1 - Al Kossow's Bitsavers

Figure 5 shows how the driver output waveform is altered. During forward bias, the transistor clips the negative level at VFB(in the range - 0.7 V to - 1.4 V) with the VIL level programmed over the range of - O. 7 V to - 6 V. During the initial reversebias, the driver output voltage Yo is given by the programmed VIH level minus the iR drop across the driver output impedance,i.e.,Vo = VIH- iRDuring the current plateau, the Vo value is essentially 0 V. The driver output voltage then recovers to the programmed valueas the diode reverse current relaxes to the dc reverse-bias level IS. In effect, the toft value is a measure of the time that theoutput waveform is distorted if the unwary engineer programs VIL values significantly below - O. 7 V.VoV,H - - - - - - - - - -----.... ----»"0"2-c:;",.. mc·:::len......S'"o3m...2r:::lV,L Vo = VIL(lH) - iRVFO: Forward-bias diode dropV,L: Address input low-level (forward-bias)FIGURE 5 - ADDRESS DRIVER OUTPUT VOLTAGEThe following equation derived from diode switching theory serves to estimate the magnitude of the toft values for input'levels below -1 V.toft = 40 Ir - Ir/(r+ 1)) 2] nswhere r = - (VIL + 1 )/VIHand VIL < -1 VThe coefficient 40 ns is a characteristic of the diode structure and physical parameters of the material. For example,VIL = - 3 V and VIH = + 3 V give toft = 20 ns. This estimate is not accurate at very small forward-bias values, because itignores the rise time of the driver's positive transition edge. As long as the predicted value is greater than or equal to the edgetransition time, the estimate is good. It is assumed that driver output impedance has the same value R at both upper andlower levels, VIIi and VIL.The distortion in driver waveform, shown in Figure 5, increases as the driver input low level is progressively driven morenegative. Depending on driver output impedence, only slight distortion is observed in the positive transition for input levelsnear -0.7 V to -1 V. This irregularity corresponds to the onset of the recovery phenomenon short-circuiting the output ofthe driver .. Significant distortion occurs at large negative values of VIL, and the test engineer must be aware of thisphenomenon to prevent erroneous conclusions as to the performance of the device.The input transistor provides great advantage to device use in a system environment. In a system, the negative undershootof an address line is caused by transient transmission-line reflections (undershoot of negative-edge transitions). Here the inputtransistor clips much of the swing below - O. 7 Von the address line. Positive-edge transition from a settled negative addresslow level, which gives rise to the forward-to reverse-bias recovery delay, does not occur in the typical system environment.9-10MOS MemoryApplications Engineering