Datasheet MIC1555BM5, MIC1557BM5 Datasheet (MICREL)

MIC1555/1557 Micrel

8kHz

Enabled

Disabled

VS
CS

MIC1557

4

3

GND

2

+5V

OUT

5

T/T

1

Output

1k

0.1µF

MIC1555/1557

IttyBitty™ RC Timer / Oscillator

General Description

The MIC1555 IttyBitty™ CMOS RC timer/oscillator and
MIC1557 IttyBitty CMOS RC oscillator are designed to pro­vide 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 (FC) pin or an open­collector 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 microsec­onds 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 shut­down. 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.

Features

• +2.7V to +18V operation

• Low current
<1µA typical shutdown mode (MIC1557)
200µA 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

• <15Ω 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

Typical Applications

+5V

MIC1555

13

VS OUT

Standby

Trigger

October 1998 1 MIC1555/1557

4

TRG
GND2THR

Monostable (One-Shot)

100µs

Output

1k

5

0.1µF

Astable (Oscillator)

MIC1555/1557 Micrel

Pin Configuration

VS

GND GND

13

22

CS

T10

45

THRTRG

T11

T/T

13

Part
Identification

OUTVS

Part Number Identification

MIC1555BM5 T10

OUT

45

MIC1557BM5 T11

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.

>2

3 CS Chip Select/Reset (Input): Active high at

<1⁄3VS.
3 5 OUT Output: CMOS totem-pole output.
4 TRG Trigger (Input): Sets output high. Active low (at ≤ 2⁄3VS nominal)
5 THR Threshold (Dominant Input): Sets output low. Active high (at ≥ 2⁄3V

nominal).

1 T/T Trigger/Threshold (Input): Internally connected to both threshold and trigger

functions. See TRG and THR.

⁄3VS. Output off when low at

.

S

MIC1555/1557 2 October 1998

MIC1555/1557 Micrel

Absolute Maximum Ratings

Supply Voltage (VS) ....................................................+22V

Threshold Voltage (V
Trigger Voltage (V

Lead Temperature (soldering 10sec.) ....................... 300°C

TRG

, V

THR

, V

) ..................................+22V

T/T

) .......................................+22V

T/T

Operating Ratings

Supply Voltage (VS) ..................................... +2.7V to +18V

Ambient Temperature Range (TA) ............. –40°C to +85°C

Package Thermal Resistance

(θJA)...................................................................220°C/W

(θJC) ..................................................................130°C/W

Electrical Characteristics

TA = 25°C, bold values indicate –40°C ≤ TA ≤ +85°C; unless noted.
Parameter Condition Min Typ Max Units

Supply current VS = 5V 240 300 µA

VS = 15V 350 400 µA

Monostable Timing Accuracy RA = 10k, C =0.1µF, VS = 5V 2 %

RA = 10k, C =0.1µF, VS = 5V 858 1161 µs

Monostable Drift over Temp VS = 5V, –55 ≤ TA ≤ +125°C, Note 1 100 ppm/°C

VS =10V, –55 ≤ TA ≤ +125°C, Note 1 150 ppm/°C

VS = 15V, –55 ≤ TA ≤ +125°C, Note 1 200 ppm/°C
Monostable Drift over Supply VS = 5V to 15V, Note 1 0.5 %/V
Astable Timing Accuracy RA = RB = 10k, C = 0.1µF, VS = 5V 2 %

RA = RB = 10k, C = 0.1µF, VS = 5V 1717 2323 µs
Maximum Astable Frequency RT = 1k, CT = 47pF, Vs = 8V 5 MHz
Astable Drift over Temp VS = 5V, –55 ≤ TA ≤ +125°C, Note 1 100 ppm/°C

VS =10V, –55 ≤ TA ≤ +125°C, Note 1 150 ppm/°C

VS = 15V, –55 ≤ TA ≤ +125°C, Note 1 200 ppm/°C
Astable Drift over Supply VS = 5V to 15V, Note 1 0.5 %/V
Threshold Voltage VS = 15V 61 67 72 %V
Trigger Voltage VS = 15V 27 32 37 %V
Trigger Current VS = 15V 50 nA
Threshold Current VS = 15V 50 nA
Chip Select on > 2/3V

off < 1/3V
Output Voltage Drop VS = 15V, I

VS = 5V, I
Output Voltage Drop VS = 15V, I

VS = 5V, I
Supply Voltage functional operation, Note 1 2.7 18 V
Output Rise Time RL = 10M, CL = 10pF, VS = 5V, Note 1 15 ns
Output Fall Time RL = 10M, CL = 10pF, VS = 5V, Note 1 15 ns

S
S

= 20mA 0.3 1.25 V

SINK

= 3.2mA 0.08 0.5 V

SINK

SOURCE

SOURCE

= 20mA 14.1 14.7 V

= 3.2mA 3.8 4.7 V

50 67 72 %V
28 33 50 %V

S
S

S
S

General Note: Devices are ESD protected, however handling precautions are recommended.
Note 1: Not tested.

October 1998 3 MIC1555/1557

MIC1555/1557 Micrel

Typical Characteristics (T

Astable Frequency

100

10

1

100k

0.1

0.01

CAPACITANCE (µF)

0.001

0.0001

1M

-1

1x1001x1011x1021x1031x1041x1051x1061x10

1x10-21x10

FREQUENCY (Hz)

On Resistance

vs. Temperature

70
60
50
40
30
20

ON RESISTANCE (Ω)

10

0

-40 -20 0 20 40 60 80 100

TEMPERATURE (°C)

RT=1k

10k

= 25°C, VIN = +5)

A

7

Pulse Width

RT=1k

10k

-1

1x1001x1011x1021x1031x1041x1051x1061x10

1x10

PERIOD (µs)

CAPACITANCE (µF)

0.001

0.0001

100

10

1

0.1

0.01

Supply Current

vs. Temperature

-40 -20 0 20 40 60 80 100

TEMPERATURE (°C)

SUPPLY CURRENT (µA)

300

280

260

240

220

200

1M

100k

On Resistance

vs. Supply Voltage

70
60
50
40
30
20

ON RESISTANCE (Ω)

10

7

0

0 5 10 15 20

SUPPLY VOLTAGE (V)

Supply Current

vs. Supply Voltage

500

400

300

200

100

SUPPLY CURRENT (µA)

0

0 5 10 15 20

SUPPLY VOLTAGE (V)

k Factors

2.0

1.9

1.8

1.7

1.6

1.5

1.4

k FACTOR

1.3

1.2

1.1

1.0
10 100 1000

Times RC

t=k2 RC

RC (µs)

f=1⁄k

1

MIC1557 Chip Select

vs. Supply Voltage

14
12
10

8
6
4
2

CHIP SELECT VOLTAGE (V)

0

ON

OFF

3 6 9 12 15 18

SUPPLY VOLTAGE (V)

RC

MIC1555/1557 4 October 1998

MIC1555/1557 Micrel

S

R

Q

T/T

OUT

VS

Bias

GND

CS

V

SUPPLY

MIC1557

Output

1k

0.1µF

8kHz

1M

Functional Diagrams

V

SUPPLY

VS

MIC1555

Bias

THR

100µs

OUT

1k

Standby

Trigger

<100µs

S

Q

R

TRG

GND

MIC1555 Block Diagram with External Components

(Monostable Configuration)

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) con­nection 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 1⁄3VS and 2⁄3VS 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 1µA. 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 (2⁄3VS) is reached, the
flip-flop is set, making the output low. THR is dominant over
TRG.

Output

0.1µF

MIC1557 Block Diagram with External Components

(Astable Configuration)

Trigger Comparator

The trigger comparator is connected to R (reset) on the RS
flip-flop. When TRG (trigger) goes below the trigger voltage
(1⁄3VS), 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 VS.

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

(threshold voltage) reaches 2⁄3

THR

VS, 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

reaches 2⁄3VS (threshold volt-

T/T

age), the output is switched off, slowly discharging the
capacitor. When V
output is switched on, causing V

decreases to 1⁄3VS (trigger voltage), the

T/T

to rise again, repeating

T/T

the cycle.

October 1998 5 MIC1555/1557

MIC1555/1557 Micrel

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 (CT) is charged through a resistor (RT)
connected to the output of an MIC1555, the trip point is
approximately 1.1RCT (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 oscilla­tions 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 (DT)
in parallel with (RT) resets the one-shot quickly.

2.7V to 18V

R

T

D

T

R

PU

THR

5

C

Trigger

1

⁄3V

IN

IN

TRG

4

C

T

MIC1555

Figure 1. One-Shot Diagram

The period of a monostable circuit is:

t = k2 RC

where:

t = period (s)
k2 = constant [from Typical Characteristics graph]
R = resistance (Ω)
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 compara­tor inputs, threshold and trigger (T/T), tied together internally.
Chip select (CS) is brought out to allow on-off control of the
oscillator.

VS

GND

CS

OUT

1

tON = 1.1RTC

OUT

3

2

T

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 (RT) is connected from the output to a
grounded timing capacitor, (CT) 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 propa­gation 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
(RA), and a second resistor (RB) in series with an opposite
facing diode connected in parallel. The frequency is then
made up of two components, the charging time (tA) and the
discharging time (tB) tA= 0.7RAC and tB= 0.7RBC. The
frequency is the reciprocal of the sum of the two times tA + tB,
so the total time is 1.4RTCT. The first half-cycle of an astable,
after power-on or CS enable, is lengthened since the capaci­tor is charging from ground instead of the 1⁄3 input trigger trip
voltage, to 1.1RC, the same as a monostable pulse.

2.7V to 18V

C

B

4

VS

R

E

T/T

1

MIC1557

1k to 1M

C

T

100pF to 10µF

GND

R

A

R

B

CS

OUT

CS

3

OUT

5

2

t

t = 0.7(RA+RB)C

T

Figure 2. Oscillator Diagram

The MIC1555 or MIC1557 can be used to construct an
oscillator.

The frequency of an astable oscillator is:

1

f

=

k RC

1

where:

f = frequency (Hz)
k1 = constant [from Typical Characteristics graph]
R = resistance (Ω)
C = capacitance (F)

MIC1555/1557 6 October 1998

MIC1555/1557 Micrel

THR

OUT

MIC1555

1

3

GND

2

VS

5

TRG

4

+5V +12V

R

L

OFF ≥ 3.3V

ON ≤ 1.6V

To use the MIC1555 as an oscillator, connect TRG to THR.

+5V

1

4

2

MIC1555

VS
TRG
GND

OUT

THR

8kHz

3

5

Output

1k

0.1µF

Figure 3. MIC1555 Oscillator Configuration

The MIC1557 features a CS input. When logic-low, CS
places the MIC1557 into a <1µA shutdown state. If unused,
the MIC1557 CS input on must be pulled up.

+5V

1M

4

3

2

MIC1557

VS
CS
GND

OUT

T/T

8kHz

5

Output

1k

1

0.1µF

Figure 4. MIC1557 Oscillator Configuration

Falling-Edge Triggered Monostable Circuit

The MIC1555 may be triggered by an ac-coupled falling­edge, 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.

Input

+5V

1M

MIC1555

13

VS OUT

4

TRG
GND2THR

100µs

Output

1N41481k

5

0.1µF

Accuracy

The two comparators in the MIC1555/7 use a resistor voltage
divider to set the threshold and trigger trip points to approxi­mately 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 1MΩ or capacitors larger than
10µF are not recommended due to leakage current inaccura­cies. Time delays greater than 10 seconds are more accu­rately 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

, 214) using a CD4020 CMOS

hex

divider. 4.5Hz may be generated with a 1µF CT and approxi­mately 156kΩ.

Inverting Schmitt Trigger

Refer to figure 7. The trip points of the MIC1555/7 are defined
as 1/3 and 2/3VS, which allows either device to be used as a
signal conditioning inverter, with hysteresis. A slowly chang­ing 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.

Figure 5. Falling-Edge Trigger Configuration

Rising-Edge Triggered Monostable Circuit

The MIC1555 may be triggered by an ac-coupled rising­edge, as shown in figure 6. The pulse begins when the ac­coupled input rises, and a diode from the output holds the
THR input low until TRG discharges to 1⁄3VS. This circuit
provides a low-going output pulse.

+5V

MIC1555

13

VS OUT

4

TRG
GND2THR

5

Figure 6. Rising-Edge Trigger Configuration

October 1998 7 MIC1555/1557

0.1µF

1N41481k

100µs

Output

Input

Figure 7. Schmitt trigger

Charge-Pump Low-Side MOSFET Drivers

A standard MOSFET requires approximately 15V to fully
enhance the gate for minimum R

. Substituting a logic-

DS(on)

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

MIC1555/1557 Micrel

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.

+12V

C1

100pF

to 10µF

TTL High = ON

+5V

MIC1557

43

VS

1

T/T

2

GND

RT 1k to 1M

CS

OUT

5

R

E

100k

N-Channel
MOSFET
(IRF540)

15V

R

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

L

low on the probe.

+5V to +18VRT 10k

1N914

10k

47k

C

T

0.01µF

5

4

2

MIC1555

THR
TRG
GND

VS

OUT

1

3

0.001µF
10k

Figure 9. Audible Voltmeter

100Ω

Package Information

1.90 (0.075) REF

0.95 (0.037) REF

3.02 (0.119)

2.80 (0.110)

0.50 (0.020)

0.35 (0.014)

1.75 (0.069)

1.50 (0.059)

1.30 (0.051)

0.90 (0.035)

0.15 (0.006)

0.00 (0.000)

SOT-23-5 (M5)

3.00 (0.118)

2.60 (0.102)

10°

0°

DIMENSIONS:

MM (INCH)

0.20 (0.008)

0.09 (0.004)

0.60 (0.024)

0.10 (0.004)

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© 1998 Micrel Incorporated

MIC1555/1557 8 October 1998