ISL88705 데이터 시트보기 (PDF) - Renesas Electronics

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ISL88705

µP Supervisor with Watchdog Timer, Power-Fail Comparator, Manual Reset and Adjustable Power-On Reset

Renesas Electronics 

ISL88705, ISL88706, ISL88707, ISL88708, ISL88716, ISL88813

VDD

VTH1

1V

>tMR

MR

tPOR

tRPD

tPOR

tPOR

RST

RST

FIGURE 1. POWER-SUPPLY MONITORING TIMING DIAGRAM (WDI TRI-STATED)

Low Voltage Monitoring

These devices monitor both the voltage level of VDD and an

auxiliary voltage on PFI.

When IC is initially biased reset is asserted until the VDD

voltage is greater than the specific IC fixed-voltage trip point

for the tPOR duration of 200ms. At any subsequent time that

VDD does not exceed its voltage threshold, reset is once

again asserted, i.e. RST is high and RST is low (see

Figure 1).

.Power Failure Monitor

These devices also have a Power-Failure Monitor that helps

to monitor an additional critical voltage on the Power-Fail

Input (PFI) pin. For example, the PFI pin could be used to

provide an early power-fail warning, detect a low-battery

condition, presence detection or simply monitor a power

supply other than +5V. The 1.25V threshold detector can be

adjusted using an external resistor divider network to provide

custom voltage monitoring of voltages greater than 1.25V,

according to Equation 1 (see Figure 2).

PFI VTH

=

1.25







-R----1--R---+--2--R-----2-

(EQ. 1)

PFO goes low whenever PFI is less than the 1.25V (or

user-set) threshold voltage.

.

R1

VIN

R2

PFI

ISL8870x

FIGURE 2. CUSTOM VTH WITH RESISTOR DIVIDER ON PFI

If using a voltage divider on the PFI input to critique an

external voltage and intending to use the MR input to initiate

resets then avoid having the PFI voltage less than PFI Vth

+2.2V as unintended PFO transition may occur when MR is

transitioning high.

Adjusting tPOR

On the ISL88707 and ISL88708, users can adjust the

Power-On Reset time-out delay (tPOR) to many times the

nominal tPOR of 200ms. To do this, connect a capacitor

between CPOR and ground (see Figure 3). For example,

connecting a 50pF capacitor to CPOR will increase tPOR from

200ms to ~1.4s. Care should be taken in PCB layout and

capacitor placement in order to reduce stray capacitance as

much as possible, which contributes to tPOR error.

FN8092 Rev 6.00

November 30, 2015

Page 7 of 16

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