8.1 Overview

The TL77xxA family of integrated-circuit supply-voltage supervisors is designed specifically for use as reset controllers in microcomputer and microprocessor systems. The supply-voltage supervisor monitors the supply for undervoltage conditions at the SENSE input. During power up, the RESET output becomes active (low) when V_CC attains a value approaching 3.6 V. At this point (assuming that SENSE is above V_IT+), the delay timer function activates a time delay, after which outputs RESET and RESET go inactive (high and low, respectively). When an undervoltage condition occurs during normal operation, RESET and RESET go active. To ensure that a complete reset occurs, the reset outputs remain active for a time delay after the voltage at the SENSE input exceeds the positive-going threshold value.

During power down and when SENSE is below V_IT−, the outputs remain active until V_CC falls below 2 V. After this, the outputs are undefined. An external capacitor (typically 0.1 μF) must be connected to REF to reduce the influence of fast transients in the supply voltage.

Five versions of this circuit are available:

• TL7705A (Vt = 4.55 V): Application in TTL-systems and microcomputer systems which require a 5 volt supply (for example, TMS7000)
• TL7709A (Vt = 7.6 V): Application in microcomputer systems using the TMS1XXXNLL
• TL7712A (Vt = 10.8 V): Application in CMOS, microprocessor, and memory circuits with a 12 volt supply.
• TL7715A (Vt = 13.5 V): Application in circuits which operate with a supply voltage of 15 V, as is found often in analog circuits.
• TL7702A (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.

8.2 Functional Block Diagram

The functional block diagram is shown for illustrative purposes only; the actual circuit includes a trimming network to adjust the reference voltage and sense-comparator trip point.

A. TL7702A: R1 = 0 Ω, R2 = open

TL7705A: R1 = 7.8 kΩ, R2 = 10 kΩ

TL7709A: R1 = 19.7 kΩ, R2 = 10 kΩ

TL7712A: R1 = 32.7 kΩ, R2 = 10 kΩ

TL7715A: R1 = 43.4 kΩ, R2 = 10 kΩ

B. Resistor values shown are nominal.

8.3 Feature Description

8.4 Device Functional Modes

Figure 7 shows the timing of the various signals. In this example the SENSE input is connected to the supply voltage V_CC as in typical applications of this device. The minimum supply voltage for which the function of this device is guaranteed is 3.6 V. After power-on, the outputs are undefined until the minimum supply voltage V_res is reached. For the TL77xxA the minimum supply voltage is V_res = 3 V (typical 2.5 V). Beyond the voltage V_res the capacitor C_T is first kept discharged, and the outputs stay in the active 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_d, 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.

A. Note: SENSE Input connected to V_CC

Figure 7. Timing Diagram