Dynamic Shift Registers - Bitsavers - Trailing-Edge

applications informationCircuit OperationInput current forced into the base of Q 1 through thecoupling capacitor CIN causes Q, to be driven intosaturation, swinging the output to V- + V CE(sat) +VDiode'When the input current has decayed, or has beenswitched, such that Q, turns off, Q 2 receives basedrive through R 2 , turning Q 2 on. This suppliescurrent to the load and the output swings positiveto V+ - VBE .It may be noted that Q, must switch off beforeQ2 begins to supply current, hence high internaltransients currents form V- to V+ cannot occur.INPUTR1250 .4~Ol,---.....-ov·:~; J~2:.--j.............-O OUTPUTiL---+--~-----ov-FIGURE 1. MH0025 Schematic (One-Half Circuit)culations to enable the fan-out to be calculatedfor any system condition.Transient CurrentThe maximum peak output current of the MH0025is given as 1.5A. Average transient current requiredfrom the driver can be calculated from:Typical rise times into 1000 pF load is 25 nsFor V+ - V- = 20V, I = O.SA.Transient Output PowerThe average transient power (Pac) dissipated, isequal to the energy needed to charge and dischargethe output capacitive load (Cd multiplied by thefrequency of operation (fl.(nPAC = C L x (V+ - V-)2 X f (2)For V+ - V- = 20V, f = 1.0 MHz, C L = 1000 pF,PAC = 400 mW .Internal Power"0" State Negligible «3 mW)"1" States:~oN(J1........s:~oN(J1("')Fan-Out CalculationThe drive capability of the MH0025 is a functionof system requirements, i.e. speed, ambient temperature,voltage swing, drive circuitry, and straywiring capacity.= 80 mW for V+ - V- = 20V, DC = 20%Package Power DissipationTotal average power = transient output power +internal power(3)The following equations cover the necessary calexampIe calcu lationHow many MM506 shift registers can be driven byan MH0025CN driver at 1 MHz using a clock pulsewidth of 200 ns, rise time 30-50 ns and 16V amplitudeover the temperature range 0-70°C?Power Dissipation:At 70°C the MH0025CN can dissipate 630 mWwhen soldered into printed circuit board.Transient Peak Current Limitation:From equation (1 L it can be seen that at 16V and30 ns, the maximum load that can be driven islimited to 2800 pF.Average I nternal Power:Equation (3), gives an average power of 50 mW at16V and a 20% duty cycle.For one half of the MH0025C, 630 mW","" 2 can bedissipated.315 mW = 50 mW + transient output power265 mW = transient output powerUsing equation (2) at 16V, 1 MHz and 250 mW,each half of the MH0025CN can drive a 975 pFload. This is, less than the load imposed by thetransient current limitation of equation (n andso a maximum load of 975 pF would prevail.From the data sheet for the MM506, the averageclock pulse load is 80 pF. Therefore the number975of devices driven is 00 or 12 registers.129