Supply Voltage Supervisor:
After power-on a digital system must normally be forced into definite initial state. For the microcomputers and
microprocessors a reset input is provided which in simple applications, an R/C network is connected. After poweron,
this circuitry maintains the logic level at this input high (or low), until the supply voltage has reached its nominal
value & internal logic of the microcomputer has executed the initialization of the system.
In the larger computers several features are provided to prevent such errors: a power-fail interrupt signals dangerous
conditions in time, the content of the memory is protected by battery back-up . In small microcomputer
systems this amount of effort is so much expensive, and in most applications not needed . It is usually sufficient if,
after a serious voltage drop, the microcomputer is forced into defined initial conditions.For implementation of this function,
whilst preventing the problems Which are mentioned above, following circuit features are required:
Accurate detection of the serious voltage drop.
Generation of reset signal while as supply voltage is not in the operational
range to prevent the undefined operations of the microcomputer.
Maintenance of reset signal for certain time after supply voltage has
returned to the nominal value, to ensure proper initialization of the circuit.
For these applications, Texas Instruments has developed a series of integrated circuits with a minimum of the
external components and without the additional adjustment, will fulfill requirements described above.
Design Tips :
The application of the supply voltage supervisors of the series TL77xx is not complicated. However it would be
noticed, that this circuit is an analog circuit, whose function and performance – e.g. the stability of the reference
voltage – may be negatively influenced by noise in the neighboring circuits. Therefore the voltage monitor should be
placed on PCB , where there are no neighboring circuits in the which switch high currents like bus
interface circuits and power switches . When laying out the layout of PCB special care should
taken with the interconnects which carry analog signals. Beside SENSE the input these are the Ct and Vref terminals.
Noise coupled into C t input will lead to a reduction of the output pulse width. Noise coupled into the Vref input or
into the filter capacitor at this input may lead to undesired triggering of the circuit and by this to an undesired RESET
pulse. Practice shows, that this malfunction when high currents flow over the interconnects of these capacitors to the
GND terminal of the voltage monitor. To avoid these effects, Ground terminals of these capacitors must be
connected by the shortest way to ground terminal of the voltage monitor in so that no currents caused by other
circuits flow over these wires. layout proposal for pcb . Furthermore the
resistors of the voltage divider at the SENSE input of the TL7702 have to be placed in so that
it dont have noise can be coupled into this circuit.
5 versions of this circuit are available:
TL7705A, TL7705B (Vt = 4.55 V): Application in TTL-systems and microcomputer
systems which require a 5 volt supply (e.g. TMS7000)
TL7709A (Vt = 7.6 V): Application in microcomputer systems using the
TL7712A (Vt = 10.8 V): Application in the CMOS, microprocessor, and memory
circuits with a 12 volt supply.
TL7715A (Vt = 13.5 V): Application in circuits which operates with supply voltage
of 15 V as is found often in the analog circuits.
TL7702A, TL7702B (Vt = 2.5 V): Application in systems where other supply
voltages are used. The required trigger level my be adjusted with an external
resistor divider at the SENSE input.
The main part of this circuit is reference voltage source which consists of toostable, temperature compensated
band gap reference. An external capacitor (typ. 0.1 F) must be connected to voltage output V ref to reduce the
influence of fast transients in the supply voltage. The voltage at SENSE input is divided by the resistor divider and
compared with the reference voltage by the comparator. To achieve high accuracy the divider is adjusted at wafer
probe. When the input voltage is sensed to be lower than threshold voltage, Thyristor is triggered, which
discharges timing capacitor C t. It is also possible to fire the thyristor via the RESIN input by a logic level (TTL
level, active low). The thyristor is turned off again when either the voltage at the SENSE input (or RESIN input)
increases beyond the threshold, or – during short supply voltage drops – the discharge current of capacitor
becomes lower than hold current of thyristor.
Q1: What are the APPLICATIONS of Supply Voltage Supervisor ?
Ans:• All DSP and Microcontroller Applications
• All FPGA/ASIC Applications
• Telecom/Wireless Infrastructure
• Industrial Equipment
• Analog Sequencing
Q2: What is the value of Adjustable Threshold?
Ans: Adjustable Threshold Down to 0.4V.
Q3: What is the value of Adjustable Delay Time range?
Ans: 1.4ms to 10s.
Q4: What is it’s Threshold Accuracy?
Ans: 0.25% .
Q5:What is the Input voltage range ?
Ans : Range of V CC –0.3 to 7.0 V.