USING THE TELCOM TC54
VOLTAGE DETECTOR
APPLICATION NOTE 2
USING THE TELCOM TC54
VOLTAGE DETECTOR
AN-2
OPERATION
The TC54VN equivalent circuit is shown in Figure 1.
The internal precision temperature-compensated reference
and comparator circuit consistently monitor V
IN
for an under-
voltage condition. When such a condition occurs, the output
is forced to the active (LOW) state and remains there until V
IN
is once again within tolerance. As shown in the timing
diagram of Figure 1, the output is forced active when V
IN
<
(–V
DET
) (Where (–V
DET
) is the threshold voltage) and returns
HIGH when V
IN
exceeds (–V
DET
) plus 5% hysteresis (ex-
ample: if (–V
DET
) = 2.0V, the output will go HIGH when V
IN
>
2.1V).
INTRODUCTION
TelCom's TC54 series of voltage detectors are de-
signed to supersede a variety of discrete comparator circuits
and bipolar technology voltage detectors.
These voltage detectors are especially suited for appli-
cation in battery powered systems because of their ex-
tremely low 1µA operating current. Each part is laser
programmed to the desired trip point voltage over the range
of 0.8V to 6.0V in 100mV steps. For complete design
flexibility, complementary or open-drain output versions are
available. TC54 package options include surface mount
SOT-23 and SOT-89 or through-hole TO-92 types.
TC54 BLOCK DIAGRAM AND TIMING
TC54VC only
2
V
IN
–
+
INPUT VOLTAGE
HYSTERESIS
VOLTAGE
DETECT VOLTAGE (- V
DET
)
MINIMUM OPERATING
VOLTAGE
GROUND LEVEL
RELEASE
VOLTAGE (+ V
DET
)
V
OUT
1
OUTPUT VOLTAGE
V
REF
V
SS
3
Figure 1.
GROUND LEVEL
TC54 AS A MICROPROCESSOR RESET
GENERATOR
Following a power outage, microprocessors require a
reset to get them back into their starting state. Commonly
used simple Schmitt trigger/capacitor circuits are inad-
equate since they do not accurately monitor the power
supply voltage tolerance. The TC54 series overcomes this
problem with precise voltage detection. Figure 2 shows a
reset circuit using a standard TC54 and external RC time
constant, replacing bipolar integrated voltage detectors
(see Table 1).
V
IN
V
IN
V
OUT
TC54VN
C
V
SS
R
Microprocessor
Circuit
Figure 2. Microprocessor Reset Circuit using TC54
and External Delay
AN2-2 8/22/97
1
USING THE TELCOM TC54
VOLTAGE DETECTOR
AN-2
Table 1. TC54 Cross Reference Listing
MFGR
Motorola
Motorola
Motorola
Motorola
Motorola
Motorola
T.I.
T.I.
PART#
MC33064P-5
MC34064P-5
MC34164P-3
MC34164P-5
MC33164P-3
MC33164P-5
TL7757ILP
TL7757CLP
TELCOM #
TC54VN4502EZB
TC54VN4502EZB
TC54VN2702EZB
TC54VN4302EZB
TC54VN2702EZB
TC54VN4302EZB
TC54VN4502EZB
TC54VN4502EZB
DESCRIPTION
4.5V trip, -40 to +85C, TO-92
4.5V trip, 0 to +70C, TO-92
2.7V trip, 0 to +70C, TO-92
4.3V trip, 0 to +70C, TO-92
2.7V trip, -40 to +85C, TO-92
4.3V trip, -40 to +85C, TO-92
4.5V trip, -40 to +85C, TO-92
4.5V trip, 0 to +70C, TO-92
TELCOM
ATTRIBUTE
500 times less power
500 times less power
10 times less power
10 times less power
10 times less power
10 times less power
2000 times less power
2000 times less power
MODIFYING THE TRIP POINT, –V
DET
Although the TC54 has a pre-programmed V
DET
, it is
sometimes necessary to make adjustments during
prototyping. This can be accomplished by connecting an
external resistor divider to a TC54 which has a V
DET
lower
than that of V
SOURCE
(Figure 3).
To maintain detector accuracy, the bleeder current
through the divider should be significantly higher than the
1µA operating current required by the TC54. A reasonable
value for this bleeder current is 100µA (100 times the 1µA
required by the TC54). For example, if (-V
DET
) = 2V and the
desired trip point is 2.5V, the value of R1 + R2 is 250K Ohms
(2.5V/100µA). The value of R1 + R2 can be rounded to the
nearest standard value and plugged into the equation of
Figure 3 to calculate values for R1 and R2. 1% tolerance
resistors are recommended.
OTHER APPLICATIONS
V
SOURCE
V
SOURCE
R2
V
IN
TC54
R1
V
SS
V
OUT
(
R1R1R2
)
= (–V
+
DET
)
Where:
V
SOURCE
= Voltage to be monitored
(-V
DET
) = Threshold Voltage Setting of TC54
Note:
In this example, V
SOURCE
must be
greater than (–V
DET
)
Figure 3. Modify trip point of TC54 using external resistor divider
AN2-2 8/22/97
2
USING THE TELCOM TC54
VOLTAGE DETECTOR
AN-2
4.3V
270Ω
V
CC
RL
V
IN
Pwr
Sply
V
OUT
TC54Vx
V
SS
VIN
V
OUT
TC54VN
V
SS
POWER GOOD
MTP3055EL
Figure 4. MOSFET low drive protection
Figure 6. Power good monitor
V
IN
TC54Vx
V
SS
V
OUT
BATLOW
Low operating power and small physical size make the
TC54 series ideal for many voltage detector applications,
such as those shown below. Figure 4 shows a low-voltage
gate drive protection circuit, which prevents overheating of
the logic-level MOSFET due to insufficient gate voltage.
When the input signal is below the threshold of the TC54VN,
its output grounds the gate of the MOSFET. Figure 5 and 6
show the TC54 in conventional voltage monitoring applica-
tions.
Figure 5. Battery voltage monitor
Sales Offices
TelCom Semiconductor
1300 Terra Bella Avenue
P.O. Box 7267
Mountain View, CA 94039-7267
TEL: 650-968-9241
FAX: 650-967-1590
E-Mail: liter@c2smtp.telcom-semi.com
AN2-2 8/22/97
TelCom Semiconductor
Austin Product Center
9101 Burnet Rd. Suite 214
Austin, TX 78758
TEL: 512-873-7100
FAX: 512-873-8236
TelCom Semiconductor H.K. Ltd.
10 Sam Chuk Street, Ground Floor
San Po Kong, Kowloon
Hong Kong
TEL: 852-2324-0122
FAX: 852-2354-9957
3
Printed in the U.S.A.
Domestic Chip Exchange
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