555 Notas De Aplicación

INTEGRATED CIRCUITS

AN170
NE555 and NE556 applications
1988 Dec

Philips
Semiconductors

Philips Semiconductors

Application note

NE555 and NE556 applications

AN170

oscillator, only one additional resistor is necessary. By proper
selection of external components, oscillating frequencies from one
cycle per half hour to 500kHz can be realized. Duty cycles can be
adjustedfrom less than one percent to 99 percent over the
frequency spectrum. Voltage control of timing and oscillation
functions is also available.

INTRODUCTION
In mid 1972, Philips Semiconductors introduced the 555 timer, a
unique functional building block that has enjoyed unprecedented
popularity. The timer’s success can be attributed to several inherent
characteristics foremost of which areversatility, stability and low
cost. There can be no doubt that the 555 timer has altered the
course of the electronics industry with an impact not unlike that of
the IC operational amplifier.

Timer Circuitry
The timer is comprised of five distinct circuits: two voltage
comparators; a resistive voltage divider reference; a bistable
flip-flop; a discharge transistor; and an output stage thatis the
“totem-pole” design for sink or source capability. Q10-Q13 comprise
a Darlington differential pair which serves as a trigger comparator.
Starting with a positive voltage on the trigger, Q10 and Q11 turn on
when the voltage at Pin 2 is moved below one third of the supply
voltage. The voltage level is derived from a resistive divider chain
consisting of R7, R8 and R9. All threeresistors are of equal value
(5kΩ). At 15V supply, the triggering level would be 5V.
When Q10 and Q11 turn on, they provide a base drive for Q15,
turning it on. Q16 and Q17 form a bistable flip-flop. When Q15 is
saturated, Q16 is ”off’ and Q17 is saturated. Q16 and Q17 will remain
in these states even if the trigger is removed and Q15 is turned ”off’.
While Q17 is saturated, Q20 and Q14 are turnedoff.

The simplicity of the timer, in conjunction with its ability to produce
long time delays in a variety of applications, has lured many
designers from mechanical timers, op amps, and various discrete
circuits into the ever increasing ranks of timer users.

DESCRIPTION
The 555 timer consists of two voltage comparators, a bistable
flip-flop, a discharge transistor, and a resistor dividernetwork. To
understand the basic concept of the timer let’s first examine the
timer in block form as in Figure 1.
The resistive divider network is used to set the comparator levels.
Since all three resistors are of equal value, the threshold comparator
is referenced internally at 2/3 of supply voltage level and the trigger
comparator is referenced at 1/3 of supply voltage. The outputs ofthe
comparators are tied to the bistable flip-flop. When the trigger
voltage is moved below 1/3 of the supply, the comparator changes
state and sets the flip-flop driving the output to a high state. The
threshold pin normally monitors the capacitor voltage of the RC
timing network. When the

The output structure of the timer is a “totem-pole” design, with Q22
and Q24 being large geometrytransistors capable of providing
200mA with a 15V supply. While Q20 is ”off’, base drive is provided
for Q22 by Q21, thus providing a high output.
For the duration that the output is in a high state, the discharge
transistor is ”off’. Since the collector of Q14 is typically connected to
the external timing capacitor, C, while Q14 is off, the timing capacitor
now can charge through the timingresistor, RA.

VCC
555 OR 1/2 556

The capacitor voltage is monitored by the threshold comparator
(Q1-Q4) which is a Darlington differential pair. When the capacitor
voltage reaches two thirds of the supply voltage, the current is
directed from Q3 and Q4 thru Q1 and Q2. Amplification of the current
change is provided by Q5 and Q6. Q5-Q6 and Q7-Q8 comprise a
diode-biased amplifier. The...