SE555-SP Datasheet, PDF (13/18 Pages) Texas Instruments – QML CLASS V PRECISION TIMER

SE555-SP
www.ti.com
SGLS401 – FEBRUARY 2010
Pulse-Width Modulation
The operation of the timer can be modified by modulating the internal threshold and trigger voltages, which is
accomplished by applying an external voltage (or current) to CONT. Figure 19 shows a circuit for pulse-width
modulation. A continuous input pulse train triggers the monostable circuit, and a control signal modulates the
ÏÏÏÏÏ _{threshold voltage. Figure 20 shows the resulting output pulse-width modulation. While a sine-wave modulation}
ÏÏÏÏÏ _{signal is shown, any wave shape could be used.}
ÏÏÏÏÏ VCC (5 V to 15 V)
ÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏ ₄
8
RL
RA
R_A = 3 kΩ
C = 0.02 µF
R_L = 1 kΩ
See Figure 18
ÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏ _Clock
Input
RESET
2
TRIG
VCC
3
OUT
Output
Modulation Input Voltage
ÏÏÏÏÏÏ ₇
ÏÏÏÏÏÏ ^DISCH
ÏÏÏÏÏÏ ^Modulation 5
Input
CONT
ÏÏÏÏÏÏ (See Note A)
THRES ⁶
GND
ÏÏÏÏÏ 1
C
Clock Input Voltage
ÏÏÏÏÏ _{NOTE A: The modulating signal can be direct or capacitively coupled to CONT.}
ÏÏÏÏÏ _{For direct coupling, the effects of modulation source voltage and}
ÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏ ^{impedance on the bias of the timer should be considered.}
Output Voltage
Capacitor Voltage
Time − 0.5 ms/div
Figure 19. Circuit for Pulse-Width Modulation
Figure 20. Pulse-Width-Modulation Waveforms
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