-register-plus- latch-type led drivers. the family includes 8-port and 16-port types, with 5.5v- or 36v-rated led outputs, with and without open-circuit led detection and watchdog. all versions operate from a 3v to 5.5v supply, and are specified over the -40? to +125? temperature range. features ? 25mb industry-standard 4-wire serial interface at 5v ? 3v to 5.5v logic supply ? 8 constant-current led outputs rated at 5.5v ? up to 55ma continuous current per output ? output current programmed by single resistor ? 3% current matching between outputs ? 6% current matching between ics ? watchdog clears display if interface fails ? reports open-circuit led faults ? -40? to +125? temperature range max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog ________________________________________________________________ maxim integrated products 1 16 15 14 13 12 11 10 9 1 2 3 4 5 6 7 8 gnd v+ set dout oe out7 out6 out5 out4 top view max6978aue MAX6978APE max6978aee tssop/pdip/ssop din clk out1 le out0 out2 out3 pin configuration ordering information 19-3578; rev 1; 3/05 for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. typical application circuit and selector guide appear at end of data sheet. applications variable message signs marquee displays point-of-order signs traffic signs gaming features architectural lighting part temp range pin-package max6978aue -40? to +125? 16 tssop-ep* MAX6978APE -40? to +125? 16 pdip max6978aee -40? to +125? 16 ssop * ep = exposed pad.
max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog 2 _______________________________________________________________________________________ absolute maximum ratings stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. voltage (with respect to gnd) v+ ............................................................................-0.3v to +6v out_ .......................................................................-0.3v to +6v din, clk, le, oe , set ................................-0.3v to (v+ + 0.3v) dout current ..................................................................?0ma out_ sink current .............................................................60ma total gnd current ...........................................................480ma continuous power dissipation 16-pin ssop (derate 7.1mw/? over t a = +70?) .......571mw 16-pin pdip (derate 10.5mw/? over t a = +70?) ......842mw 16-pin tssop (derate 21.3mw/? over t a = +70?) ..............................................................1702mw operating temperature range .........................-40? to +125? junction temperature ......................................................+150? storage temperature range .............................-65? to +150? lead temperature (soldering, 10s) .................................+300? electrical characteristics ( typical operating circuit , v+ = 3v to 5.5v, t a = t min to t max , unless otherwise noted. typical values are at v+ = 5v, t a = +25?.) (note 1) parameter symbol conditions min typ max units operating supply voltage v+ 3.0 5.5 v output voltage v out 5.5 v standby current (interface idle, all output ports high impedance, r set = 360 ? ) i + all logic inputs at v+ or gnd, dout unloaded 4.5 5.6 ma standby current (interface running, all output ports high impedance, r set = 360 ? ) i + f clk = 5mhz, oe = v+, din and le = v+ or gnd, dout unloaded 4.7 6 ma supply current (interface idle, all output ports active low, r set = 360 ? ) i + all logic inputs at v+ or gnd, dout unloaded 10 25 ma input high voltage din, clk, le, oe v ih 0.7 v+ v input low voltage din, clk, le, oe v il 0.3 v+ v hysteresis voltage din, clk, le, oe ? v i 0.8 v input leakage current din, clk, le, oe i ih , i il -1 +1 ? output-high voltage dout v oh i source = 4ma v + - 0.5v v output low voltage v ol i sink = 4ma 0.5 v output current out_ i out v+ = 3v to 5.5v, v out = 0.8v to 2.5v, r set = 360 ? 42 50 56 ma output leakage current out_ i leak oe = v+, v out = v+ 1 a watchdog timeout t wd 1s
max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog _______________________________________________________________________________________ 3 5v timing characteristics ( typical operating circuit , v+ = 4.5v to 5.5v, t a = t min to t max , unless otherwise noted.) (note 1) parameter symbol conditions min typ max units clk clock period t cp 40 ns clk pulse-width high t ch 19 ns clk pulse-width low t cl 19 ns din setup time t ds 4ns din hold time t dh 8ns dout propagation delay t do 12 32 ns dout rise and fall time t dr , t df c dout = 10pf, 20% to 80% 10 ns le pulse-width high t lw 20 ns le pulse width (fault-detection data valid) t lf 20 ns le setup time t ls 10 ns le rising to out_ rising delay t lrr (note 2) 100 ns le rising to out_ falling delay t lrf (note 2) 280 ns clk rising to out_ rising delay t crr (note 2) 100 ns clk rising to out_ falling delay t crf (note 2) 310 ns oe rising to out_ rising delay t oe h 100 ns oe falling to out_ falling delay t oe l 300 ns led output out_ turn-on fall time t f 80% to 20% 200 ns led output out_ turn-off rise time t r 20% to 80% 120 ns
max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog 4 _______________________________________________________________________________________ 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.0 3.5 4.0 4.5 5.0 5.5 supply current vs. supply voltage (interface idle, all outputs off, r set = 720 ? ) max6978 toc01 supply voltage (v) supply current (ma) t a = +125 c t a = +85 c t a = +25 c t a = -40 c 4.2 4.5 4.4 4.3 4.6 4.7 4.8 4.9 5.0 5.1 5.2 3.0 4.0 3.5 4.5 5.0 5.5 max6978 toc02 supply voltage (v) supply current (ma) t a = +125 c t a = +85 c supply current vs. supply voltage (interface idle, all outputs off, r set = 360 ? ) t a = -40 c t a = +25 c 0 3 9 6 12 15 3.0 4.0 3.5 4.5 5.0 5.5 max6978 toc03 supply voltage (v) supply current (ma) supply current vs. supply voltage (interface idle, all outputs on, r set = 720 ? ) t a = +125 c t a = +85 c t a = +25 c t a = -40 c t ypical operating characteristics (t a = +25?, unless otherwise noted.) 3.3v timing characteristics ( typical operating circuit , v+ = 3v to 5.5v, t a = t min to t max , unless otherwise noted.) (note 1) parameter symbol conditions min typ max units clk clock period t cp 52 ns clk pulse-width high t ch 24 ns clk pulse-width low t cl 24 ns din setup time t ds 4ns din hold time t dh 8ns dout propagation delay t do 12 48 ns dout rise and fall time c dout = 10pf, 20% to 80% 10 ns le pulse-width high t lw 20 ns le pulse width (fault-detection data valid) t lw 20 ns le setup time t ls 15 ns le rising to out_ rising delay 100 ns le rising to out_ falling delay 310 ns clk rising to out_ rising delay 100 ns clk rising to out_ falling delay 330 ns oe rising to out_ rising delay t oe h 100 ns oe falling to out_ falling delay t oe l 330 ns led output out_ turn-on fall time t f 80% to 20% 200 ns led output out_ turn-off rise time t r 20% to 80% 120 ns note 1: all parameters tested at t a = +25?. specifications over temperature are guaranteed by design. note 2: see figure 3.
max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog _______________________________________________________________________________________ 5 0 3 9 6 12 15 3.0 4.0 3.5 4.5 5.0 5.5 max6978 toc04 supply voltage (v) supply current (ma) supply current vs. supply voltage (interface idle, all outputs on, r set = 360 ? ) t a = +125 c t a = +85 c t a = +25 c t a = -40 c 0 10 5 20 15 25 30 01.0 1.5 0.5 2.0 2.5 3.0 port output current vs. port output voltage (r set = 720 ? , v+ = 3.3v) max6978 toc05 port output voltage (v) port output current (ma) t a = +125 c t a = +85 c t a = +25 c t a = -40 c 0 20 10 40 30 50 60 01.0 1.5 0.5 2.0 2.5 3.0 port output current vs. port output voltage (r set = 360 ? , v+ = 3.3v) max6978 toc06 port output voltage (v) port output current (ma) t a = +125 c t a = +85 c t a = +25 c t a = -40 c 0 10 5 20 15 25 30 01.0 1.5 0.5 2.0 2.5 3.0 port output current vs. port output voltage (r set = 720 ? , v+ = 5.0v) max6978 toc07 port output voltage (v) port output current (ma) t a = +125 c t a = +85 c t a = +25 c t a = -40 c 0 20 10 40 30 50 60 01.0 1.5 0.5 2.0 2.5 3.0 port output current vs. port output voltage (r set = 360 ? , v+ = 5.0v) max6978 toc08 port output voltage (v) port output current (ma) t a = +125 c t a = +85 c t a = +25 c t a = -40 c 26.0 25.5 25.0 24.5 24.0 3.0 4.0 3.5 4.5 5.0 5.5 port output current vs. supply voltage (r set = 720 ? , v out = 2v) max6978 toc09 supply voltage (v) port output current (ma) t a = +125 c t a = +85 c t a = +25 c t a = -40 c 52 51 50 49 48 3.0 4.0 3.5 4.5 5.0 5.5 port output current vs. supply voltage (r set = 360 ? , v out = 2v) max6978 toc10 supply voltage (v) port output current (ma) t a = +125 c t a = +85 c t a = +25 c t a = -40 c 0 10 30 20 40 50 0400 0 2000 6000 8000 10,000 port output current vs. r set resistance (v+ = 5.0v) max6978 toc11 r set resistance ( ? ) port output current (ma) t ypical operating characteristics (continued) (t a = +25?, unless otherwise noted.)
max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog 6 _______________________________________________________________________________________ pin description pin name function 1 gnd ground 2 din serial-data input. data is loaded into the internal 8-bit shift register on clk? rising edge. 3 clk serial-clock input. data is loaded into the internal 8-bit shift register on clk? rising edge. 4le load-enable input. data is loaded transparently from the internal shift register to the output latch while le is high. data is latched into the output latch on le's falling edge, and retained while le is low. 5?2 out0?ut7 led driver outputs. out0?ut7 are open-drain, constant-current sinking outputs rated to 5.5v. 13 oe output-enable input. high forces outputs out0?ut7 to high impedance, without altering the contents of the output latches. low enables outputs out0?ut7 to follow the state of the output latches. 14 dout serial-data output. data is clocked out of the 8-bit internal shift-register to dout on clk? rising edge. 15 set led current setting. connect set to gnd through a resistor (r set ) to set the maximum led current. 16 v+ positive supply voltage. bypass v+ to gnd with a 0.1? ceramic capacitor. pad exposed pad* exposed pad on package underside. connect to gnd. serial-to-parallel shift register latches out0 out1 out2 dout din clk out7 constant-current sinks out5 out6 power-on reset v+ current reference v+ oe gnd gnd le d0 d7 d0 d7 d0 d7 r set max6978 fault detection watch dog figure 1. max6978 block diagram * tssop package only.
detailed description the max6978 led driver comprises a 4-wire serial interface driving eight constant-current sinking open- drain output ports. the outputs drive leds in either sta- tic or multiplex applications (figure 1). the constant- current outputs are guaranteed for current accuracy not only with chip-supply voltage variations (5v ?0% and 3v to 5.5v), but also over a realistic range of driver output voltage drop (0.5v to 2.5v). the drivers use cur- rent-sensing feedback circuitry (not simple current mir- rors) to ensure very small current variations over the full allowed range of output voltage (see the typical operating characteristics ). the 4-wire serial interface comprises an 8-bit shift reg- ister and an 8-bit transparent latch. the shift register is written through a clock input clk and a data input din and the data propagates to a data output dout. the data output allows multiple drivers to be cascaded and operated together. the contents of the 8-bit shift regis- ter are loaded into the transparent latch through a latch-enable input le. the latch is transparent to the shift register outputs when high, and latches the current state on the falling edge of le. each driver output is an open-drain constant-current sink that should be connected to the cathode of either a single led or a series string of multiple leds. the led anode can be connected to a supply voltage of up to 5.5v, independent of the max6978 supply, v+. the constant-current capability is up to 55ma per output, set for all eight outputs by an external resistor, r set . initial power-up and operation an internal reset circuit clears the internal registers of the max6978 on power-up. all outputs out0?ut7, therefore, initialize high impedance, and the interface output dout initializes low, regardless of the initial logic levels of the clk, din, oe , and le inputs. 4-wire serial interface the serial interface on the max6978 is a 4-wire serial interface using four inputs (din, clk, le, oe ) and a data output (dout). this interface is used to write dis- play data to the max6978. the serial-interface data word length is 8 bits, d0?7. the functions of the five interface pins are as follows. din is the serial-data input, and must be stable when it is sampled on the rising edge of clk. data is shifted in, msb first. this means that data bit d7 is clocked in first, followed by 7 more data bits, finishing with the lsb d0. clk is the serial-clock input, which shifts data at din into the max6978 8-bit shift register on its rising edge. le is the latch load input of the max6978, which trans- fers data from the max6978 8-bit shift register to its 8-bit latch when le is a high (transparent latch), and latches the data on the le falling edge of le (figure 2). the ris- ing edge of le loads fault data into the max6978 8-bit shift register, replacing bits d6 and d5 with fault data. this fault data is automatically shifted out through dout when the next display data is shifted in. the fourth input provides output-enable control of the output drivers. oe is high to force outputs out0?ut7 high impedance, without altering the contents of the output latches, and low to enable outputs out0?ut7 to follow the state of the output latches. max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog _______________________________________________________________________________________ 7 table 1. 4-wire serial-interface truth table shift-register contents latch contents output contents serial data input din clock input clk d 0 d 1 d 2 ? n-1 d n load input le d 0 d 1 d 2 ? n-1 d n blanking input oe h hr 1 r 2 ? n-2 r n-1 l lr 1 r 2 ? n-2 r n-1 x r 0 r 1 r 2 ? n-1 r n xxxxx h r 0 r 1 r 2 ? n-1 r n p 1 p 2 p 3 ? n-1 p n l p 0 p 1 p 2 ? n-1 p n l p 0 p 1 p 2 p n - 1 p n xxxxx h hi-z hi-z hi-z hi-z hi-z l = low-logic level h = high-logic level x = don? care p = present state r = previous state
max6978 oe is independent of the operation of the serial inter- face. data can be shifted into the serial-interface shift register and latched regardless of the state of oe . dout is the serial-data output, which shifts data out from the max6978? 8-bit shift register on the rising edge of clk. data at din is propagated through the shift reg- ister and appears at dout eight clock cycles later. watchdog the max6978 includes a watchdog circuit that monitors the clk, din, and le inputs. if there is no transition on any one of these inputs for nominally 1s, then the output latches are cleared and outputs out0?ut7 go high impedance like the initial power-up condition. this turns off all leds connected to the outputs. the shift-register data does not change, just the output-latch data. the watchdog is intended to be used as a fail-safe fea- ture for applications, which prefer a blank display to an incorrect display if the serial interface fails. when the watchdog triggers, the outputs remain off until the dri- ver output latches are updated with data turning them on. recovery is therefore automatic if the transmission failure is temporary, because the max6978 does not lock up in the watchdog timeout state. the max6978 operates correctly when the serial interface is next acti- vated, and the watchdog circuit is reset and starts monitoring the serial interface again. the watchdog function requires no software change to the application driving the max6978. led fault detection the max6978 includes circuitry that detects open-cir- cuit leds automatically. an open-circuit fault occurs when an output is programmed to sink current but less than about 50% of the programmed current flows. open circuits are checked just after the rising edge of le, when new led data is loaded into the output latch- es from the shift register(s). if one or more output port is detected with an open-circuit fault, then the 2 bits, d6 and d5, in the serial-interface shift register are both set high. if no open-circuit faults are detected, then d6 and d5 are both cleared low. the data in other 6-bit posi- tions in the serial-interface shift register is not altered. fault status is automatically shifted out on dout when the next data transmission is shifted in. le is normally taken high after all 8 bits of new led data have been clocked into the shift register, and so at that time, dout is outputting data bit d7. on the next two rising edges of clk, the 2 fault status bits, d6 and d5, are clocked out in that order, followed by the remaining 5 unchanged data bits d4 to d0. a typical fault-detecting application tests all the shifted out data. bits d0?4 and d7 are checked against the originally transmitted data to check data link integrity. bits d5 and d6 are checked first to see that they con- 8-port, 5.5v constant-current led driver with led fault detection and watchdog 8 _______________________________________________________________________________________ . d7 d6 t ds t dh t cl t cp t do t oew t oel t f t r t oeh t ls t lw t ch d1 d0 d7 le clk din dout oe out_ 80% 20% t lf figure 2. 4-wire serial-interface timing diagram
tain the same data (validating the status), and second whether faults are reported by the actual logic level. the fault status bit identifies which driver chip has at least one open-circuit led. it is possible to determine how many leds driven by a chip are at fault by running eight test patterns, each of which lights a different sin- gle led. the fault status bit then effectively identifies only one output at a time. applications information selecting external component r set to set led output current the max6978 uses an external resistor r set to set the led current for outputs out0?ut7. the minimum allowed value of r set is 307.6 ? , which sets the output currents to 55ma. the maximum allowed value of r set is 1.5k ? . the reference value, 360 ? , sets the output currents to 50ma. to set a different output current, use the formula: r set = 18,000 / i out where i out is the desired output current in ma. computing power dissipation the upper limit for power dissipation (p d ) for the max6978 is determined from the following equation: p d = (v+ x i+) + (v out x duty x i out x n) where: v+ = supply voltage i+ = operating supply current when sinking i out led drive current into n outputs duty = pwm duty cycle applied to oe n = number of max6978 outputs driving leds at the same time (maximum is 8) v out = max6978 port output voltage when driving load led(s) i out = led drive current programmed by r set p d = power dissipation, in mw if currents are in ma dissipation example: i out = 47ma, n = 8, duty = 1, v out = 2v, v+ = 5.25v p d = (5.25v x 25ma) + (2v x 1 x 47ma x 8) = 0.883w thus, for a 16-pin tssop package (t ja = 1 / 0.0213 = +46.95?/w from the absolute maximum ratings ), the maximum allowed ambient temperature t a is given by: t j(max) = t a + (p d x t ja ) = +150? = t a + (0.883 x 46.95?/w) so t a = +108.5?. overtemperature cutoff the max6978 contains an internal temperature sensor that turns off all outputs when the die temperature exceeds approximately +165?. the outputs are enabled again when the die temperature drops below approximately +140?. register contents are not affected, so when a driver is overdissipating the exter- nal symptom will be the load leds cycling between on and off as the driver repeatedly overheats and cools, alternately turning the leds off and then back on again. power-supply considerations the max6978 operates with a chip supply v+, and one or more led supplies. bypass each supply to gnd with a 0.1? capacitor as close to the max6978 as pos- sible. this is normally adequate for static led driving. for multiplex or pwm applications, it is necessary to add an additional bulk electrolytic capacitor of 4.7? or more to each supply for every 4 to 16 max6978s. the necessary capacitance depends on the led load cur- rent, pwm switching frequency, and serial interface speed. inadequate v+ decoupling can cause timing problems, and very noisy led supplies can affect led current regulation. for the tssop version, connect the underside exposed pad to gnd. chip information transistor count: 2382 process: bicmos max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog _______________________________________________________________________________________ 9 le out_ le out_ clk out_ clk out_ t crr t crf t lrr t lrf figure 3. le and clk to out_ timing
c mosi out0 out1 out2 out3 out4 out5 out6 out7 sclk din clk 0v gnd miso dout 0v 360 ? oe enable +5v v+ 5v latch load set max6978 t ypical application circuit selector guide part no. of outputs max output voltage (v) max output current led fault detection watchdog max6968 max6977 yes max6978 8 5.5 yes yes max6970 max6981 yes max6980 836 yes yes max6969 max6984 yes max6979 16 5.5 yes yes max6971 max6982 yes max6983 16 36 55ma yes yes max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog 10 ______________________________________________________________________________________
max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog ______________________________________________________________________________________ 11 tssop 4.4mm body.eps e 1 1 21-0108 package outline, tssop, 4.40 mm body, exposed pad xx xx pa cka ge information (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation go to www.maxim-ic.com/packages .)
max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog 12 ______________________________________________________________________________________ package information (continued) (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation go to www.maxim-ic.com/packages .) ssop.eps package outline, ssop, 5.3 mm 1 1 21-0056 c rev. document control no. approval proprietary information title: notes: 1. d&e do not include mold flash. 2. mold flash or protrusions not to exceed .15 mm (.006"). 3. controlling dimension: millimeters. 4. meets jedec mo150. 5. leads to be coplanar within 0.10 mm. 7.90 h l 0 0.301 0.025 8 0.311 0.037 0 7.65 0.63 8 0.95 max 5.38 millimeters b c d e e a1 dim a see variations 0.0256 bsc 0.010 0.004 0.205 0.002 0.015 0.008 0.212 0.008 inches min max 0.078 0.65 bsc 0.25 0.09 5.20 0.05 0.38 0.20 0.21 min 1.73 1.99 millimeters 6.07 6.07 10.07 8.07 7.07 inches d d d d d 0.239 0.239 0.397 0.317 0.278 min 0.249 0.249 0.407 0.328 0.289 max min 6.33 6.33 10.33 8.33 7.33 14l 16l 28l 24l 20l max n a d e a1 l c h e n 1 2 b 0.068
max6978 8-port, 5.5v constant-current led driver with led fault detection and watchdog maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 ____________________ 13 2005 maxim integrated products printed usa is a registered trademark of maxim integrated products, inc. package information (continued) (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation go to www.maxim-ic.com/packages .) pdipn.eps

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