October 1998
1
MIC1555/1557
MIC1555/1557
Micrel
MIC1555/1557
IttyBittyTM RC Timer / Oscillator
General Description
The MIC1555 IttyBittyTM CMOS RC timer/oscillator and
MIC1557 IttyBitty CMOS RC oscillator are designed to pro-
vide rail-to-rail pulses for precise time delay or frequency
generation.
The devices are similar in function to the industry standard
"555", without a frequency control (F
C
) pin or an open-
collector discharge (D) pin. The threshold pin (TH) has
precedence over the trigger (TR) input, ensuring that the
BiCMOS output is off when TR is high.
The MIC1555 may be used as an astable (oscillator) or
monostable (one-shot) with separate threshold and trigger
inputs. In the one-shot mode, the output pulse width is
precisely controlled by an external resistor and a capacitor.
Time delays may be accurately controlled from microsec-
onds to hours. In the oscillator mode, the output is used to
provide precise feedback, with a minimum of one resistor and
one capacitor producing a 50% duty cycle square wave.
The MIC1557 is designed for astable (oscillator) operation
only, with a chip select/reset (CS) input for low power shut-
down. One resistor and one capacitor provide a 50% duty
cycle square wave. Other duty-cycle ratios may be produced
using two diodes and two resistors.
The MIC1555/7 is powered from a +2.7V to +18V supply
voltage.
The MIC1555/7 is available in the SOT-23-5 5-lead package,
and is rated for 40
C to +85
C ambient temperature range.
Typical Applications
Features
+2.7V to +18V operation
Low current
<1
A typical shutdown mode (MIC1557)
200
A typical (TRG and THR low) at 3V supply
Timing from microseconds to hours
"Zero" leakage trigger and threshold inputs
50% square wave with one Resistor, one Capacitor
Threshold input precedence over trigger input
<15
output on resistance
No output cross-conduction current spikes
<0.005%/
C temperature stability
<0.055%/V supply stability
Small SOT-23-5 surface mount package
Applications
Precision timer
Pulse generation
Sequential timing
Time-delay generation
Missing pulse detector
Micropower oscillator to 5MHz
Charge-pump driver
LED blinker
Voltage converter
Linear sweep generator
Variable frequency and duty cycle oscillator
Isolated feedback for power supplies
Ordering Information
Part Number
Temp. Range
Package
Marking
MIC1555BM5
40
C to +85
C
SOT-23-5
T10
MIC1557BM5
40
C to +85
C
SOT-23-5
T11
Standby
Trigger
VS
OUT
MIC1555
1
3
GND
2
THR
5
+5V
Output
TRG
4
100s
1k
0.1F
Monostable (One-Shot)
8kHz
Enabled
Disabled
VS
CS
MIC1557
4
3
GND
2
+5V
OUT
5
T/T
1
Output
1k
0.1F
Astable (Oscillator)
MIC1555/1557
Micrel
MIC1555/1557
2
October 1998
Pin Configuration
VS
THR
TRG
OUT
T10
Part
Identification
1
3
4
5
T/T
OUT
VS
CS
T11
1
3
4
5
2
2
GND
GND
SOT-23-5 (M5)
Pin Description
Pin Number
Pin Number
Pin Name
Pin Function
MIC1555
MIC1557
1
4
VS
Supply (Input): +2.7 to +18V supply.
2
2
GND
Ground: Supply return.
3
CS
Chip Select/Reset (Input): Active high at
>2
/
3
V
S
. Output off when low at
<
1
/
3
V
S
.
3
5
OUT
Output: CMOS totem-pole output.
4
TRG
Trigger (Input): Sets output high. Active low (at
2
/
3
V
S
nominal)
.
5
THR
Threshold (Dominant Input): Sets output low. Active high (at
2
/
3
V
S
nominal).
1
T/T
Trigger/Threshold (Input): Internally connected to both threshold and trigger
functions. See TRG and THR.
Part Number
Identification
MIC1555BM5
T10
MIC1557BM5
T11
October 1998
3
MIC1555/1557
MIC1555/1557
Micrel
Electrical Characteristics
T
A
= 25
C, bold values indicate 40
C
T
A
+85
C; unless noted.
Parameter
Condition
Min
Typ
Max
Units
Supply current
V
S
= 5V
240
300
A
V
S
= 15V
350
400
A
Monostable Timing Accuracy
R
A
= 10k, C =0.1
F, V
S
= 5V
2
%
R
A
= 10k, C =0.1
F, V
S
= 5V
858
1161
s
Monostable Drift over Temp
V
S
= 5V, 55
T
A
+125
C, Note 1
100
ppm/
C
V
S
=10V, 55
T
A
+125
C, Note 1
150
ppm/
C
V
S
= 15V, 55
T
A
+125
C, Note 1
200
ppm/
C
Monostable Drift over Supply
V
S
= 5V to 15V, Note 1
0.5
%/V
Astable Timing Accuracy
R
A
= R
B
= 10k, C = 0.1
F, V
S
= 5V
2
%
R
A
= R
B
= 10k, C = 0.1
F, V
S
= 5V
1717
2323
s
Maximum Astable Frequency
R
T
= 1k, C
T
= 47pF, V
s
= 8V
5
MHz
Astable Drift over Temp
V
S
= 5V, 55
T
A
+125
C, Note 1
100
ppm/
C
V
S
=10V, 55
T
A
+125
C, Note 1
150
ppm/
C
V
S
= 15V, 55
T
A
+125
C, Note 1
200
ppm/
C
Astable Drift over Supply
V
S
= 5V to 15V, Note 1
0.5
%/V
Threshold Voltage
V
S
= 15V
61
67
72
%V
S
Trigger Voltage
V
S
= 15V
27
32
37
%V
S
Trigger Current
V
S
= 15V
50
nA
Threshold Current
V
S
= 15V
50
nA
Chip Select
on >
2
/
3
V
S
50
67
72
%V
S
off <
1
/
3
V
S
28
33
50
%V
S
Output Voltage Drop
V
S
= 15V, I
SINK
= 20mA
0.3
1.25
V
V
S
= 5V, I
SINK
= 3.2mA
0.08
0.5
V
Output Voltage Drop
V
S
= 15V, I
SOURCE
= 20mA
14.1
14.7
V
V
S
= 5V, I
SOURCE
= 3.2mA
3.8
4.7
V
Supply Voltage
functional operation, Note 1
2.7
18
V
Output Rise Time
R
L
= 10M, C
L
= 10pF, V
S
= 5V, Note 1
15
ns
Output Fall Time
R
L
= 10M, C
L
= 10pF, V
S
= 5V, Note 1
15
ns
General Note: Devices are ESD protected, however handling precautions are recommended.
Note 1:
Not tested.
Absolute Maximum Ratings
Supply Voltage (V
S
) .................................................... +22V
Threshold Voltage (V
THR
, V
T/T
) .................................. +22V
Trigger Voltage (V
TRG
, V
T/T
) ....................................... +22V
Lead Temperature (soldering 10sec.) ....................... 300
C
Operating Ratings
Supply Voltage (V
S
) ..................................... +2.7V to +18V
Ambient Temperature Range (T
A
) ............. 40
C to +85
C
Package Thermal Resistance
(
JA
) ................................................................... 220
C/W
(
JC
) .................................................................. 130
C/W
MIC1555/1557
Micrel
MIC1555/1557
4
October 1998
Typical Characteristics
(T
A
= 25
C, V
IN
= +5)
0.0001
0.001
0.01
0.1
1
10
100
1x10
-1
1x10
0
1x10
1
1x10
2
1x10
3
1x10
4
1x10
5
1x10
6
1x10
7
CAPACITANCE (
F)
PERIOD (
s)
Pulse Width
100k
10k
R
T
=1k
1M
0
10
20
30
40
50
60
70
-40 -20
0
20
40
60
80 100
ON RESISTANCE (
)
TEMPERATURE (
C)
On Resistance
vs. Temperature
200
220
240
260
280
300
-40 -20
0
20
40
60
80 100
SUPPLY CURRENT (
A)
TEMPERATURE (
C)
Supply Current
vs. Temperature
0
10
20
30
40
50
60
70
0
5
10
15
20
ON RESISTANCE (
)
SUPPLY VOLTAGE (V)
On Resistance
vs. Supply Voltage
0
100
200
300
400
500
0
5
10
15
20
SUPPLY CURRENT (
A)
SUPPLY VOLTAGE (V)
Supply Current
vs. Supply Voltage
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
10
100
1000
k FACTOR
RC (
s)
k Factors
Times RC
f=1/k
1
RC
t=k
2
RC
0.0001
0.001
0.01
0.1
1
10
100
1x10
-2
1x10
-1
1x10
0
1x10
1
1x10
2
1x10
3
1x10
4
1x10
5
1x10
6
1x10
7
CAPACITANCE (
F)
FREQUENCY (Hz)
Astable Frequency
1M
R
T
=1k
10k
100k
0
2
4
6
8
10
12
14
3
6
9
12
15
18
CHIP SELECT VOLTAGE (V)
SUPPLY VOLTAGE (V)
MIC1557 Chip Select
vs. Supply Voltage
ON
OFF
October 1998
5
MIC1555/1557
MIC1555/1557
Micrel
Functional Description
Refer to the block diagrams.
The MIC1555/7 provides the logic for creating simple RC
timer or oscillator circuits.
The MIC1555 has separate THR (threshold) and TRG
(trigger) connections for monostable (one-shot) or astable
(oscillator) operation.
The MIC1557 has a single T/T (threshold and trigger) con-
nection for astable (oscillator) operation only. The MIC1557
includes a CS (chip select/reset) control.
Supply
VS (supply) is rated for +2.7V to +18V. An external capacitor
is recommended to decouple noise.
Resistive Divider
The resistive voltage divider is constructed of three equal
value resistors to produce
1
/
3
V
S
and
2
/
3
V
S
voltage for trigger
and threshold reference voltages.
Chip Select/Reset (MIC1557 only)
CS (chip select/reset) controls the bias supply to the oscillator's
internal circuitry. CS must be connected to CMOS logic-high
or logic-low levels. Floating CS will result in unpredictable
operation. When the chip is deselected, the supply current is
less than 1
A. Forcing CS low resets the MIC1557 by setting
the flip flop, forcing the output low.
Threshold Comparator
The threshold comparator is connected to S (set) on the RS
flip-flop. When the threshold voltage (
2
/
3
V
S
) is reached, the
flip-flop is set, making the output low. THR is dominant over
TRG.
Functional Diagrams
S
R
Q
T/T
OUT
VS
Bias
GND
CS
V
SUPPLY
MIC1557
Output
1k
0.1F
8kHz
1M
MIC1557 Block Diagram with External Components
(Astable Configuration)
S
R
Q
OUT
GND
Output
TRG
THR
Standby
Trigger
VS
V
SUPPLY
MIC1555
1k
0.1F
<100s
Bias
100s
MIC1555 Block Diagram with External Components
(Monostable Configuration)
Trigger Comparator
The trigger comparator is connected to R (reset) on the RS
flip-flop. When TRG (trigger) goes below the trigger voltage
(
1
/
3
V
S
), the flip-flop resets, making the output high.
Flip-Flop and Output
A reset signal causes Q to go low, turning on the P-channel
MOSFET and turning off the N-channel MOSFET. This
makes the output rise to nearly V
S
.
A set signal causes Q to go high, turning off the P-channel
MOSFET, and turning on the N-channel MOSFET, grounding
OUT.
Basic Monostable Operation
Refer to the MIC1555 functional diagram.
A momentary low signal applied to TRG causes the output to
go high. The external capacitor charges slowly through the
external resistor. When V
THR
(threshold voltage) reaches
2
/
3
V
S
, the output is switched off, discharging the capacitor.
During power-on, a single pulse may be generated.
Basic Astable Operation
Refer to the MIC1557 functional diagram.
The MIC1557 starts with T/T low, causing the output to go
high. The external capacitor charges slowly through the
external resistor. When V
T/T
reaches
2
/
3
V
S
(threshold volt-
age), the output is switched off, slowly discharging the
capacitor. When V
T/T
decreases to
1
/
3
V
S
(trigger voltage), the
output is switched on, causing V
T/T
to rise again, repeating
the cycle.
MIC1555/1557
Micrel
MIC1555/1557
6
October 1998
Application Information
Basic Monostable (One-Shot) Circuit
A monostable oscillator produces a single pulse each time
that it is triggered, and is often referred to as a "one-shot". The
pulse width is constant, while the time between pulses
depends on the trigger input. One-shots are generally used
to "stretch" incoming pulses, of varying widths, to a fixed
width. The IttyBitty MIC1555 is designed for monostable
operation, but may also be connected to provide astable
oscillations. The pulse width is determined by the time it takes
to charge a capacitor from ground to a comparator trip point.
If the capacitor (C
T
) is charged through a resistor (R
T
)
connected to the output of an MIC1555, the trip point is
approximately 1.1RC
T
(the same time as the initial power-on
cycle of an astable circuit.) If the trigger pulse of an MIC1555
remains low longer than the output pulse width, short oscilla-
tions may be seen in the output of a one-shot circuit, since the
threshold pin has precedence over the trigger pin. These
occur since the output goes low when the threshold is
exceeded, and then goes high again as the trigger function is
asserted. AC coupling the input with a series capacitor and
a pull-up resistor, with an RC time constant less than the
pulse width, will prevent these short oscillations. A diode (D
T
)
in parallel with (R
T
) resets the one-shot quickly.
CS
OUT
THR
TRG
VS
GND
D
T
R
T
OUT
C
IN
R
PU
C
T
MIC1555
Trigger
1
/
3
V
IN
t
ON
= 1.1R
T
C
T
2.7V to 18V
1
3
2
4
5
Figure 1. One-Shot Diagram
The period of a monostable circuit is:
t = k
2
RC
where:
t = period (s)
k
2
= constant [from Typical Characteristics graph]
R = resistance (
)
C = capacitance (F)
Basic Astable (Oscillator) Circuits
An astable oscillator switches between two states, "on" and
"off", producing a continuous square wave. The IttyBitty
MIC1557 is optimized for this function, with the two compara-
tor inputs, threshold and trigger (T/T), tied together internally.
Chip select (CS) is brought out to allow on-off control of the
oscillator.
The MIC1555 may also be used as an astable oscillator by
tying the threshold and trigger pins together, forming a T/T
pin. If a resistor (R
T
) is connected from the output to a
grounded timing capacitor, (C
T
) the voltage at their junction
will ramp up from ground when the output goes high. If the T/
T pin is connected to this junction, the output will switch low
when the ramp exceeds
2
/
3
of the input voltage. The junction's
voltage ramps down toward ground while the output is low.
When the ramp is below
1
/
3
of the input voltage, the output
switches to high, and the junction ramps up again.
The continuing frequency of an MIC1555/7 astable oscillator
depends on the RC time constant, and is approximately 0.7/
RC below 1MHz. At frequencies above 1MHz the RC
multiplier increases as capacitance is decreased, and propa-
gation delay becomes dominant. Non-symmetrical oscillator
operation is possible at frequencies up to 5MHz.
If a duty cycle other than 50% is desired, a low-power signal
diode may be connected in series with the timing resistor
(R
A
), and a second resistor (R
B
) in series with an opposite
facing diode connected in parallel. The frequency is then
made up of two components, the charging time (t
A
) and the
discharging time (t
B
) t
A
= 0.7R
A
C and t
B
= 0.7R
B
C. The
frequency is the reciprocal of the sum of the two times t
A
+ t
B
,
so the total time is 1.4R
T
C
T
. The first half-cycle of an astable,
after power-on or CS enable, is lengthened since the capaci-
tor is charging from ground instead of the 1/3 input trigger trip
voltage, to 1.1RC, the same as a monostable pulse.
R
A
1k to 1M
CS
OUT
T/T
VS
GND
OUT
MIC1557
2.7V to 18V
C
B
R
E
C
T
100pF to 10F
5
3
2
4
1
CS
t = 0.7(R
A
+R
B
)C
T
t
R
B
Figure 2. Oscillator Diagram
The MIC1555 or MIC1557 can be used to construct an
oscillator.
The frequency of an astable oscillator is:
f
1
k RC
1
=
where:
f = frequency (Hz)
k
1
= constant [from Typical Characteristics graph]
R = resistance (
)
C = capacitance (F)
October 1998
7
MIC1555/1557
MIC1555/1557
Micrel
Accuracy
The two comparators in the MIC1555/7 use a resistor voltage
divider to set the threshold and trigger trip points to approxi-
mately
2
/
3
and
1
/
3
of the input voltage, respectively. Since the
charge and discharge rates of an RC circuit are dependent on
the applied voltage, the timing remains constant if the input
voltage varies. If a duty cycle of exactly 50% (or any other
value from 1 to 99%), two resistors (or a variable resistor) and
two diodes are needed to vary the charge and discharge
times. The forward voltage of diodes varies with temperature,
so some change in frequency will be seen with temperature
extremes, but the duty cycle should track. For absolute timing
accuracy, the MIC1555/7 output could be used to control
constant current sources to linearly charge and discharge the
capacitor, at the expense of added components and board
space.
Long Time Delays
Timing resistors larger than 1M
or capacitors larger than
10
F are not recommended due to leakage current inaccura-
cies. Time delays greater than 10 seconds are more accu-
rately produced by dividing the output of an oscillator by a
chain of flip-flop counter stages. To produce an accurate
one-hour delay, for example, divide an 4.55Hz MIC1557
oscillator by 16,384 (4000
hex
, 2
14
) using a CD4020 CMOS
divider. 4.5Hz may be generated with a 1
F C
T
and approxi-
mately 156k
.
Inverting Schmitt Trigger
Refer to figure 7. The trip points of the MIC1555/7 are defined
as
1
/
3
and
2
/
3
V
S
, which allows either device to be used as a
signal conditioning inverter, with hysteresis. A slowly chang-
ing input on T/T will be converted to a fast rise or fall-time
opposite direction rail-to-rail output voltage. This output may
be used to directly drive the gate of a logic-level P-channel
MOSFET with a gate pull-up resistor. This is an inverted logic
low-side logic level MOSFET driver. A standard N-channel
MOSFET may be driven by a second MIC1555/7, powered by
12V to 15V, to level-shift the input.
THR
OUT
MIC1555
1
3
GND
2
VS
5
TRG
4
+5V +12V
R
L
OFF
3.3V
ON
1.6V
Figure 7. Schmitt trigger
Charge-Pump Low-Side MOSFET Drivers
A standard MOSFET requires approximately 15V to fully
enhance the gate for minimum R
DS(on)
. Substituting a logic-
level MOSFET reduces the required gate voltage, allowing
an MIC1557 to be used as an inverting Schmitt Trigger,
described above. An MIC1557 may be configured as a
voltage quadrupler to boost a 5V input to over 15V to fully
enhance an N-channel MOSFET which may have its drain
To use the MIC1555 as an oscillator, connect TRG to THR.
VS
TRG
MIC1555
1
4
GND
2
+5V
OUT
3
THR
5
Output
8kHz
1k
0.1F
Figure 3. MIC1555 Oscillator Configuration
The MIC1557 features a CS input. When logic-low, CS
places the MIC1557 into a <1
A shutdown state. If unused,
the MIC1557 CS input on must be pulled up.
VS
CS
MIC1557
4
3
GND
2
+5V
OUT
5
T/T
1
Output
8kHz
1M
1k
0.1F
Figure 4. MIC1557 Oscillator Configuration
Falling-Edge Triggered Monostable Circuit
The MIC1555 may be triggered by an ac-coupled falling-
edge, as shown in figure 5. The RC time constant of the
input capacitor and pull-up resistor should be less than the
output pulse time, to prevent multiple output pulses. A
diode across the timing resistor provides a fast reset at the
end of the positive timing pulse.
VS
OUT
MIC1555
1
3
GND
2
THR
5
+5V
Output
TRG
4
100s
1M
1N4148
1k
0.1F
Input
Figure 5. Falling-Edge Trigger Configuration
Rising-Edge Triggered Monostable Circuit
The MIC1555 may be triggered by an ac-coupled rising-
edge, as shown in figure 6. The pulse begins when the ac-
coupled input rises, and a diode from the output holds the
THR input low until TRG discharges to
1
/
3
V
S
. This circuit
provides a low-going output pulse.
VS
OUT
MIC1555
1
3
GND
2
THR
5
+5V
Output
TRG
4
100s
1N4148
1k
0.1F
Input
Figure 6. Rising-Edge Trigger Configuration
MIC1555/1557
Micrel
MIC1555/1557
8
October 1998
connected to a higher voltage, through a high-side load. A
TTL high signal applied to CS enables a 10kHz oscillator,
which quickly develops 15V at the gate of the MOSFET,
clamped by a zener diode. A resistor from the gate to ground
ensures that the FET will turn off quickly when the MIC1557
is turned off.
VS
CS
MIC1557
4
3
GND
2
+5V
OUT
5
T/T
1
TTL High = ON
15V
+12V
R
L
N-Channel
MOSFET
(IRF540)
R
E
100k
C1
100pF
to 10F
R
T
1k to 1M
Figure 8. Charge-Pump
Audible Voltmeter
If an additional charge or discharge source is connected to
the timing capacitor, the frequency may be shifted by turning
the source on or off. An MIC1555 oscillator, powered by the
circuit under test, may be used to drive a small loudspeaker
or piezo-electric transducer to provide a medium frequency
for an open or high impedance state at the probe. A high tone
is generated for a high level, and a lower frequency for a logic
low on the probe.
THR
OUT
MIC1555
1
3
GND
2
VS
5
TRG
4
100
+5V to +18V
R
T
10k
10k
10k
47k
C
T
0.01F
1N914
0.001F
Figure 9. Audible Voltmeter
Package Information
0.20 (0.008)
0.09 (0.004)
0.60 (0.024)
0.10 (0.004)
3.02 (0.119)
2.80 (0.110)
10
0
3.00 (0.118)
2.60 (0.102)
1.75 (0.069)
1.50 (0.059)
0.95 (0.037) REF
1.30 (0.051)
0.90 (0.035)
0.15 (0.006)
0.00 (0.000)
DIMENSIONS:
MM (INCH)
0.50 (0.020)
0.35 (0.014)
1.90 (0.075) REF
SOT-23-5 (M5)
MICREL INC.
1849 FORTUNE DRIVE
SAN JOSE, CA 95131
USA
TEL
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or
other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.
1998 Micrel Incorporated
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