TL7702 [TI]

Supply voltage Supervisor; 电源电压监控器


型号：	TL7702
厂家：	TEXAS INSTRUMENTS
描述：	Supply voltage Supervisor 电源电压监控器监控
文件：	总17页 (文件大小：343K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

Supply voltage Supervisor

TL77xx Series

Author: Eilhard Haseloff

Literature Number: SLVAE04

March 1997

IMPORTANT NOTICE

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orders, that the information being relied on is current.

TI warrants performance of its semiconductor products and related software to the

specifications applicable at the time of sale in accordance with TI’s standard warranty.

Testing and other quality control techniques are utilized to the extent TI deems

necessary to support this warranty. Specific testing of all parameters of each device is

not necessarily performed, except those mandated by government requirements.

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TI SEMICONDUCTOR PRODUCTS ARE NOT DESIGNED, INTENDED,

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Inclusion of TI products in such applications is understood to be fully at the risk of the

customer. Use of TI products in such applications requires the written approval of an

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In order to minimize risks associated with the customer’s applications, adequate design

and operating safeguards should be provided by the customer to minimize inherent or

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or relating to any combination, machine, or process in which such semiconductor

products or services might be or are used.

Contents

1. Introduction............................................................................................................................1

2. Circuit Description..................................................................................................................2

3. Application examples.............................................................................................................5

4. Design Tips..........................................................................................................................12

5. Summary .............................................................................................................................14

List of Figures

1 Reset circuit.................................................................................................................................1

2 Functional Diagram......................................................................................................................3

3 Diagram for Calculation of C_t.......................................................................................................3

4 Timing diagram............................................................................................................................4

5 TL7705A in 5 Volt Microcomputer Applications...........................................................................6

6 Voltage Supervision of a multiple power supply ..........................................................................6

7 Typical Trigger Sensitivity at the SENSE input of the voltage monitor TL7705A.........................7

8 Circuit for Reduced Trigger Sensitivity ........................................................................................8

9 Modified Output Circuit ................................................................................................................9

10Circuit Diagram for Data Protection in a battery buffered memory............................................10

11Typical Variation of the Reference Voltage V_refversus supply Voltage Variations .....................10

12Typical Variation of the Reference Voltage V_refversus Ambient Temperature Variations .........11

13Buffered Circuit for the Reference Voltage................................................................................12

14Printed Circuit Layout for the supply voltage supervisor ...........................................................13

15Series Resistor at the C_tinput of the TL770xB..........................................................................14

Supply voltage Supervisor TL77xx Series

iii

Literature Number: SLVAE04

Supply Voltage Supervisor TL77xx Series

ABSTRACT

After power-on a digital system must be forced into a definite initial state. For microcomputers and microprocessors a

Reset input is provided to which, in simple applications, an R/C network is connected. After power-on, this circuitry

maintains the logic level at this input high (or low), until the supply voltage has reached its nominal value, and the

internal logic of the microcomputer has executed the Reset. However, this simple circuit does not work well under all

conditions, or during short drops of the supply voltage. This applications report describes the operation and the

application of the supply voltage supervisor circuits series TL77xx, which has been specially designed to solve these

applications problems.

1. Introduction

After power-on a digital system must normally be forced into a definite initial state. For microcomputers

and microprocessors a reset input is provided to which, in simple applications, an R/C network is

connected. After power-on, this circuitry maintains the logic level at this input high (or low), until the supply

voltage has reached its nominal value, and the internal logic of the microcomputer has executed the

initialization of the system.

Figure 1. Reset circuit

However, this circuit does not work well during short drops of the supply voltage. In this case, the capacitor

C must be quickly discharged via the diode D, which will occur only if the supply voltage V goes below the

threshold voltage (1 ¼ 2 Volt) of the Reset input. A small decrease of V under the recommended

minimum supply voltage can destroy the content of the memory and registers and yet not activate the reset

circuit. This may have catastrophic consequences.

The following program example (8080 assembler code) tests an input:

WAIT

LOOP

LXI

LDAX

ANI

B,INPUT

MASK

LOOP

;load address

;read input

;mask bit

;test

JNZ

If, during the execution of the loop, the content of the B/C register is affected by a short voltage drop

(causing an incorrect input to be read), an incorrect condition will be tested and an incorrect decision will

be the consequence. Alternatively the addressed input will show a value which can never give a positive

test result: the execution of the program seems to be stopped.

Supply voltage Supervisor TL77xx Series

Circuit Description

In 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 a battery back-up, and so on. In

small microcomputer systems this amount of effort is too expensive, and in most applications also not

required. It is usually sufficient if, after a serious voltage drop, the microcomputer is forced into a defined

initial condition. To implement this function, whilst preventing the problems mentioned above, the following

circuit features are required:

- Accurate detection of a serious voltage drop.

- Generation of a reset signal while as the supply voltage is not in the operational

range, to prevent undefined operations of the microcomputer.

- Maintenance of the reset signal for a certain time after the supply voltage has

returned to its nominal value, to ensure proper initialization of the circuit.

For these applications, Texas Instruments has developed a series of integrated circuits which, with a

minimum of external components and without additional adjustment, will fulfill the requirements described

above.

2. Circuit Description

The main part of this circuit is a reference voltage source, which consists of a very stable, temperature-

compensated band gap reference. An external capacitor (typ. 0.1mF) must be connected to the voltage

output V_ref, to reduce the influence of fast transients in the supply voltage. The voltage at the SENSE input

is divided by a resistor divider and compared with the reference voltage by a comparator. To achieve high

accuracy, this divider is adjusted at wafer probe. When the input voltage is sensed to be lower than the

threshold voltage, the thyristor is triggered, which discharges the timing capacitor C. 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 (orRESIN input) increases beyond the threshold, or - during

short supply voltage drops - the discharge current of the capacitor becomes lower than the hold current of

the thyristor.

Thereafter, the capacitor is recharged by a current source 100mA, the charge time being calculated as

follows:

t_d= 1.3×10⁴×C_t

C_tin F, t in s

The magnitude of charge current and therefore also the delay _dt time is determined by the tolerance of the

resistors in the integrated circuit. These tolerances, caused by the semiconductor manufacturing process,

are not negligible. Therefore the delay time may vary -50% to +100 %. However, for the applications

discussed here this will not be a restriction. The diagram in figure 3 shows the typical delay time _dt versus

the capacitance of the external capacitor C.

Literature Number: SLVAE04

Circuit Description

Note:

R_1typ

R_2typ

TL7702

TL7705

TL7709

TL7712

TL7715

9.0 kW

20.4 kW

35.6 kW

46.8 kW

10.0 kW

Figure 2. Functional Diagram

Figure 3. Diagram for Calculation of C_t

Supply voltage Supervisor TL77xx Series

Circuit Description

An additional comparator compares the voltage at the capacitor with the reference voltage and forces the

outputs into the active state as long as the voltage at the capacitor is lower than the reference voltage.

Figure 4 shows the timing of the various signals. In this example the SENSE input is connected to the

supply voltage V_ccas in typical applications of this device. The minimum supply voltage for which the

function of this device is guaranteed is 3.6V. After power-on the outputs are undefined until the minimum

supply voltage V is reached. For the TL77xxA the minimum supply voltage is V = 3.0 V (typical 2.5 V),

res

for the TL77xxB is V_res= 1.0 V. Also, when using the TL77xxB it has to be noted, that with such low supply

voltages the function of the reset input of the following circuit may not be guaranteed.

Beyond the voltage V the capacitor C is first kept discharged, and the outputs stay in the active

res

state(RESET = High, RESET = Low). When the input voltage becomes higher than the threshold voltage

V_t, the thyristor is turned off and the capacitor is charged. After a delay, t, the voltage at the capacitor

passes the trigger level of the output comparator and the outputs become inactive. The circuit to be

initialized is now set to a defined state and starts the correct operation.

Note: SENSE Input connected to V_cc

Figure 4. Timing diagram

The thyristor is triggered again during voltage drops below the threshold voltage V. and the reset

sequence starts again. Also now the outputs stays in the active state for the time _dt after the return of the

supply voltage to its required value. A hysteresis V at the input comparator prevents oscillation of the input

circuit when the input voltage rises or falls slowly.

The time t_d- and also the capacitor C - are determined by the requirements of the following circuitry. In

TTL or CMOS logic circuits, theoretically a reset time of 20 to 50ns is sufficient. For proper operation,

microcomputers require a reset signal which lasts for several machine cycles and is thus of the order of 10

to 200 ms, according to the type of microcomputer in use. In a practical application, the delay time will be

determined by characteristics of the power supply. Care has to be taken, that during and shortly after

Literature Number: SLVAE04

Application examples

power-on, short voltage fluctuations do not repetitively reset the system. Delay times of 10 to 20ms or

even up to 500 ms will usually avoid these problems. Owing to an internal limitation of discharge current of

the timing capacitor C_t, there is no upper limit for the size of this capacitor.

3. Application examples

Five versions of this circuit are available:

- TL7705A, TL7705B (V_t= 4.55 V): Application in TTL-systems and micro-

computer systems which require a 5 volt supply (e.g. TMS7000)

- TL7709A (V_t= 7.6 V): Application in microcomputer systems using the

TMS1XXXNLL.

- TL7712A (V_t= 10.8 V): Application in CMOS, microprocessor, and memory

circuits with a 12 volt supply.

- TL7715A (V_t= 13.5 V): Application in circuits which operate with a supply voltage

of 15 V, as is found often in analog circuits.

- TL7702A, TL7702B (V_t= 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.

Since for most applications the circuits are already adjusted to the appropriate voltage levels, these

devices are easy to use. Figure 5 shows the initialization circuit diagrams for TMS7000 microcomputer

system with supply voltage V = 5 V. The external components required are the decoupling capacitor C

ref

for the reference voltage and the timing capacitor C. The outputs of the TL77xx are open collector outputs.

In figure 5 therefore a pull-up resistor is shown at the RESET output to ensure the correct High level.

Figure 5. TL7705A in 5 Volt Microcomputer Applications

Supply voltage Supervisor TL77xx Series

Application examples

Figure 6. Voltage Supervision of a multiple power supply

In larger systems, where several supply voltages are required, it is necessary to supervise all supply

voltages which may cause dangerous conditions in case of power failure. In the circuit diagram of figure 6,

two TL7712A's are used to monitor the positive and the negative 12 volt supplies. Their outputs are fed to

the RESIN input of the TL7705A, which monitors the 5 volt supply. The output of this device provides a

reset signal, which becomes active whenever any one of the three supply voltages fails. A reset signal can

be generated manually via a switch which is connected to the voltage monitor of the positive 12V supply.

When designing a supply voltage monitor the designer has to take care, that when only one of the supply

voltages becomes marginal or fails, a defined reset signal is generated (at least if the main supply voltage

5 V is still available). Therefore the circuit which monitors the 12V supply voltage is supplied by the 5V

supply. The reset signal of the circuit which monitors the negative supply is fed via a resistor divider to the

base of the transistor BC546, which controls the RESIN input of the TL7705A. The voltage divider is

designed so that a reset is generated even if the negative supply fails totally.

The capacitor which determines the delay of the two circuits which monitors the both 12v supplies, can be

chosen short (in the example shown here it is 0.01mF). The output of these circuits has only to trigger the

third monitor TL7705A. The final duration of the reset signal will be determined by the capacitor Cof the

last mentioned circuit.

These supply voltage supervisor circuits were designed to detect supply voltage drops as short as

>300 ns. In figure 7 the minimum pulse width t_dminat the SENSE input is shown versus the amplitude of the

Literature Number: SLVAE04

Application examples

supply voltage drop DV_ccwhich is required to trigger the voltage monitor. The sensitivity of the other circuits

(TL7702, TL7712 etc.) can be calculated as proportional to the trigger voltage ratio.

0 V

Vcc

0,5

1,5

2,5

3,5

-0,2 V

-0,4 V

-0,6 V

-0,8 V

-1 V

t^d/ s

-1,2 V

-1,4 V

-1,6 V

-1,8 V

-2 V

Figure 7. Typical Trigger Sensitivity at the SENSE input of the

voltage monitor TL7705A

Figure 8. Circuit for Reduced Trigger Sensitivity

Supply voltage Supervisor TL77xx Series

Application examples

In applications where this performance is not required, sensitivity can be reduced by placing an R-C filter in

front of SENSE input. To avoid a unacceptable change of the threshold voltage of circuits with a fixed

threshold voltage (TL7705, TL7709, TL7712, TL7715) the value of the resistor should be a few 10W only.

In application like this it may be better to use the TL7702A. With this circuit the threshold voltage can be

set to the desired value by a high impedance voltage monitor. A small filter capacitor C at the SENSE

input reduces the trigger sensitivity (figure 8).

In some applications it is necessary to keep the output of the reset circuit active even if no supply voltage

is applied. Under normal conditions the output transistor of the TL77xxA is turned off (inactive state), when

the supply voltage is lower than 3V (V_cc> 1 V for the TL77xxB). In figure 9 a P-channel field effect

transistor is connected to theRESET output. This transistor conducts when the supply voltage drops below

3 V i.e. at a gate source voltage of -3V or less. To ensure that the transistor is switched off when the

supply voltage has reached the nominal value, the gate has to be at least 6V more positive than the

source (or the required high level e.g. 2.4V). In figure 9 therefore the voltage monitor is supplied by supply

voltage of 12 V. Since the requirements for this supply voltage in terms of stability are not high, this supply

voltage for example can be taken from the filter capacitor in front of the voltage regulator in the power

supply.

Figure 9. Modified Output Circuit

A further application for these integrated circuits is in battery-buffered memory systems. When the line

voltage fails, the content of the memory must not be corrupted by a random write operation of the

microcomputer. These uncontrolled write operations may take place at low supply voltage even if a reset

signal is applied to the processor. Generally, it is sufficient to switch the chip select line into the inactive

state (some memories require that the write line also be disabled). A switch, which consists of transistor Q

and diode D₁, is inserted into the chip select line of the memory. Under normal operation (line voltage

present) the output of the TL7705B is turned off (high); the transistor Q draws its base current from

transistor Q₂and resistor R₁. When the chip select line is switched from high to low by the supervising

microprocessor, the transistor conducts and the CS input of the memory goes low and the memory is

enabled. Because of the small DC load of the resistor R, the saturation voltage of the transistor (and

Literature Number: SLVAE04

Application examples

therefore the shift of the low level at the CS input) is very small (typ. 40mV). When the chip select line is

switched high again by the processor, the transistor Q is turned off (the influence of the inverse current

gain is negligible); the diode D conducts and charges the circuit capacitance. In the case of a power-

failure the TL7705B is triggered and its RESET output becomes low. The base of transistor Q can no

longer draw current. Thus the CS input of the memory is separated from the chip select line.

Figure 10. Circuit Diagram for Data Protection in a battery buffered

memory

Vref

2,513 V

2,512 V

2,511 V

2,51 V

2,509 V

2,508 V

2,507 V

2,506 V

2,505 V

Vcc

0 V

5 V

10 V

15 V

20 V

Supply voltage Supervisor TL77xx Series

Application examples

Figure 11. Typical Variation of the Reference Voltage V_refversus

supply Voltage Variations

As has already been mentioned the supply voltage supervisors of the series TL77xxA incorporate

extremely stable reference voltage source which can be accessed at the V terminal. This voltage source

ref

can also be used when in other applications a constant voltage source is required. As shown in picture 11,

the reference voltage V varies less than 10 mV, when the supply voltage is changed from 3.5¼ 18 V.

ref

The same stability of the reference voltage is maintained, when the ambient temperature is changed.

Figure 12 shows the typical characteristic. The references voltage varies only 16mV, when the ambient

temperature is changed from -40 ¼ 85 °C.

Vref

2,522 V

2,52 V

2,518 V

2,516 V

2,514 V

2,512 V

2,51 V

2,508 V

2,506 V

-40

-20

100 T

/ C

Figure 12. Typical Variation of the Reference Voltage V_refversus

Ambient Temperature Variations

When using the integrated reference voltage to supply other circuits, the designer has to consider that

maximum current available from this voltage source is in the order of 100mA only. With higher loads, the

stability of the reference voltage suffers. For higher currents a buffer in the form of an operational amplifier

connected as an emitter follower is recommended (figure 13). If the voltage monitor section of the circuit is

not used in this application, the capacitor C is not required. This terminal may then be left open. The inputs

SENSE and RESIN are connected to ground.

Literature Number: SLVAE04

Design Tips

Figure 13. Buffered Circuit for the Reference Voltage

4. Design Tips

The application of the supply voltage supervisors of the series TL77xx is not complicated. However it

should 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 the printed circuit board, 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 the printed circuit board special care should taken with the interconnects which carry analog signals.

Beside the SENSE input these are the C and V_refterminals. Noise coupled into the C input will lead to a

reduction of the output pulse width. Noise coupled into the V input or into the filter capacitor at this input

ref

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, the GND terminals of these capacitors must be

connected by the shortest way to the GND terminal of the voltage monitor in so that no currents caused by

other circuits flow over these wires. Figure14 show a layout proposal for the printed circuit board.

Furthermore the resistors of the voltage divider at the SENSE input of the TL7702 (R and R₃in figure 14)

have to be placed in so, that no noise may be coupled into this circuit.

Supply voltage Supervisor TL77xx Series

Design Tips

Figure 14. Printed Circuit Layout for the supply voltage supervisor

When using the supply voltage supervisor TL770xB in certain applications a current limiting

resistor R_t(figure 15) in series to the capacitor C_t(pin 3) is required. With this circuit the timing

capacitor will be charged up to the supply voltage V_ccrespectively an internal clamping (= 7.1 V),

where the smaller of the these two voltages determines the final voltage at the capacitor. When

the supply voltage drops quickly down to a voltage which is lower than the current voltage at the

capacitor C_t, a parasitic current path may be turned on which in turn erroneously activates the

outputs. This effect is avoided when the mentioned parasitic current I_pstays below 1 mA.

Considering this the resistor R_tis calculated as follows:

V_{Ct max}- V

R = ³

1mA

where V_t= threshold voltage of the SENSE input

V_Ctmax= V_ccmaxor 7 V, the lower of these values apply

When using the TL7705B (V_t= 4.55 V) with a maximum supply voltage V_cc= 5.5 V, one gets:

5.5V - 4.55V

R = ³

= 950

1mA

Literature Number: SLVAE04

Summary

Figure 15. Series Resistor at the C_tinput of the TL770xB

To a small degree this resistor influences the delay time t. In most applications however this effect can be

neglected, because - when determining the length of the reset pulse - a large reserve is taken into

account.

When designing a supply voltage supervision circuit often an analysis of the noise on the supply lines is

required. The probes of the oscilloscope usually used for this kind of measurement mostly are not capable

of performing the measurements correctly. The main reason is the ground wire attached to the probe. This

wire often acts as an antenna which receives all the noise generated by the surrounding circuits. This

leads to a wrong display on the oscilloscope screen. More accurate results are found, when the signal to

be measured is taken via a 0.1mF capacitor (to block the DC voltage) directly soldered to the point of

measurement. The other end of the capacitor is connected to a coaxcable, whose shield is connected by

the shortest path to the next ground reference point. The coaxcable leads to the input of the oscilloscope,

where the cable has to be terminated correctly to avoid line reflections.

5. Summary

Monitoring of the supply voltage is absolutely mandatory to guarantee a correct initialization of the circuit

and to detect undefined operating condition e.g. an undervoltage. This report shows several application

examples for the supply voltage supervisor circuits of the series TL77xxA. Owing their very accurate

threshold voltage, they can be easily designed into systems, replacing expensive discrete circuits .

Supply voltage Supervisor TL77xx Series

TL7702 [TI]

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