RB

Modulation

Input

(see Note A)

CONT

TRIG

RESET

VCC

OUT

DISCH

VCC (5 V to 15 V)

RL

RA

C

GND

THRES

NOTE A: The modulating signal can be direct or capacitively coupled

to CONT. For direct coupling, the effects of modulation

source voltage and impedance on the bias of the timer

should be considered.

Pin numbers shown are for the D, JG, P, PS, and PW packages.

4

8

3

7

6

2

5

Output

NA555, NE555, SA555, SE555

SLFS022I – SEPTEMBER 1973 – REVISED SEPTEMBER 2014

www.ti.com

Typical Applications (continued)

Figure 20. Circuit for Pulse-Position Modulation

9.2.3.1 Design Requirements

Both DC and AC coupled modulation input will change the upper and lower voltage thresholds for the timing

capacitor. Both frequency and duty cycle will vary with the modulation voltage.

9.2.3.2 Detailed Design Procedure

The nominal output frequency and duty cycle can be determined using formulas in A-stable Operation section. RL

improves VOH, but it is not required for TTL compatibility.

16

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