Driving multiple logic mosfets and cmos inputs from 7555 timer?

[rmdr]

I'm designing a 12-hr CMOS clock and using a 7555 timer as a PWM generator (1.4kHz) for brightness control. The PWM output drives 8 loads, four of which are logic compatible 2n7002ps NMOS gates (30pF at 0.6nc) and the other four are CMOS inputs (three 7-segment decoder blanking inputs and one AND gate input). I may have gone overboard with the load resistors at 7.5k each. My reasoning was that the 7555 output source current is limited to 10mA before the output voltage drops significantly. At 8 loads, the total current is limited to 8*(5/7.5k) = 5.3mA. If the gate capacitance was large enough, this would for a very brief insignificant period of time during the transient on the gate, make the 7555 output voltage equal to 2V.

Now I don't think I really need these gate resistors because the capacitance is so small, and I calculated that with 7.5k gate resistors, the gate voltage would reach 5V in 5*RC = 5*7.5k*30pF = 1.125 us, 0.315% of my PWM switching time (0.157% of the period), so 7.5k would suffice. I believe without the resistors, the current should not be a big issue, but I still added them due to multiple loads.

Do I really need gate resistors to limit the current, even for 8 loads (highlighted in red in attached schematic), since the input capacitances are so small? The same applies for the gate resistors coming from the second counter bit 0, AM/PM indicator flip-flop, and second reset indicator from the XOR (all highlighted in green in attached schematic) since they all at <1Hz.

[Zero999]

The problem is the switching speed will be low. Reduce the 7k5 resistors to something like 100R for a start. You could add an emitter followed, with a diode between the base (cathode) and emitter (anode) to speed up the on time, but it will reduce the output voltage by 0.6V.

[David Hess]

At such a low speed, the 7555 should drive that set of loads just fine.

[T3sl4co1l]

You could use a second quad AND to avoid the discrete NANDs (and half as many 2N7002s), or really the gate will drive LEDs just fine by itself, then no transistors needed at all.

I'd avoid 74AHC, regular HC is fine here.  No need for anything faster.  (Is AHC faster, or just unclamped input? I forget.)

Also could use a second '3G14 to buffer the 555 output, or a hex (HC14) to do both in one chip.

Tim

[David Hess]

I'd avoid 74AHC, regular HC is fine here.  No need for anything faster.  (Is AHC faster, or just unclamped input? I forget.)

AHC is about 50% faster than HC.