1
USING THE TELCOM TC54
VOLTAGE DETECTOR
AN-2
AN2-2 8/22/97
APPLICATION NOTE 2
USING THE TELCOM TC54
VOLTAGE DETECTOR
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).
INPUT VOLTAGE
DETECT VOLTAGE (- VDET)
MINIMUM OPERATING
VOLTAGE
GROUND LEVEL
HYSTERESIS
VOLTAGE
OUTPUT VOLTAGE
RELEASE
VOLTAGE (+ VDET)
GROUND LEVEL
Figure 1.
TC54 BLOCK DIAGRAM AND TIMING
Figure 2. Microprocessor Reset Circuit using TC54
and External Delay
TC54VN
V
IN
V
OUT
V
IN
V
SS
Microprocessor
Circuit
R
C
V
OUT
TC54VC only
V
SS
1
3
+
–
V
IN
2
V
REF
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.
OPERATION
The TC54VN equivalent circuit is shown in Figure 1.
The internal precision temperature-compensated reference
and comparator circuit consistently monitor VIN for an under-
voltage condition. When such a condition occurs, the output
is forced to the active (LOW) state and remains there until VIN
is once again within tolerance. As shown in the timing
diagram of Figure 1, the output is forced active when VIN <
(–VDET) (Where (–VDET) is the threshold voltage) and returns
HIGH when VIN exceeds (–VDET) plus 5% hysteresis (ex-
ample: if (–VDET) = 2.0V, the output will go HIGH when VIN >
2.1V).
2
USING THE TELCOM TC54
VOLTAGE DETECTOR
AN-2
AN2-2 8/22/97
( )
VSOURCE = (–VDET)
Where:
VSOURCE = Voltage to be monitored
(-VDET) = Threshold Voltage Setting of TC54
Note: In this example, VSOURCE must be
greater than (–VDET)
R1
R1 + R2
Table 1. TC54 Cross Reference Listing
TELCOM
MFGR PART# TELCOM # DESCRIPTION ATTRIBUTE
Motorola MC33064P-5 TC54VN4502EZB 4.5V trip, -40 to +85C, TO-92 500 times less power
Motorola MC34064P-5 TC54VN4502EZB 4.5V trip, 0 to +70C, TO-92 500 times less power
Motorola MC34164P-3 TC54VN2702EZB 2.7V trip, 0 to +70C, TO-92 10 times less power
Motorola MC34164P-5 TC54VN4302EZB 4.3V trip, 0 to +70C, TO-92 10 times less power
Motorola MC33164P-3 TC54VN2702EZB 2.7V trip, -40 to +85C, TO-92 10 times less power
Motorola MC33164P-5 TC54VN4302EZB 4.3V trip, -40 to +85C, TO-92 10 times less power
T.I. TL7757ILP TC54VN4502EZB 4.5V trip, -40 to +85C, TO-92 2000 times less power
T.I. TL7757CLP TC54VN4502EZB 4.5V trip, 0 to +70C, TO-92 2000 times less power
MODIFYING THE TRIP POINT, –VDET
Although the TC54 has a pre-programmed VDET, 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 VDET lower
than that of VSOURCE (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 (-VDET) = 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
Figure 3. Modify trip point of TC54 using external resistor divider
TC54
VIN VOUT
VSS
R1
VSOURCE
R2
3
USING THE TELCOM TC54
VOLTAGE DETECTOR
AN-2
AN2-2 8/22/97
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 4. MOSFET low drive protection
Figure 5. Battery voltage monitor
Figure 6. Power good monitor
VIN
V
CC
V
OUT
V
SS
4.3V 270Ω
RL
MTP3055EL
TC54VN
V
IN
V
OUT
V
SS
BATLOW
TC54Vx
V
IN
V
OUT
V
SS
Pwr
Sply POWER GOOD
TC54Vx
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
Printed in the U.S.A.
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
TelCom Semiconductor
Austin Product Center
9101 Burnet Rd. Suite 214
Austin, TX 78758
TEL: 512-873-7100
FAX: 512-873-8236
