Dynamic Shift Registers - Bitsavers - Trailing-Edge

application information (con't)The total average power dissipated by the MH0026is the sum of the DC power and AC transientpower. The total must be less than given packagepower ratings.Since the device dissipates only 2mW with outputvoltage high (MaS logic "0"), the dominatingfactor in average DC power is duty cycle or thepercent of time in output voltage low state (MaSlogic "1 "). Percent of total power contributed byP De is usually neglible in shift register applicationswhere duty cycle is less than 25%. PDe dominatesin RAM address line driver applications whereduty cycle can exceed 50%.3.1 DC Power (per driver)DC Power is given by:PDe = (V+ - V-) X (IS(Low») X( ON time )OFF time-ON timeor PDe = (Output Low Power) X (Duty Cycle)+ _ Vswhere: IS(Low) = Is@V -V = 20VExample 1: (V+ = +5V, V- = -12Vja) Duty cycle = 25%, thereforePDC = 17V X 40mA X 17/20 X 25%P DC = 145mW worst·case, each sidePDC = 109mW typicallyb) Duty cycle = 5%PDC = 21mWc) See graph on page 132The above illustrates that for shift register applica·tions, the minimum clock width allowable for thegiven type of shift register should be used in orderto drive the largest number of registers per clockdriver.Example 2: (V+ = +17V, V- = GNDj:a) Duty cycle = 50%P De = 290mW worst-casePDe = 218mW typicallyb) Duty cycle = 100%PDC = 580mWThus for RAM address line applications, packagetype and heat sink technique will limit drivecapability rather than AC power.3.2 AC Transient Power (per driver)AC Transient power is given by:P =AC(V+ - V-)2 X f X Cwhere: f = frequency of operationCL = Load capacitance (includ ing allstrays and wiring)Example 3: (V+ = +5V, V- = -12VjPAC = 17 X 17 X f(MHz) X 10 6 XC L (nF) X 10- 9PAC = 290mW per MHz per 1000pFThus at 5MHz, a 1000pF load will cause any driverto dissipate one and one half watts. For long shiftregisters, a driver with the highest package powerrating will drive the largest number of bits for thelowest cost per bit.3.3 Package SelectionPower ratings are based on a maximum junctionrating of 175°C. The following guidelines aresuggested for package selection. Graphs on page 132illustrate derating for various operating temperatures.3.31 TO-5 rH") Package: Rated at 600mW stillair (derate at 4.0mWtC above 25°C) and 900mWwith clip on heat sink (derate at 6.0mWtC above25° C). This popular hermetic package is recommendedfor small systems. Low cost (about 10¢)clip-on-heat sink increases driving capability by50%.3.32 8-Pin ("N") Molded Mini-DIP: Rated at600mW still air (derate at 4.0mWtC above 25°C)and 1.0 watt soldered to PC board (derate at6.6mWtC). Constructed with a special copperlead frame, this package is recommended formedium size commercial systems particularlywhere automatic insertion is used. (Please note forprototype work, that this package is only rated at600mW when mounted in a socket and not onewatt until it is soldered down.)3.33 TO-8 ("G") Package: Rated at 1.5 wattsstill air (derate at 10mWtC above 25°C) and 2.3watts with clip on heat sink (Thermalloy type215-1.9 or equivalent-derate at 15mWtC).Selected for its power handling capability andmoderate cost, this hermetic package will drivevery large systems at the lowest cost per bit.L~J:ooNm.......~J:ooNm(')135