Soft Turn on OCD Circuit
Original Publication Date: 1988-Jan-01
Included in the Prior Art Database: 2005-Feb-14
An OCD (Off Chip Driver) buffer transistor in MOS chips must have a large W/L ratio to drive a large capacitive load at a minimum delay. The large transistor, however, causes Ldi/dt noise because of its sharp current rising. The voltage spike on the ground line appears at an early stage of a transistor's turn-on process. OCD circuits are disclosed which alleviate Ldi/dt noise by controlling the initial current ramp, yet they maintain a high-speed switching performance at a large capacitive load. Fig. 1 shows a first OCD circuit. T1, T5, T7 and T8 are PMOSFETs. T2, T3, T4, T6 and T9 are NMOSFETs. CL is a load capacitor. Although Fig. 1 shows the NMOSFET pull down circuit of an OCD, a similar PMOSFET pull-up portion is assumed. T1, T3 and T4 act as a constant current source.