LM555
Timer
General Description
The LM555 is a highly stable device for generating accurate
time delays or oscillation. Additional terminals are provided
for triggering or resetting if desired. In the time delay mode of
operation, the time is precisely controlled by one external re-
sistor and capacitor. For astable operation as an oscillator,
the free running frequency and duty cycle are accurately
controlled with two external resistors and one capacitor. The
circuit may be triggered and reset on falling waveforms, and
the output circuit can source or sink up to 200mA or drive
TTL circuits.
Features
nDirect replacement for SE555/NE555
nTiming from microseconds through hours
nOperates in both astable and monostable modes
nAdjustable duty cycle
nOutput can source or sink 200 mA
nOutput and supply TTL compatible
nTemperature stability better than 0.005% per ˚C
nNormally on and normally off output
nAvailable in 8-pin MSOP package
Applications
nPrecision timing
nPulse generation
nSequential timing
nTime delay generation
nPulse width modulation
nPulse position modulation
nLinear ramp generator
Schematic Diagram
DS007851-1
February 2000
LM555 Timer
© 2000 National Semiconductor Corporation DS007851 www.national.com
Connection Diagram
Ordering Information
Package Part Number Package Marking Media Transport NSC Drawing
8-Pin SOIC LM555CM LM555CM Rails M08A
LM555CMX LM555CM 2.5k Units Tape and Reel
8-Pin MSOP LM555CMM Z55 1k Units Tape and Reel MUA08A
LM555CMMX Z55 3.5k Units Tape and Reel
8-Pin MDIP LM555CN LM555CN Rails N08E
Dual-In-Line, Small Outline
and Molded Mini Small Outline Packages
DS007851-3
Top View
LM555
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Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage +18V
Power Dissipation (Note 3)
LM555CM, LM555CN 1180 mW
LM555CMM 613 mW
Operating Temperature Ranges
LM555C 0˚C to +70˚C
Storage Temperature Range −65˚C to +150˚C
Soldering Information
Dual-In-Line Package
Soldering (10 Seconds) 260˚C
Small Outline Packages
(SOIC and MSOP)
Vapor Phase (60 Seconds) 215˚C
Infrared (15 Seconds) 220˚C
See AN-450 “Surface Mounting Methods and Their Effect
on Product Reliability” for other methods of soldering
surface mount devices.
Electrical Characteristics (Notes 1, 2)
(T
A
= 25˚C, V
CC
= +5V to +15V, unless othewise specified)
Parameter Conditions Limits Units
LM555C
Min Typ Max
Supply Voltage 4.5 16 V
Supply Current V
CC
= 5V, R
L
=∞
V
CC
= 15V, R
L
=∞
(Low State) (Note 4)
3
10 6
15 mA
Timing Error, Monostable
Initial Accuracy 1%
Drift with Temperature R
A
= 1k to 100kΩ, 50 ppm/˚C
C = 0.1µF, (Note 5)
Accuracy over Temperature 1.5 %
Drift with Supply 0.1 %/V
Timing Error, Astable
Initial Accuracy 2.25 %
Drift with Temperature R
A
,R
B
= 1k to 100kΩ, 150 ppm/˚C
C = 0.1µF, (Note 5)
Accuracy over Temperature 3.0 %
Drift with Supply 0.30 %/V
Threshold Voltage 0.667 x V
CC
Trigger Voltage V
CC
= 15V 5 V
V
CC
= 5V 1.67 V
Trigger Current 0.5 0.9 µA
Reset Voltage 0.4 0.5 1 V
Reset Current 0.1 0.4 mA
Threshold Current (Note 6) 0.1 0.25 µA
Control Voltage Level V
CC
= 15V
V
CC
=5V 9
2.6 10
3.33 11
4V
Pin 7 Leakage Output High 1 100 nA
Pin 7 Sat (Note 7)
Output Low V
CC
= 15V, I
7
= 15mA 180 mV
Output Low V
CC
= 4.5V, I
7
= 4.5mA 80 200 mV
LM555
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Electrical Characteristics (Notes 1, 2) (Continued)
(T
A
= 25˚C, V
CC
= +5V to +15V, unless othewise specified)
Parameter Conditions Limits Units
LM555C
Min Typ Max
Output Voltage Drop (Low) V
CC
= 15V
I
SINK
= 10mA 0.1 0.25 V
I
SINK
= 50mA 0.4 0.75 V
I
SINK
= 100mA 2 2.5 V
I
SINK
= 200mA 2.5 V
V
CC
=5V
I
SINK
= 8mA V
I
SINK
= 5mA 0.25 0.35 V
Output Voltage Drop (High) I
SOURCE
= 200mA, V
CC
= 15V 12.5 V
I
SOURCE
= 100mA, V
CC
= 15V 12.75 13.3 V
V
CC
= 5V 2.75 3.3 V
Rise Time of Output 100 ns
Fall Time of Output 100 ns
Note 1: All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is func-
tional, but do not guarantee specific performance limits. Electrical Characteristics state DC andAC electrical specifications under particular test conditions which guar-
antee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is
given, however, the typical value is a good indication of device performance.
Note 3: For operating at elevated temperatures the device must be derated above 25˚C based on a +150˚C maximum junction temperature and a thermal resistance
of 106˚C/W (DIP), 170˚C/W (S0-8), and 204˚C/W (MSOP) junction to ambient.
Note 4: Supply current when output high typically 1 mA less at VCC =5V.
Note 5: Tested at VCC = 5V and VCC = 15V.
Note 6: This will determine the maximum value of RA+R
Bfor 15V operation. The maximum total (RA+R
B
)is20MΩ.
Note 7: No protection against excessive pin 7 current is necessary providing the package dissipation rating will not be exceeded.
Note 8: Refer to RETS555X drawing of military LM555H and LM555J versions for specifications.
LM555
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Typical Performance Characteristics
Minimuim Pulse Width
Required for Triggering
DS007851-4
Supply Current vs.
Supply Voltage
DS007851-19
High Output Voltage vs.
Output Source Current
DS007851-20
Low Output Voltage vs.
Output Sink Current
DS007851-21
Low Output Voltage vs.
Output Sink Current
DS007851-22
Low Output Voltage vs.
Output Sink Current
DS007851-23
LM555
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Typical Performance Characteristics (Continued)
Output Propagation Delay vs.
Voltage Level of Trigger Pulse
DS007851-24
Output Propagation Delay vs.
Voltage Level of Trigger Pulse
DS007851-25
Discharge Transistor (Pin 7)
Voltage vs. Sink Current
DS007851-26
Discharge Transistor (Pin 7)
Voltage vs. Sink Current
DS007851-27
LM555
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Applications Information
MONOSTABLE OPERATION
In this mode of operation, the timer functions as a one-shot
(
Figure 1
). The external capacitor is initially held discharged
by a transistor inside the timer. Upon application of a nega-
tive trigger pulse of less than 1/3 V
CC
to pin 2, the flip-flop is
set which both releases the short circuit across the capacitor
and drives the output high.
The voltage across the capacitor then increases exponen-
tially for a period of t = 1.1 R
A
C, at the end of which time the
voltage equals 2/3 V
CC
. The comparator then resets the
flip-flop which in turn discharges the capacitor and drives the
output to its low state.
Figure 2
shows the waveforms gener-
ated in this mode of operation. Since the charge and the
threshold level of the comparator are both directly propor-
tional to supply voltage, the timing internal is independent of
supply.
During the timing cycle when the output is high, the further
application of a trigger pulse will not effect the circuit so long
as the trigger input is returned high at least 10µs before the
end of the timing interval. However the circuit can be reset
during this time by the application of a negative pulse to the
reset terminal (pin 4). The output will then remain in the low
state until a trigger pulse is again applied.
When the reset function is not in use, it is recommended that
it be connected to V
CC
to avoid any possibility of false trig-
gering.
Figure 3
is a nomograph for easy determination of R, C val-
ues for various time delays.
NOTE: In monostable operation, the trigger should be driven
high before the end of timing cycle.
ASTABLE OPERATION
If the circuit is connected as shown in
Figure 4
(pins 2 and 6
connected) it will trigger itself and free run as a multivibrator.
The external capacitor charges through R
A
+R
B
and dis-
charges through R
B
. Thus the duty cycle may be precisely
set by the ratio of these two resistors.
In this mode of operation, the capacitor charges and dis-
charges between 1/3 V
CC
and 2/3 V
CC
. As in the triggered
mode, the charge and discharge times, and therefore the fre-
quency are independent of the supply voltage.
DS007851-5
FIGURE 1. Monostable
DS007851-6
VCC = 5V Top Trace: Input 5V/Div.
TIME = 0.1 ms/DIV. Middle Trace: Output 5V/Div.
RA= 9.1kΩBottom Trace: Capacitor Voltage 2V/Div.
C = 0.01µF
FIGURE 2. Monostable Waveforms
DS007851-7
FIGURE 3. Time Delay
DS007851-8
FIGURE 4. Astable
LM555
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Applications Information (Continued)
Figure 5
shows the waveforms generated in this mode of
operation.
The charge time (output high) is given by:
t
1
= 0.693 (R
A
+R
B
)C
And the discharge time (output low) by:
t
2
= 0.693 (R
B
)C
Thus the total period is:
T=t
1
+t
2
= 0.693 (R
A
+2R
B
)C
The frequency of oscillation is:
Figure 6
may be used for quick determination of these RC
values.
The duty cycle is:
FREQUENCY DIVIDER
The monostable circuit of
Figure 1
can be used as a fre-
quency divider by adjusting the length of the timing cycle.
Figure 7
shows the waveforms generated in a divide by three
circuit.
PULSE WIDTH MODULATOR
When the timer is connected in the monostable mode and
triggered with a continuous pulse train, the output pulse
width can be modulated by a signal applied to pin 5.
Figure
8
shows the circuit, and in
Figure 9
are some waveform
examples.
DS007851-9
VCC = 5V Top Trace: Output 5V/Div.
TIME = 20µs/DIV. Bottom Trace: Capacitor Voltage 1V/Div.
RA= 3.9kΩ
RB=3kΩ
C = 0.01µF
FIGURE 5. Astable Waveforms
DS007851-10
FIGURE 6. Free Running Frequency
DS007851-11
VCC = 5V Top Trace: Input 4V/Div.
TIME = 20µs/DIV. Middle Trace: Output 2V/Div.
RA= 9.1kΩBottom Trace: Capacitor 2V/Div.
C = 0.01µF
FIGURE 7. Frequency Divider
DS007851-12
FIGURE 8. Pulse Width Modulator
DS007851-13
VCC = 5V Top Trace: Modulation 1V/Div.
TIME = 0.2 ms/DIV. Bottom Trace: Output Voltage 2V/Div.
RA= 9.1kΩ
C = 0.01µF
FIGURE 9. Pulse Width Modulator
LM555
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Applications Information (Continued)
PULSE POSITION MODULATOR
This application uses the timer connected for astable opera-
tion, as in
Figure 10
, with a modulating signal again applied
to the control voltage terminal. The pulse position varies with
the modulating signal, since the threshold voltage and hence
the time delay is varied.
Figure 11
shows the waveforms
generated for a triangle wave modulation signal.
LINEAR RAMP
When the pullup resistor, R
A
, in the monostable circuit is re-
placed by a constant current source, a linear ramp is gener-
ated.
Figure 12
shows a circuit configuration that will perform
this function.
Figure 13
shows waveforms generated by the linear ramp.
The time interval is given by:
V
BE
≅0.6V
DS007851-14
FIGURE 10. Pulse Position Modulator
DS007851-15
VCC = 5V Top Trace: Modulation Input 1V/Div.
TIME = 0.1 ms/DIV. Bottom Trace: Output 2V/Div.
RA= 3.9kΩ
RB=3kΩ
C = 0.01µF
FIGURE 11. Pulse Position Modulator
DS007851-16
FIGURE 12.
DS007851-17
VCC = 5V Top Trace: Input 3V/Div.
TIME = 20µs/DIV. Middle Trace: Output 5V/Div.
R1= 47kΩBottom Trace: Capacitor Voltage 1V/Div.
R2= 100kΩ
RE= 2.7 kΩ
C = 0.01 µF
FIGURE 13. Linear Ramp
LM555
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Applications Information (Continued)
50% DUTY CYCLE OSCILLATOR
For a 50% duty cycle, the resistors R
A
and R
B
may be con-
nected as in
Figure 14
. The time period for the output high is
the same as previous, t
1
= 0.693 R
A
C. For the output low it
is t
2
=
Thus the frequency of oscillation is
Note that this circuit will not oscillate if R
B
is greater than 1/2
R
A
because the junction of R
A
and R
B
cannot bring pin 2
down to 1/3 V
CC
and trigger the lower comparator.
ADDITIONAL INFORMATION
Adequate power supply bypassing is necessary to protect
associated circuitry. Minimum recommended is 0.1µF in par-
allel with 1µF electrolytic.
Lower comparator storage time can be as long as 10µs
when pin 2 is driven fully to ground for triggering. This limits
the monostable pulse width to 10µs minimum.
Delay time reset to output is 0.47µs typical. Minimum reset
pulse width must be 0.3µs, typical.
Pin 7 current switches within 30ns of the output (pin 3) volt-
age.
DS007851-18
FIGURE 14. 50% Duty Cycle Oscillator
LM555
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Physical Dimensions inches (millimeters) unless otherwise noted
Small Outline Package (M)
NS Package Number M08A
8-Lead (0.118” Wide) Molded Mini Small Outline Package
NS Package Number MUA08A
LM555
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
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COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
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can be reasonably expected to cause the failure of
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Molded Dual-In-Line Package (N)
NS Package Number N08E
LM555 Timer
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
