tpo

tpo

RESET

Vhys

VCC

1.5 V

T

T

Vs’

12

TL7700

SLVS220G – JULY 1999 – REVISED AUGUST 2016

www.ti.com

Product Folder Links: TL7700

Submit Documentation Feedback

Copyright © 1999–2016, Texas Instruments Incorporated

Feature Description (continued)

8.3.3 Output Pulse-Duration Setting

Constant-current charging starts on the timing capacitor when the sensing-line voltage reaches the TL7700

sense voltage. When the capacitor voltage exceeds the threshold level of the output drive comparator, RESET

changes from a low to a high level. The output pulse duration is the time between the point when the sense-pin

voltage exceeds the threshold level and the point when the RESET output changes from a low level to a high

level. When the TL7700 is used for system power-on reset, the output pulse duration, tpo, must be set longer

than the power rise time. The value of tpo is:

tpo = Ct × 10

5 seconds

where

•

Ct is the timing capacitor in farads

(3)

There is a limit on the device response speed. Even if Ct = 0, tpo is not 0, but approximately 5 µs to 10 µs.

Therefore, when the TL7700 is used as a comparator with hysteresis without connecting Ct, switching speeds

(tr/tf, tpo/tpd, and so forth) must be considered.

8.4 Device Functional Modes

Figure 20 describes how the RESET output pin responds to a change in the voltage at the sense pin. When the

sense pin drops below 500 mV, the RESET pin is pulled low.

The sense voltage, Vs', is different from the SENSE terminal input voltage, Vs. Vs normally is 500 mV for triggering.

Figure 20. VCC RESET Response and Timing