SRAM:Output Circuit [4]

Output Circuit [4]

The key issue for designing the high-speed SRAM with byte-wide organization is noise reduction. There are two kinds of noise: VDD noise and GND noise. In the high-speed SRAM with byte-wide organization, when the output transistors drive a large load capacitance, the noise is generated and multiplied by 8 because eight outputs may change simultaneously. It is a fundamentally serious problem for the data zero output. That is to say, when the output NMOS transistor drives the large load capacitance, the GND potential of the chip goes up because of the peak current and the parasitic inductance of the GND line. Therefore, the address buffer and the ATD circuit are influenced by the GND bounce, and unnecessary signals are generated.

Figure 52.18 shows a noise-reduction output circuit. The waveforms of the noise-reduction output circuit and conventional output circuit are shown in Figure 52.19. In the conventional circuit, nodes

A and B are connected directly as shown in Figure 52.18. Its operation and characteristics are shown by the dotted lines in Figure 52.18. Due to the high-speed driving of transistor M4, the GND potential goes up, and the valid data are delayed by the output ringing. A new noise-reduction output circuit consists of one PMOS transistor, two NMOS transistors, one NAND gate, and the delay part (its characteristics are shown by the solid lines in Figure 52.19). The operation of this circuit is explained as follows. The control signals CE and OE are at high level and signal WE is at low level in the read operation. When the data zero output of logical high level is transferred to node C, transistor M1 is cut off, and M2 raises node A to the middle level. Therefore, the peak current that flows into the GND line through transistor M4 is reduced to less than one half that of the conventional circuit because M4 is driven by the middle level. After a 5-ns delay from the beginning of the middle level, transistor M3 raises node A to the VDD level. As a result, the conductance of M4 becomes maximum, but the peak current is small because of the low output voltage. Therefore, the increase of GND potential is small, and the output ringing does not appear.