low dropout positive voltage et- ams1084m-xx regulator september 2005 inv 400 description the ams1084m series of positive adjustable and fixed regulators ar e designed to provide 5a with high efficiency. all internal circuitry is designed to operate down to 1.3v input to output differential. on-chip trimming adjusts the reference voltage to 1%. applications ? high efficiency linear regulators ? post regulators for switching supplies ? adjustable power supply features ? adjustable or fixed output ? output current of 5a ? low dropout, 1.5v max at 5a output current ? 0.04% line regulation ? 0.2 % load regulation ? 100% thermal limit burn-in ? fast transient response typical application data pin description ams1084m-3.3 a ms1085 4.8v 4.95v ams1084m beijing estek electronics co.,ltd page 1 of 3
absolute maximum ratings symbol parameter value unit v cc power dissipation internally limited w v in input voltage 15 v operation junction temperature range control section 0 to 125 t j power transistor 0 to 150 0 c t stg storage temperature range -65 to +150 0 c t lead lead temperature (soldering 10 sec) 300 0 c electrical characteristics i load = 0ma and t j = +25 0 c (unless otherwise noted) parameter device test conditions min. typ. max. unit v in =5v, i load = 10ma 1.238 1.250 1.262 reference voltage note 1 ams1084m v in ? v out = 1.5v to 10v, i load = 10ma to 5a * 1.225 1.250 1.275 v v in ? v out = 1.5v, variator from nominal v out -1 - +1 output voltage note 1 all fixed versions v in ? v out = 1.5v to 10v i load = 0ma to 5a, variator from nominal v out * -2 - +2 % line regulation note 1 all i load = 10ma, v in ? v out = 1.5v to 10v * - 0.04 0.20 load regulation note 1 all v in ? v out = 1.5v i load = 10ma to 5a * - 0.2 0.40 % minimum load current ams1084m v in =5v, v adj = 0v * - 3 7 ma ground pin current all fixed versions v in ? v out = 1.5v i load = 10ma to 5a * - 7 10 ma adjust pin current ams1084m v in ? v out = 1.5v to 10v i load = 10ma * - 40 90 a current limit all v in ? v out = 1.5v * 5 6.5 - a ripple rejection note 2 all v in ? v out = 3v i load = 5a * 60 72 - db dropout voltage note 1,3 all i load = 5a * - 1.3 1.5 v temperature coefficient all v in ? v out = 1.5v, i load = 10ma * - 0.005 - %/ 0 c the * denotes the specifications whic h apply over the full temperatur e range (see previous table, t j ) notes: 1 : low duty pulse testing with kelvin connections required. 2 : 120hz input ripple (c adj for adj = 25f, c out = 25f) 3 : ? v out , ? v ref = 1% application information the ams1084m series of adjustable and fi xed regulators are easy to use and have a ll the protection feat ures expected in high performance voltage regulat ors: short circuit protec tion and thermal shut-down. pin compatible with older three terminal adjustable regulators, thes e devices offer the advantage of a lower dropout voltage, more precise reference tolerance and improv ed reference stability with temperature. stability the circuit design used in the ams1084m series requires t he use of an output c apacitor as part of the device frequency compensation. the addition of 150f aluminum electrolyt ic or a 22f solid tantal um on the output will ensure stability for all operating conditions. when the adjustment terminal is bypassed wi th a capacitor to improve the ripple re jection, the require ment for an output capacitor increases. the value of 22f t antalum or 150f aluminum covers all ca ses of bypassing the adjustment terminal. without bypassing the adjustment terminal smaller capacitors can be used with equally good results. beijing estek electronics co.,ltd page 2 of 3 et- ams1084m-xx
to ensure good transient response with heavy load current changes capacitor values on the order of 100f are used in the output of many r egulators. to further improv e stability and transient respons e of these devices larger values of output c apacitor can be used. protection diodes unlike older regulators, t he ams1084m family does not need any protection diodes between the adjustment pin and the output and from the output to the input to prevent over-stressing the die. internal resistors are limiting the in ternal current pat hs on the ams1084m adjustment pin, therefore even with capacitors on the adjustment pin no protection diode is needed to ensure device safety under short-circuit conditions. diodes between the input and out put are not usually needed. microsecond surge currents of 50a to 100a can be handled by the internal diode bet ween the input and out put pins of the device. in normal operations it is diffi cult to get those values of surge cu rrents even with the use of large output capacitances. if high value output capacit ors are used, such as 1000f to 5000f and the input pin is instantaneously shorted to ground, damage can occu r. a diode from output to input is recommended, when a crowbar circuit at the input of the ams1084m is used. normal power supply cycling or even plugging and unpluggi ng in the system will not generate current large enough to do any damage. the adjustment pin can be driven on a transi ent basis 25v, with respect to the output without any dev ice degradation. as with any ic regulator, none the protection circuitry will be f unctional and the internal transis tors will break down if the maximum input to output volt age differential is exceeded. ripple rejection the ripple rejection values are measured with the adjustment pin bypassed. the im pedance of the adjust pi n capacitor at the ripple frequency should be less than t he value of r1 (normally 100 ? to 120 ? ) for a proper bypassing and ripple rejection approaching the values shown. the size of the required adjust pin capacitor is a function of the input ripple frequency. if r1=100 ? at 120hz the adjust pin capacitor shoul d be 25f. at 10khz only 0.22f is needed. the ripple rejection will be a function of output voltage, in circuits without an adjust pin bypa ss capacitor. the output rippl e will increase directly as a ratio of t he output voltage to t he reference voltage (v out / v ref ). output voltage the ams1084m series develops a 1.25v reference voltage bet ween the output and the adjust terminal. placing a resistor between these two terminals causes a constant current to flow through r1 and down through r2 to set the overall output voltage. this current is normally the specified minimum load current of 10ma. because i adj is very small and constant it represents a small error and it can usually be ignored. load regulation true remote load sensing it is not possibl e to provide, becaus e the ams1084m is a three terminal device. the resistance of the wire connecting t he regulator to the load will limit the load regulation. the data sheet specif ication for load regulation is measured at the bottom of the package. negative side sensing is a true ke lvin connection, with the bottom of the output divider returned to t he negative side of the load. the best load regulation is obtained when the top of the resistor divider r1 is connected directly to the case not to the load. if r1 were connected to the load, the effective resistance between the regulator and the load would be: connected as shown fig.3, r is not multiplied by the divider ratio. usi ng 16-gauge wire the parasitic line resistance is about 0.004 ? per foot, translating to 4mv/ft at 1a load current. it is important to keep the positiv e lead between regulator and load as short as possible and use large wire or pc board traces. ams1084m ams1084m ams1084m beijing estek electronics co.,ltd page 3 of 3 et- ams1084m-xx
|
