DEVICE DESCRIPTION
These devices are precision timing circuits for
generation of accurate time delays or
oscillation. Advanced circuit design means that
these devices can operate from a single battery
cell with the minimum of quiescent current.
In monostable mode time delays are
controlled by a single resistor and capacitor
network. In astable mode the frequency and
d u t y c y c l e c a n b e a c c u r a t e l y a n d
independently controlled with two external
resistors and one capacitor.
The threshold and trigger levels are normally
set as a proportion of V
CC
by internal resistors.
These levels can be programmed by the use
of the control input pin.
When the trigger input reduces to a value
below the trigger level, the flip-flop is set and
the output goes high. With the trigger input
above the trigger level and the threshold input
above the threshold level, the flip-flop is reset
and the output goes low. The reset pin has
priority over all the other inputs and is used
to start new timing cycles. A low on the reset
input causes the flip-flop to reset forcing the
output low. Whenever the output is forced
low then the internal discharge transistor is
turned on.
FEATURES
·
0.9V supply operating voltage guaranteed
·
Pin connections comparable with 555
series timers
·
Very low quiescent current 74
µ
A
·
SO8 and DIL8 packages
·
Operating temperature range
compatible with battery technologies
APPLICATIONS
·
Portable and battery powered
equipment
·
Low voltage and low power systems
PRECISION SINGLE CELL TIMER
ISSUE 2 - MAY 1998
ZSCT1555
7
DISCHARGE
6
THRESH
2
TRIGGER
5
CONTROL
8
VCC
4
RESET
3
OUTPUT
1
GND
SCHEMATIC DIAGRAM
4-311
ABSOLUTE MAXIMUM RATINGS
Supply Voltage
9V
Input Voltages
9V
(Cont, Reset, Thres, Trig)
Output Current
100mA
Operating Temperature
-20 to 100°C
Storage Temperature
-55 to 150°C
Power Dissipation (T
amb
=25°C)
DIL8
625mW
SO8
625mW
Recommended Operating Conditions
Supply Voltage 0.9V(min) 6V(max)
Input Voltages
6V(max)
(Cont, Reset, Thres, Trig)
Output
Current Sink
100mA(max)
Source
150
µ
A(max)
ELECTRICAL CHARACTERISTICS
TEST CONDITIONS (Unless otherwise stated):T
amb
=
25°C,V
CC
=
1.5V
SYMBOL PARAMETER
CONDITIONS
LIMITS
UNITS
MIN.
TYP.
MAX.
V
CC
Supply Voltage
0.9
6
V
I
CC
Supply Current
no load
V
CC
=
5V, no load
74
150
120
200
mA
V
TH
Threshold Voltage
V
CC
= 5V
1.195
3.9
1.22
4
1.245
4.1
V
I
TH
Threshold Current (Note 1)
0
20
100
nA
V
TR
Trigger Voltage
V
CC
= 5V
0.2
0.57
0.25
0.62
0.3
0.67
V
I
TR
Trigger Current
0
-35
-100
nA
t
PD
Trigger Propagation delay
Delay from trigger
to output
2
µ
s
V
RS
Reset Voltage
0.1
0.2
0.4
V
I
RS
Reset Current
Reset @ 0V
0
-5
-10
µ
A
I
DS
Discharge switch Off-state
current
0
10
100
nA
V
DS
Discharge switch On-state
voltage
I
DS
= 0.2mA
V
CC
= 5V, I
DS
= 0.3mA
0
0
180
240
225
350
mV
V
CT
Control Voltage (Open Circuit)
V
CC
= 5V
1.195
3.9
1.22
4
1.245
4.1
V
V
OL
Output Voltage (Low)
I
OL
=10mA
I
OL
=50mA
V
CC
=5V, I
OL
=10mA
V
CC
=5V, I
OL
=100mA
0
0
0
0
0.15
0.45
0.13
0.65
0.3
0.65
0.3
1
V
V
OH
Output Voltage (High)
I
OH
= 100
µ
A
V
CC
= 5V, I
OH
= 150
µ
A
1
4.5
1.1
4.6
1.5
5
V
ZSCT1555
4-312
SYMBOL PARAMETER
CONDITIONS
LIMITS
UNITS
MIN.
TYP.
MAX.
t
R
Output pulse rise time
C
L
= 10pF
V
CC
=5V, C
L
=10pF
1.6
1.2
µ
s
t
F
Output pulse fall time
C
L
= 10pF
V
CC
=5V, C
L
=10pF
240
24
ns
t
IA
(m)
t
V
(m)
t
T
(m)
Timing error, Monostable
Initial accuracy (Note 2)
Drift with supply voltage
Drift with temperature
RA= 10 to 50 k
RB= 10 to 50 k
C
T
= 68nF
1.6
0.262
100
%
%/V
ppm/°C
t
IA
(a)
t
V
(a)
t
T
(a)
Timing error, Astable
Initial accuracy (Note 2)
Drift with supply voltage
Drift with temperature
RA= 10 to 50 k
RB= 10 to 50 k
C
T
= 68nF
4.8
0.662
150
%
%/V
ppm/°C
f
A
Astable maximum frequency
RA=20 k
RB= 10 k
C
T
=47pF
330
kHz
Note 1: This will influence the maximum values of RA and RB (RA
MAX
=10M
,RB
MAX
=1.5M
)
Note 2: Is defined as the difference between the measured value and the average value of a
random sample taken on a batch basis
ZSCT1555
ELECTRICAL CHARACTERISTICS (Continued)
TEST CONDITIONS (Unless otherwise stated):T
amb
=25°C,V
CC
=1.5V
4-313
ZSCT1555
0
1
2
3
4
5
0.80
0.85
0.90
0.95
1.00
1.05
P
u
l
s
e
D
ura
t
i
o
n
r
e
l
a
t
i
v
e
t
o
Vcc=5v
Supply Voltage (V)
Normalized Output Pulse Duration
v Supply Voltage
6
-20°C
+25°C
+100°C
200
160
120
80
40
0
1.0
2.0
3.0
4.0
5.0
0.0
Supply Curr
ent
(µA
)
Supply Voltage (V)
Supply Current v Supply Voltage
8
6
4
2
0
0
0.2
Prop
a
g
a
t
ion Del
a
y
(
µs)
Lowest Voltage Level of Trigger Pulse (xVcc)
Output Propagation Delay
Vcc=5v
0.1
+100°C
+25°C
-20°C
Rout/Vcc=1K
5
4
3
2
1
0
0
0.1
0.2
Prop
a
g
a
t
i
o
n Del
a
y
(
µs)
Lowest Voltage Level of Trigger Pulse (xVcc)
Output Propagation Delay
Vcc=1.5v
+25°C
-20°C
Rout/Vcc=1K
5
4
3
2
1
0
0
0.1
+100°C
+25°C
-20°C
Minimum
P
ulse Widt
h
(µ
s)
Lowest Voltage Level of Trigger Pulse (xVcc)
Minimum Pulse Width Required for
Triggering
Vcc=1.5v
Rout/Vcc = 1K
5
4
3
2
1
0
0
0.2
+100°C
+25°C
-20°C
Minimum
P
ulse
W
i
d
th
(
µs)
Lowest Voltage Level of Trigger Pulse (xVcc)
Minimum Pulse Width Required for
Triggering
Vcc=5v
0.1
Rout/Vcc = 1K
+100°C
TYPICAL CHARACTERISTICS
4-314
ZSCT1555
Vcc=5v
-20°C
+25°C
+100°C
1
0.1
0.01
0.001
Sink Current (mA)
Discharge Transistor Voltage
v Sink Current
10
1
0.1
0.01
Dis
charge T
ransis
t
or V
o
l
t
age (
V
)
Vcc=1.5v
-20°C
+25°C
+100°C
1
0.1
0.01
0.001
Sink Current (mA)
Discharge Transistor Voltage
v Sink Current
10
1
0.1
0.001
Dischar
ge T
r
an
s
i
s
t
or
V
o
l
t
age (V
)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.01
0.1
1
10
100
Low Level Output Current (mA)
Output Low Voltage Drop v
Output Current
Low
Level O
u
t
put
V
o
l
t
age (
V
)
Vcc=1.5v
-20°C
+25°C
+100°C
1.0
0.8
0.6
0.4
0.2
0.0
0.01
0.1
1
10
100
Low Level Output Current (mA)
Output Low Voltage Drop v
Output Current
Low
Level O
u
t
put
V
o
l
t
age (
V
)
-20°C
+25°C
+100°C
Vcc=5v
0.5
0.4
0.3
0.2
0.1
0.0
-20°C
+25°C
+100°C
Vcc=5v
V
cc -
V
o
u
t
(V
)
0.001
0.01
0.1
1.0
High Level Output Current (mA)
Output High Voltage Drop v
Output Current
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.1
0.01
0.001
-20°C
+25°C
+100°C
Vcc=1.5v
High-Level Output Current (mA)
Output High Voltage Drop v
Output Current
V
cc
- V
o
u
t
(
V
)
TYPICAL CHARACTERISTICS
4-315
FUNCTIONAL DIAGRAM
FUNCTIONAL TABLE
RESET
TRIGGER VALUE THRESHOLD
VOLTAGE
OUTPUT
DISCHARGE
SWITCH
Low
N/A
N/A
Low
On
High
<V
CC
/5
N/A
High
Off
High
>V
CC
/5
>4V
CC
/5
Low
On
High
>V
CC
/5
<4V
CC
/5
As Previously established
POWER DERATING TABLE
Package
TA
25°C
Power Rating
Derating Factor
Above TA=25°C
T
A
=70°C Power
Rating
T
A
=85°C Power
Rating
N8
625mW
6.25mW/°C
330mW
250mW
D8
625mW
6.25mW/°C
330mW
250mW
ZSCT1555
4-316
APPLICATIONS INFORMATION
Many configurations of the ZSCT1555 are
possible. The following gives a selection of a
few of these using the most basic monostable
and astable connections. The final application
example in astable mode shows the device
optimum use for low voltage and power
economy in a single cell boost converter.
Monostable Operation
Figure 1 shows connection of the timer as a
one-shot whose pulse period is independent
of supply voltage. Initially the capacitor is held
discharged. The application of a negative
going trigger pulse sets an internal flip flop
which allows the capacitor to start to charge
up via RA and forces the output high. The
voltage on the capacitor increases for time t,
where t = 1.63RAC
T
, at the end of this period
the voltage on the capacitor is 0.8 V
CC
. At this
point the flip flop resets, the capacitor is
discharged and the output is driven low.
Figure 2 shows the timing diagram for this
function. During the output high period
further trigger pulses are locked out however
the circuit can be reset by application of a
negative going pulse on the reset pin. Once
the output is driven low it remains in this state
until the application of the next trigger pulse.
If the reset function is not used then it is
recommended to connect to V
CC
to eliminate
any possibility of false triggering.
Figure 3 gives an easy selection of RA and C
T
values for various time delays.
This configuration of circuit can be used as a
frequency divider by adjusting the timing
period. Figure 4 indicates a divide by three.
Figure 1
Figure 2
Figure 3
100
10
1
0.1
0.01
0.001
10us
100us
1ms
10ms 100ms 1s
10s
C
-
C
a
p
acit
ance
(
uF)
Time Delay
100k
1M
10M
R
A
Figure 4
ZSCT1555
4-317
Figure 5 shows the monostable mode used as
a pulse width modulator. Here the trigger pin
is supplied with a continuous pulse train, the
resulting output pulse width is modulated by
a signal applied to the control pin.
Figure 6 shows typical waveform examples.
Astable operation
The configuration of Figure 7 produces a free
running multivibrator circuit whose frequency
is independent of supply voltage. The ratio of
resistors RA and RB precisely sets the circuit
duty cycle. The capacitor is charged and
discharged between thresholds at 0.2V
CC
and
0.8V
CC
. Oscillation frequency (f) and duty cycle
(d) can be calculated using the following
equations:-
f = 0.62/(RA + 2RB)C
T
d = RB /(RA + 2RB)
Figure 8 shows the waveforms generated in
this mode of operation.
Figure 5
Figure 6
Figure 8
ZSCT1555
Figure 7
4-318
ZSCT1555
Figure 9 gives an easy selection for RA, RB and
C
T
values.
Similar to the PWM circuit of Figure 5 the astable
circuit can be configured with modulation of the
control input as shown in Figure 10. The result is a
pulse position modulated, PPM, circuit where the
pulse position is altered by the control input voltage.
Figure 11 shows the result of modulation with
a triangle wave input to the control pin.
Figure 11
(RA+2RB)
100
10
1
0.1
0.01
0.001
0.1
1
10
100
1k
10k
100k
C - Cap
ac
ita
n
c
e
(
u
F
)
Free Running Frequency (Hz)
10M
1M
100k
Figure 10
Figure 9
4-319
Fi
gu
re
12
ZSCT1555
The circuit of Figure 12 shows the device in
astable mode operating as part of a single
cell boost converter. This circuit generates
a 5 volt supply from a single battery cell. The
circuit output voltage is maintained down
to 0.9 volts input and power economy is
optimised for extended battery life.
4-320
ORDERING INFORMATION
Part Number
Package
Part Mark
ZSCT1555D8
DIL8
ZSCT1555
ZSCT1555N8
SO8
ZSCT1555
CONNECTION DIAGRAM
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