te c h t m T8308 tm technology, inc. reserves the right p. 1 publication date: oct. 2014 to change products or specifications without notice . revision:c pwm control 800ma step-down converter features ? wide input voltage range: 7v to 30v ? led output current up to 800ma ? soft-start ? single pin on/off and brightness control using dc voltage or pwm ? high efficiency (up to 97%) ? up to 1mhz switching frequency ? typical 5% output current accurac y ? sot-89 and sop-8 lead-free package applications ? led/display back light driver ? lightings ? portable communication devices ? handheld electronics general description the T8308 is a continuous mode inductive step-down converter, designed for driving single or multiple series connected leds efficiently from a voltage source higher than the led voltage. the device operates from an input supply between 7v and 30v and provides an externally adjustable output current of up to 800ma. depending upon supply voltage and external components, this can provide up to 24 watts of output power. the T8308 includes the output switch and a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current. the T8308 is available in sot-89 and sop-8 lead-free package. part number examples part no. package T8308-ax sot-89 T8308-ad sop-8
te c h t m T8308 tm technology, inc. reserves the right p. 2 publication date: oct. 2014 to change products or specifications without notice . revision:c pin arrangement(top view) pin description symbol sot-89-5 sop-8 description lx 1 3 drain of ndmos switch gnd 2 5.6 ground en 3 1 enable control signal, h: active, l: power d own isense 4 8 connect resistor rs from this pin to vin to define nominal average output current i outnom =0.1/rs vin 5 7 input voltage (7v to 30v). decouple to ground with 47uf or higher x7r ceramic capacitor close to device.
te c h t m T8308 tm technology, inc. reserves the right p. 3 publication date: oct. 2014 to change products or specifications without notice . revision:c absolute maximum ratings parameter symbol value unit voltage on input pin relative to gnd vin -0.3 to +4 0 v isense pin voltage -5 to +0.3 v en pin voltage -0.3 to +6 v operating junction temperature rang t a -40 to 125 maximum soldering temperature (at lead, 10sec) t lead 300 storage temperature rang t s -65 to +150 sot-89 0.5 power dissipation, pd @ ta=25 sop-8 0.8 w sot-89 200 package thermal resistance, ja sop-8 178 /w electrical characteristics (ta=-40 to 85 unless otherwise noted. typical values are at ta=2 5 , vdd=24v) symbol description conditions min. typ. max. unit vin input voltage 7 - 30 v v iru internal regulator start-up threshold vin rising 5.65 v v ird internal regulator shutdown vin falling 5.55 v i q off quiescent supply current with output off dim pin grounded 170 ua i q on quiescent supply current with output switching dim pin floating fsw= 250khz 1.8 5 ma i sense mean current sense threshold voltage (defines led current setting accuracy) measured on isense pin with respect to vin, vadj = 1.25v 95 100 105 mv i senth sense threshold hysteresis 15 % isense isense pin input current vsense = vin -0.1 10 ua v ih en dim input voltage high 2 v v il en dim input voltage low 0.5 v i lxm continuous lx switch current 0.8 a r lx lx switch on resistance @i lx =0.8a 0.6 i lxi lx switch leakage current 5 ua t tp thermal shutdown protect 160 f lx operating frequency dim pin floating, l=33uh(0.093ohm) iout=0.8a @vled=3.6v driving 1led 280 khz t onmin minimum switch on time lx switch on 240 n s
te c h t m T8308 tm technology, inc. reserves the right p. 4 publication date: oct. 2014 to change products or specifications without notice . revision:c t offmin minimum switch off time lx switch off 200 n s t pwminr recommended minimum switch on time lx switch 'on' or off 800 n s f lxmax recommended maximum operating frequency 1 mhz d lx recommended duty cycle range of output switch at flxmax 0.2 0.9 notes : **parameters are not tested at production. paramete rs are guaranteed by design, characterization and p rocess control.block diagram block diagram
te c h t m T8308 tm technology, inc. reserves the right p. 5 publication date: oct. 2014 to change products or specifications without notice . revision:c functional description the device, in conjunction with the coil (l1) and c urrent sense resistor (rs), forms a selfoscillating continuous-mode buck converter. device operation operation can be best understood by assuming that t he en pin of the device is unconnected and the voltage on this pin (v en ) appears directly at the (+) input of the comparat o r when input voltage v in is first applied, the initial current in l1 and rs is zero and there is no output from the current sense circuit. under this c ondition, the (-) input to the comparator is at ground and its output is high. this turns mn on and switches the lx pin low, causing current to flow from v in to ground, via rs, l1 and the led(s). the current rises at a rate determined by v in and l1 to produce a voltage ramp (v sense ) across rs. the supply referred voltage v sense is forced across internal resistor r1 by the current s ense circuit and produces a proportional current in internal resistors r2 and r3. this produces a groun d referred rising voltage at the (-) input of the comparator . when mn is off, the current in l1 continues to flow via d1 and the led(s) back to v in . the current decays at a rate determined by the le d(s) and diode forward voltages to produce a falling voltage at the input of the comparator. switching thresholds define an average vsense switching threshold of 100 mv (measured on the isense pin with respect to vin). the average output current ioutnom is then defined by this voltage and rs according to: ioutnom = 100mv/rs nominal ripple current is 15mv/rs output shutdown the output of the low pass filter drives the shutdo wn circuit. when the input voltage to this circuit falls below the threshold, the internal regulator a nd the output switch are turned off. the voltage reference remains powered during shutdown to provid e the bias current for the shutdown circuit. quiescent supply current during shutdown is nominal ly 35ua and switch leakage is below 5ua.
te c h t m T8308 tm technology, inc. reserves the right p. 6 publication date: oct. 2014 to change products or specifications without notice . revision:c applications information setting nominal average output current with externa l resistor r s the nominal average output current in the led(s) is determined by the value of the external current sense resistor (r s ) connected between v in and i sense and is given by: i outnom = 0.1/r s [for rs 0.066 ohm] the table below gives values of nominal average out put current for several preferred values of current setting resistor (r s ) in the typical application circuit : rs (ohm) 0.125 0.2 0.285 nominal average output current (ma) 800 500 350 the above values assume that the en pin is floating . note: that rs = 0.125 ohm is the minimum allowed va lue of sense resistor under these conditions to maintain switch current below the specified maximum value. shutdown mode taking the en pin to a voltage below 0.5v for more than approximately 100us, will turn off the output and supply current will fall to a low standb y level of 35ua nominal.
te c h t m T8308 tm technology, inc. reserves the right p. 7 publication date: oct. 2014 to change products or specifications without notice . revision:c capacitor selection a low esr capacitor should be used for input decoup ling, as the esr of this capacitor appears in series with the supply source impedance and lowers overall efficiency. this capacitor has to supply the relatively high peak current to the coil and sm ooth the current ripple on the input supply. a minimum value of 47uf is acceptable if the input so urce is close to the device, but higher values will improve performance at lower input voltages, e specially when the source impedance is high. in order to avoid high frequency noise influence and i mprove circuit stability, it is recommended to shunt a value of 0.22uf capacitor. the input capaci tor should be placed as close as possible to the ic. for maximum stability over temperature and volt age, capacitors with x7r, x5r, or better dielectric are recommended. capacitors with y5v die lectric are not suitable for decoupling in this application and should not be used. inductor selection recommended inductor values for the T8308 are in th e range 33uh to 100uh. higher values of inductance are recommended at higher supply voltage s in order to minimize errors due to switching delays, which result in increased ripple and lower efficiency. higher values of inductance also result in a smaller change in output current over the supp ly voltage range. the inductor should be mounted as close to the device as possible with low resista nce connections to the lx and vin pins. the chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean output curre nt. the inductor value should be chosen to maintain ope rating duty cycle and switch 'on'/'off' times within the specified limits over the supply voltage and load current range. lx switch on time : t on = l ? i / (v in C v led - i avg (r s +r l +r lx )) , note: t onmin > 240ns lx switch off time : t off = l ? i / (v led + v d + i avg (r s +r l )) , note: t offmin > 200ns where: l is the coil inductance (h) ? i is the coil peak-peak ripple current (a) {intern ally set to 0.3 x i avg } rl is the coil resistance (ohm) r s is the current sense resistance i avg is the required led current (a) v in is the supply voltage (v) v led is the total led forward voltage (v) r lx is the switch resistance (ohm) {=0.3 ohm nominal} v d is the diode forward voltage at the required load current (v)
te c h t m T8308 tm technology, inc. reserves the right p. 8 publication date: oct. 2014 to change products or specifications without notice . revision:c typical application circuits * note : when outputs the big current, the noise ar e big, this and the system environment and pcb layo ut have the influential, may defer to the actual need to increa se the capacitor filtration noise.
te c h t m T8308 tm technology, inc. reserves the right p. 9 publication date: oct. 2014 to change products or specifications without notice . revision:c package dimensions sot-89 d polished(2x) c h e l b b1 1 2 3 10'(2x) e e1 a dimension in mm dimension in inch symbol min. max. min. max. a 1.40 1.60 0.055 0.063 b 0.44 0.56 0.017 0.022 b1 0.36 0.48 0.014 0.019 c 0.35 0.44 0.013 0.017 d 4.40 4.60 0.173 0.181 e 2.29 2.60 0.090 0.102 h 3.94 4.25 0.155 0.167 e 1.50 bsc 0.059 bsc e1 3.00 bsc 0.118 bsc l 0.89 1.2 0.035 0.047
te c h t m T8308 tm technology, inc. reserves the right p. 10 publication date: oct. 2014 to change products or specifications without notice . revision:c package dimensions 8-lead sop a 1 a b b 1 c c 1 c 2 d e f h t h e r m a l p a d * j k dimension in mm dimension in inch symbol min. typ. max. min. typ. max. a 5.70 6.00 6.30 0.224 0.236 0.248 a1 3.75 3.95 4.10 0.148 0.156 0.164 b - - 5.13 - - 0.202 b1 - 1.27 - - 0.050 - c - - 1.80 - - 0.071 c1 1.35 1.55 1.75 0.052 0.061 0.069 c2 0 - 0.15 0.000 - 0.006 d 0.31 0.41 0.51 0.012 0.016 0.020 e 0.30 0.50 0.70 0.012 0.020 0.028 f 0.10 0.15 0.25 0.004 0.006 0.010 j 2.23 ref 0.088 ref k 2.97 ref 0.117 ref h 0~8 0~8 *note : the thermal pad on the ics bottom has to be mo unted on the copper foil. to eliminate the noise influence, the thermal p ad is suggested to be connected to gnd on pcb. in addition, desired thermal conductivity will be improved, if a heat-conducting copper foil on pcb is soldered with thermal pad. the thermal pad e nhances the power dissipation. as a result, a large amount of current can be sunk safely in one p ackage.
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