10 Power Supply Recommendations

VS

10 k Ÿ

0.1

F

µ

SENSE

RESIN

GND

REF

RESET

RESET

VCC

CT

To System

RESET

R T

C T

To System

RESET

(see text)

7

2

1

3

5

6

8

4

10 k Ÿ

Reset Input

(from system)

System

Supply

Copyright © 2016, Texas Instruments Incorporated

Figure 10-1. System Reset Controller With Undervoltage Sensing

When the TL770xB SENSE terminal is used to monitor VCC, TI recommends a current-limiting resistor in series

with CT. During normal operation, the timing capacitor is charged by the onboard current source to

approximately VCC or an internal voltage clamp (≈7.1-V Zener), whichever is less. When the circuit then is

subjected to an undervoltage condition during which VCC is rapidly slewed down, the voltage on CT exceeds that

on VCC. This forward biases a secondary path internally, which falsely activates the outputs. A fault is indicated

when VCC drops below V(CT), not when VSENSE falls below VT–.

Adding the external resistor, RT, prevents false triggering. Its value is calculated as follows:

(V(CT) – VT-) / RT

(1)

where

• V(CT) = VCC or 7.1 V, whichever is less

• VT– = 4.55 V (nom)

• RT = value of series resistor required

For VCC = 5 V

(5 – 4.55) / RT < 1 mA

(2)

Therefore,

RT > 450 Ω

(3)

Using a 20%-tolerance resistor, RT should be greater than 560 Ω.

Adding this series resistor changes the duration of the reset pulse by no more than 10%. RT extends the

discharge of CT, but also skews the V(CT) threshold. These effects tend to cancel one another. The precise

percentage change can be derived theoretically, but the equation is complicated by this interaction and is

dependent upon the duration of the supply-voltage fault condition.

Both outputs of the TL770xB must be terminated with similar value resistors, even when only one is being used.

This prevents unwanted plateauing in either output waveform during switching, which may be interpreted as an

undefined state or delay system reset.

TL7702B, TL7705B, TL7733B

SLVS037O – SEPTEMBER 1989 – REVISED DECEMBER 2020

www.ti.com

14

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