3.0V/3.3V Microprocessor Supervisory Circuits
MAX690T/S/R, 704T/S/R, 802T/S/R, 804鈥?06T/S/R
Backup-Battery Switchover
In the event of a brownout or power failure, it may be
necessary to preserve the contents of RAM. With a
backup battery installed at VBATT, the devices auto-
matically switch RAM to backup power when V
CC
falls.
This family of 碌P supervisors (designed for 3.3V and 3V
systems) doesn鈥檛 always connect VBATT to V
OUT
when
VBATT is greater than V
CC
. VBATT connects to V
OUT
(through a 140鈩?switch) when V
CC
is below V
SW
and
VBATT is greater than V
CC
, or when V
CC
falls below
1.75V (typ) regardless of the VBATT voltage. This is
done to allow the backup battery (e.g., a 3.6V lithium
cell) to have a higher voltage than V
CC
.
Switchover at V
SW
(2.40V) ensures that battery-backup
mode is entered before V
OUT
gets too close to the 2.0V
minimum required to reliably retain data in CMOS RAM.
Switchover at higher V
CC
voltages would decrease
backup-battery life. When V
CC
recovers, switchover is
deferred until V
CC
rises above the reset threshold
(V
RST
) to ensure a stable supply. V
OUT
is connected to
V
CC
through a 3鈩?PMOS power switch.
__________Applications Information
These 碌P supervisory circuits are not short-circuit
protected. Shorting V
OUT
to ground鈥攅xcluding power-
up transients such as charging a decoupling
capacitor鈥攄estroys the device. Decouple both V
CC
and VBATT pins to ground by placing 0.1碌F capacitors
as close to the device as possible.
Using a SuperCap
as a Backup Power Source
SuperCaps鈩?are capacitors with extremely high
capacitance values (e.g., order of 0.47F) for their size.
Figure 3 shows two ways to use a SuperCap as a
backup power source. The SuperCap may be
connected through a diode to the 3V input (Figure 3a)
or, if a 5V supply is also available, the SuperCap may
be charged up to the 5V supply (Figure 3b) allowing a
longer backup period. Since VBATT can exceed V
CC
while V
CC
is above the reset threshold, there are no
special precautions when using these 碌P supervisors
with a SuperCap.
Manual Reset
鈥撯€?/div>
鈥?/div>
A logic鈥?on M R asserts reset. Reset remains 鈥?/div>
low
asserted
鈥?鈥?/div>
鈥撯€?/div>
while M R is low, and for t
WP
(200ms) after MR returns
high. This input has an internal 70碌A pull-up current, so
鈥撯€?/div>
鈥?/div>
it can be left open if it is not used. M R can be driven with
TTL or CMOS logic levels, or with open-drain/collector
outputs. 鈥?Connect a normally open momentary switch
鈥撯€?/div>
from M R to GND to create a manual-reset function;
external debounce circuitry is not required.
Operation without a Backup
Power Source
These 碌P supervisors were designed for battery-
backed applications. If a backup battery is not used,
connect both VBATT and V
OUT
to V
CC
, or use a
different 碌P supervisor such as the MAX706T/S/R or
MAX708T/S/R.
Replacing the Backup Battery
The backup power source can be removed while V
CC
remains valid, if VBATT is decoupled with a 0.1碌F
capacitor to ground, without danger of triggering
鈥撯€斺€斺€斺€?/div>
鈥?/div>
RESET/ R E S E T . As long as V
CC
stays above V
SW
,
battery-backup mode cannot be entered.
Table 1. Input and Output Status in
Battery-Backup Mode
PIN NAME
V
OUT
V
CC
PFI
鈥?鈥斺€?/div>
鈥?/div>
PFO
WDI
鈥撯€?/div>
鈥?/div>
MR
鈥?鈥?鈥?/div>
鈥?鈥斺€?/div>
RESET
RESET
VBATT
STATUS
Connected to VBATT through an internal
140鈩?switch
Disconnected from V
OUT
The power-fail comparator is disabled when
V
CC
< V
SW
Logic low when V
CC
< V
SW
or PFI < V
PFT
The watchdog timer is disabled
Disabled
Low logic
High impedance
Connected to V
OUT
Adding Hysteresis
to the Power-Fail Comparator
The power-fail comparator has a typical input
hysteresis of 10mV. This is sufficient for most applica-
tions where a power-supply line is being monitored
through an external voltage divider (see the section
Monitoring an Additional Power Supply).
If additional noise margin is desired, connect a resistor
鈥?鈥斺€?/div>
鈥?/div>
between P F O and PFI as shown in Figure 4a. Select
the ratio of R1 and R2 such that PFI sees 1.237V (V
PFT
)
when V
IN
falls to its trip point (V
TRIP
). R3 adds the
hysteresis and will typically be more than 10 times the
value of R1 or R2. The hysteresis window extends both
above (V
H
) and below (V
L
) the original trip point (V
TRIP
).
鈩?SuperCap is a trademark of Baknor Industries.
8
_______________________________________________________________________________________
MAX802SCPA PDF文件相關型號
MAX802SCSA,MAX804SMJA
MAX802SCPA相關型號PDF文件下載
-
型號
版本
描述
廠商
下載
-
英文版
Microprocessor Supervisory Circuits
-
英文版
8-Pin µP Supervisory Circuits with ±1.5% Re...
-
英文版
?P Reset IC with Watchdog and Battery Switchover
-
英文版
3-Pin Microprocessor Reset Circuits
-
英文版
3.0V/3.3V Microprocessor Supervisory Circuits
Maixm
-
英文版
3.0V/3.3V Microprocessor Supervisory Circuits
Maixm
-
英文版
3.0V/3.3V Microprocessor Supervisory Circuits
Maixm
-
英文版
?P Reset IC with Watchdog and Battery Switchover
-
英文版
3.0V/3.3V Microprocessor Supervisory Circuits
-
英文版
Full-Featured µP Supervisory Circuit with ±1.5&...
-
英文版
8-Pin レP Supervisory Circuits with 【1.5eset Accuracy
-
英文版
3-Pin Microprocessor Reset Monitors
ONSEMI
-
英文版
3-pin microprocessor resets
Philips
-
英文版
PIN CONFIGURATION
-
英文版
3-Pin Microprocessor Reset Monitors
ONSEMI
-
英文版
3-Pin Microprocessor Reset Monitors
ONSEMI [ON...
-
英文版
4-Pin レP Voltage Monitors with Manual Reset Input
-
英文版
4-Pin レP Voltage Monitors with Manual Reset Input
MAXIM [Max...
-
英文版
4-Pin μP Voltage Monitors with Manual Reset Input
Maixm
-
英文版
Low-Cost, uP Supervisory Circuits
MAXIM
1
2
3
4
5
6
7
8
9
10
11
12
