ISL88705 INTERSIL [Intersil Corporation], ISL88705 Datasheet - Page 7

• Manufacturer Part Number: ISL88705
• Description: uP Supervisor with Watchdog Timer, Power-Fail Comparator, Manual Reset and Adjustable Power-On Reset
• Manufacturer: INTERSIL [Intersil Corporation]

Low Voltage Monitoring
These devices monitor both the voltage level of V
auxiliary voltage on PFI.
When IC is initially biased reset is asserted until the V
voltage is greater than the specific IC fixed-voltage trip point
for the t
V
again asserted, i.e. RST is high and RST is low.
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 the following formula (See Figure 1):
PFI V
PFO goes low whenever PFI is less than the 1.25V (or user-
set) threshold voltage.
DD
FIGURE 1. CUSTOM V
does not exceed its voltage threshold, reset is once
TH
POR
= 1.25 (R1 + R2/R2)
V
IN
V
RST
RST
MR
DD
duration of 200ms. At any subsequent time that
R
1
V
1V
TH1
ISL88705, ISL88706, ISL88707, ISL88708, 8SL88716, ISL88813
R
2
TH
WITH RESISTOR DIVIDER ON PFI
FIGURE 2. POWER-SUPPLY MONITORING TIMING DIAGRAM (WDI TRI-STATED)
7
PFI
t
POR
ISL8870X
t
RPD
DD
and an
DD
t
POR
Adjusting t
On the ISL88707 and ISL88708, users can adjust the Power
On Reset timeout delay (t
of 200ms. To do this, connect a capacitor between C
ground (see Figure 3). For example, connecting a 50pF
capacitor to CPOR will increase t
Care should be taken in PCB layout and capacitor placement in
order to reduce stray capacitance as much as possible, which
contributes to t
14
12
10
FIGURE 3. ADJUSTING t
8
6
4
2
0
0
NORMALIZED t
OPEN = 200ms
>t
10
MR
POR
POR
t
20
POR
error.
30
POR
POR
C
ISL88707, ISL88708
40
POR
vs C
C
) to many times the nominal t
POR
POR
50
POR
POR
(pF)
(pF)
WITH A CAPACITOR
60
from 200ms to ~1.4s.
70
80
December 6, 2006
90
POR
100
FN8092.3
and
POR