8281/8282 11/18/2002 intended for analog and digital sate llite receivers, the low noise block converter regulator (lnbr) is a monolithic linear and switching voltage regulator, specifically designed to provide the power and the interface signals to the lnb downconverter via the coaxial cable. if the device is in stand-by mode (en terminal low), the regulator output is disabled. this is to allow the antenna downconverters to be supplied/controlled by other satellite receivers sharing the same coaxial cable. in this mode the device will limit reverse current. the a8281 is supplied in a 16-lead plastic soic with copper batwing tab (suffix ?lb?). the a8282 is supplied is a 24-lead plastic soic with copper batwing tab (suffix ?lb?). oper ating temperature range is standard classification (suffix "s"). the a8282 is available for improved power dissipation as well as allowing direct replacement of allegr o?s first generation lnbr device, the a8283slb, with the exception of the bypass switch. features ? lnb selection and stand-by function ? built-in tone oscillator factory trimmed to 22 khz facilitates diseqc? encoding ? tracking switch-mode power converter for lowest dissipation ? externally adjustable short-circuit protection ? lnb short-circuit protection and diagnostics ? auxiliary modulation input ? internal over temperature protection ? reverse current protection ? cable length compensation (a8282 only) this device incorporates features that have patents pending. always order by complete part number: part number package A8282SLBTR 24 lead soic tape/reel a8282slb 24 lead soic a8281slb 16 lead soic absolute maximum ratings at t a = +25 c load supply vo ltage, vin ...........................4 7 v output current, i out ............ internally limited* output voltage, v out ........................ -1v to 22 v switching node, lx ....................................... -1v logic input ..................................... -0.3 v to 7 v package power dissipation (t a = +25 c), p d a8281slb.......................... 56 c/w** a8282slb.......................... 50 c/w** operating temperature range, t a ................................ -20 c to +85 c junction temperature, t j ......................... +150 c storage temperature range, t s ............................... -55 c to +150 c * output current rating may be limited by duty cycle, ambient temperature, and heat sinking. under any set of conditions, do not exceed the specified current rating or a junction temperature of 150 c. ** measured on a pcb with 2 oz copper with ground area of 1 square inch. lnb supply and control voltage regulator
8281/8282 diseq termination charge pump lx buck converter 100uh tsd overcurrent lnb vbulk sense vin - + 135mv - + - + 900mv over- current output voltage select extm tcap vsel1 fault gnd olf vpump cpump pumpx 352khz divide by 2 ent 5v regulator divide by 16 en 22 khz tone generation +vin 200 mohm 10nf 100nf .1uf .1uf 100uf 100uf (8282 only) vsel0 (8282 only) llc (8282 only) vint .1uf 15 180uh .22uf 4.7uf output voltage select table ? a8282slb vsel0 vsel1 llc vlnb l l l 13 l l h 14 l h l 18 l h h 19 h l l 12 h l h 13 h h l 20 h h h 21 output voltage select table ? a8281slb vsel1 vlnb l 13 h 18
8281/8282 electrical characteristics at t j = +125c, c lnb = 100nf v in =v inmin to 47v (unless noted otherwise) limits characteristics symbol test conditions min. typ. max. units v in supply voltage range v inmax 47 v v inmin 4.5+vo output voltage v lnb relative to voltage select table i load = 6ma to 750ma -4.5 0 4.5 % output voltage v lnb ent=h, i load =12ma to 750ma average voltage of lnb -4.5 0 4.5 % vil 0.8 v logic input voltage vih 2.0 v logic input current iih vih=5 v < 1.0 10 a supply current i cc en = l .25 1 ma i ccen en = h, i load = 0 ma 6 10 ma buck switch on resistance r dsbuck t j = 25 c, i out =750ma .57 .67 ? t j =125 c, i out =750ma .8 .94 ? buck switch current limit i blim 1 2.5 a switching frequency fo f tone * 16 320 352 384 khz linear regulator voltage drop ? v buck v sense ?vo, ent = l, i load =750ma 700 900 1100 mv linear regulator on resistance r dslnb i load = 750ma, vin = 18v, t j = 25 c output select = 18v .375 ? tone characteristics tone frequency f tone ent=h 20 22 24 khz tone amplitude a tone ent = h, i load = 12ma to 750ma 0.4 .65 0.9 v pp tone duty cycle dc tone i load = 12ma to 750ma 40 60 % tone rise or fall time tr, tf ent=h, i load = 12ma to 750ma 5 10 15 s external modulation gain g extm ? v out / ? v extm , f = 22 khz square wave, i load = 12ma to 750ma 4 5.0 6 v/v extm input range ? v extm ac coupled 100 125 mvpp external modulation impedance z extm f = 22khz 4 10 k ? notes: 1. typical data is for design information only. 2. negative current is defined as coming out of (sourcing) the specified device pin.
8281/8282 electrical characteristics at t j = +125c, c lnb = 100nf v in =v inmin to 47v (unless noted otherwise) limits characteristics symbol test conditions min. typ. max. units protection circuitry output leakage current ioz voh=5.5 v < 1.0 10 a overload flag terminal logic low vol iol=8 ma 0.28 .5 v output reverse current i or en=l, v lnb = 22 v v in =22 v or v in floating 1.0 5 ma current limiting threshold v omth 115 135 155 mv thermal shutdown threshold t j 165 c thermal shutdown hysteresis ? t j 20 c notes: 1. typical data is for design information only. 2. negative current is defined as coming out of (sourcing) the specified device pin.
8281/8282 functional description buck regulator. a current-mode buck converter provides the linear regulator a supply voltage that tracks the requested lnb output voltage. the buck converter operates at 16 times the internal tone frequency, nominally 352khz. the tracking regulator provides minimum power dissipation across the range of output voltages by adjusting the sense pin voltage 900 mv nominally above the lnb output selected. the tracking regulator also provides adequate headroom for tone injection. linear regulator. the output linear regulator will sink and source current. this allows tone modulation into a capacitive load of 100nf for the output current range of 12ma to 750ma. slew rate control. the programmed output voltage rise and fall times can be set by an internal 25 k ? resistor and an external capacitor locat ed on the tcap terminal. the range of acceptable capacitor values is 4.7nf to 47nf. this feature only affects the turn on and programmed voltage rise and fall times. modulation is unaffected by the choice of tcap. if lnb output voltage rise and fall time is not a concern, the tcap terminal should use a 100nf ceramic as a default value to minimize output noise. if a small value capacitor value is used, the rise time will be limited by the time required to charge the vbulk capacitor. short circuit limit regulator. the lnb output is current limited. the short-circuit protection threshold is set by the value of an external resistor, r sense in conjunction with an internal 135mv+/- 20mv reference voltage, v omth . i om = v omth /r sense the sense resistor should be chosen based on maximum dc plus ac (tone), load current required, internal v omth tolerance, and sense resistor accuracy. for 750ma applications, a precision 140mohm resistor is recommended. for 500ma applications the resistor value can be raised to 200mohms. in operation, the short-circuit protection produces current limiting at the input due to the tracking converter. if the output is shorted, the linear regulator will limit the output current to i om . fault output. short-circuit or thermal shutdown will cause the olf terminal, an open-drain diagnostic output flag, to go low. internal tone modulation. the ent (tone enable) terminal activates the internal tone signal modulating the dc output with a 650mv peak to peak, trapezoidal waveform. the internal oscillator is factory trimmed to provide a tone of 22 khz +/- 2 khz. no further adjustment is required. burst coding of the 22 khz tone can be accomplished, due to the fast response of the ent input and rapid tone response. this allows implementation of the diseqc? protocols. external tone modulation. to improve design flexibility and to allow impl ementation of proposed lnb remote control standards, an analog modulation input terminal is available (extm). an appropriate dc blocking capacitor must be used to couple the modulating signal source to the extm terminal. if external modulation is not used, the extm terminal should be bypassed to ground via a .1uf ceramic capacitor. the input amplitude should stay within 100 to 125mvpp to guarantee the diseqc? amplitude specification over the output current range.
8281/8282 typical application diagram optional diseq termination c lnb 15 ohm 180uh .22uf extm vint pumpx vpump cpump gnd gnd ent enb vsel0 vsel1 llc rs l1 c bulk c in c tcap +30v d1 c int c1 c2 c3 f-connecter vin d2 control inputs c byp tcap r1 n/c olf vbulk n/c n/c gnd gnd sense lnb lx vin tcap description representative component c1-3, c byp, c lnb .1uf/50v ceramic x7r/x5r c in 100uf low esr electrolytic 50v nichicon uhd1h101mpt c bulk 100uf low esr electrolytic/35v nichicon uhc1v101 c int 4.7uf/16v tantulum/electrolytic d1 1.5a schottky/40v or 50v sanken ek04 d2 1a silicon diode/25v sanken eu01 l1 100uh (750ma max iload) 100uh (500ma max iload) tdk tsl1112-101k1r4 falco d08018, coilcraft dr0808, tdk tsl0808-101kr80 l2 180uh (750ma iload) tdk tsl1112s-181k1r0-pf rs 140-200mohm sense resistor .25w meritek cr04r140f c tcap 10nf ceramic x7r/x5r r1 1m 5%
8281/8282 component selection. input capacitor (c in ). an electrolytic should be located as close to the device as possible. the input current is a square wave with fast rise and fall times so the capacitor must be able to handle the rms current without excessive temperature rise. the value of the capacitor is not as important as the esr. the power dissipated in the input electrolytic is pd(c in ) = i rms ^2*esr the worse case irms is with maximum i load , minimum vin, and maximum v out (highest switch duty cycle). choose a capacitor with a ripple current rating greater than i load * 1.2*v out(max) /v in(min) buck inductor (l1). a 100uh power inductor is appropriate for all operating conditions. th e rated saturation current of the inductor must be > 1.3a. the dc resistance should be less than 350mohms, the smaller the better to maximize efficiency. clamp diode (d1). a schottky diode is required for the switching node lx. the diode should be rated at 1.5 times the maximum load current. output capacitor (c bulk ). a low esr electrolytic is recommended to minimize the vpp ripple voltage. less than 40mv vpp is a reasonable goal. vpp =esr*i ripple i ripple = (v bulk *(1-v bulk /v in ))/(l1*352khz) layout notes. 1. use a star ground approach. connect the common ground to the ground plane at the device ground pins. the soic-24 has 2 pins on each side connected to the package power ground tab. this allows the analog and power ground to be kept separate on the pcb up to the device. 2. keep the sense resistor pcb trace as short and wide as possible to lower trace resistance. 3. connect the bypass capacitors as close to the device as possible. the lower valued ceramic capacitors should be closer to the device than electrolytics. 4. if using an unshielded power inductor for the buck switch, place as far away from the device as possible. the resulting emi can result in additional noise on the lnb output. 5. place the tcap capacitor as close to the device as possible. 6. a two-sided board with ground planes on both sides of the pcb will help optimize the power dissipation. typically several copper vias under the device are used to connect the ground planes and enhance thermal performance. noise immunity. lnb systems can have a 50mv peak specification for noise on the coaxial cable. this is easily achievable with the a8282 with proper layout and following a few guidelines. 1. use a low esr capacitor for vbulk, 400mohm maximum is recommended. 2. the lnb output is sensitive to the tcap reference pin. keep the pcb traces short and location of the bypass capacitor close to the device. this pin is a high impedance node and noise can be induced from the proximity to an unshielded inductor. if the inductor can not be placed far enough away to avoid this noise pickup, it is important to ensure that the induced voltage is out of phase with the switching node lx. rotating the inductor can change the phase of the induced voltage. 3. be sure to place a 1uf to 10uf capacitor on internal reference vint. 4. bypass the extm pin with a .1uf ceramic to gnd. 5. increasing output capacitance will attenuate noise, however this must be traded off with the requirement for low cable capacitance for 22khz-tone transmission.
8281/8282 power dissipation. the power dissipated and operating junction temperature of the 8281 and 8282 can be estimated to ensure the device operates within desired thermal budget. the total chip power is contributed by three components. pd_bias = v in * (i ccen - 4ma) pd_buck = i load ^2 * r dsbuck * v bulk /v in pd_lin = ? v buck * i load p tot = pd_bias + pd_buck + pd_lin where v bulk = ? v buck + i load * r sense + v lnb i ccen , ? v buck , and r dsbuck can be taken from the specification table. r dsbuck is a function of junction termperature. the r dson will rise approximately 2.7mohm/c. the junction temperature can be estimated by: t j = p tot * r ?ja + t a or t j = p tot * r ?jt + t tab where r ?jt = 5.6c /w t a = ambient temperature c r ?ja = 50c /w for a8282slb 56c /w for a8281slb r ?ja numbers for a typical two sided, 2 oz. copper, pc board layout with copper ground plane of 1 square inch. additional copper ground plane area, multi- level boards, etc can reduce the effective r ?ja . diseqc tm . the 22khz tone is specified to be compatible to coaxial cable bus standards available from www.eutelsat.com . the a8282 lnb output will be able to drive the diseqc termination network. this terminator typically consists of a 180uh inductor, used to pass the dc current with minimal loss, and a 15 ohm parallel resistor to provide the recommended source impedance at 22khz. unidirectional communication systems such as diseqc 1.0 do not need this termination and the lnb output can be directly connected to the coaxial cable. 13v to 18v transition . the lnb output can be rapidly switched between a high and low setting as a method of receiver to lnb communicati on. the tcap capacitor will control the slew rate based on the rc charging. t rise/fall = 25k * c tcap * ln(v1/v2) small values of tcap are us ed when the transition time is desired to be less than a millisecond. in this case, the minimum rise time is limited by the charge time of the switching regulators output capacitor. this is dependent on the lnb load current, peak current limit in the buck switch, and the output amplitude change. t rise = c_bulk * (v2-v1)/(i_ave) where i_ave is the average curre nt available to charge the output capacitor and can be estimated by: i_ave = 1.4a - i_load note that this is only a limitation due to the ability to charge the output capacitor on a low to high change of the lnb voltage. for high to low transitions, the output voltage will be slew limited by tcap. the minimum value for tcap should be 4.7nf.
8281/8282 output voltage adjust. it is possible to fine tune the lnb output voltage to comply with the directtv specification by connecting a 1m resistor from pin vint to tcap. the lnb is output gained up by 6 from the tcap voltage as shown. the 1m resistor pulls the lnb voltage up 440mv from the 13v nominal setting of the voltage select dac by sourcing approximately 2.76a into the tcap node. tcap lnb lnb voltage select vint 25k 1m internal 5v reference + x 6 _ capacitive loading. the linear regulator sink current is limited which can cause overshoot of the 22khz tone. this effect only appears with low levels of output current combined with high values of output capacitance. this relationship is chart below. points above the line will not have excessive overshoot. iout(ma) vs cload (uf) 0.1 0.22 0.3 0.47 1 1.5 0 20 40 60 80 100 120 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7
8281/8282 pin name pin description 8281slb so-16 8282slb so-24 extm external modulation input 1 24 olf overload flag output 2 2 vbulk tracking supply voltage to linear regulators 3 3 gnd ground tab 4 6,7 sense current limit setup resistor 5 8 lnb output voltage to lnb 6 9 lx inductor drive point 7 10 vin supply input voltage 8 11 tcap capacitor for setting the rise and fall time of the outputs 9 12 llc logic input: increases output voltage by 1 v for line length - 13 vsel1 logic input: output voltage select 10 14 vsel0 logic input: output voltage select - 15 en logic input: enables switcher and outputs 11 16 ent logic input: enable internal modulation 12 17 gnd ground tabs 13 18,19 cpump high side of charge-pump cap 14 20 vpump gate supply voltage for high side drivers 15 21 pumpx charge-pump drive 16 22 vint internal regulated supply - 23 n/c no connect 1,4,5
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