MAX690A(1993) 데이터 시트보기 (PDF) - Maxim Integrated
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MAX690A Datasheet PDF : 12 Pages
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Microprocessor Supervisory Circuits
VBATT
VCC
+5V
SW1
SW2
SW3
SW4
D1 D2
SUBSTRATE
MAX690A MAX802L MAX805L D3
MAX692A MAX802M
CONDITION
VOUT
SW1/SW2 SW3/SW4
+
0.1F
VCC
VOUT
MAX690A
MAX802L
VBATT MAX805L RESET
(RESET)
TO STATIC RAM
TO µP
GND
VCC > Reset Threshold
Open
Closed
VCC < Reset Threshold and
Open
VCC > VBATT
Closed
VCC < Reset Threshold and Closed
Open
VCC < VBATT
RESET THRESHOLD = 4.65V IN MAX690A/MAX802L/MAX805L.
RESET THRESHOLD = 4.4V IN MAX692A/MAX802M
Figure 3. Backup-Battery Switchover Block Diagram
need not be dedicated to this function though, as it is
completely separate from the rest of the circuitry. The
external voltage divider drives PFI to sense the unregu-
lated DC input to the +5V regulator (see Typical
Operating Circuit). The voltage-divider ratio can be
chosen such that the voltage at PFI falls below 1.25V
just before the +5V regulator drops out. PFO then trig-
gers an interrupt which signals the µP to prepare for
power-down.
To conserve backup-battery power, the power-fail
detector comparator is turned off and PFO is forced
low when VBATT connects to VOUT.
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 automati-
cally switch RAM to backup power when VCC fails.
As long as VCC exceeds the reset threshold, VOUT connects
to VCC through a 5Ω PMOS power switch. Once VCC falls
below the reset threshold, VCC or VBATT (whichever is high-
er) switches to VOUT. Unlike the MAX690/MAX692, the
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L don’t
always connect VBATT to VOUT when VBATT is greater than
VCC. VBATT connects to VOUT (through an 80Ω switch) only
when VCC is below the reset threshold and VBATT is greater
( ) ARE FOR MAX805L ONLY.
Figure 4. Using a SuperCap as a Backup Power Source with a
MAX690A/MAX802L/MAX805L and a +5V ±5% Supply
than VCC.
When VCC exceeds the reset threshold, it is connected to
the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
substrate, regardless of the voltage applied to VBATT
(Figure 3). During this time, the diode (D1) between VBATT
and the substrate will conduct current from VBATT to VCC if
Table 1. Input and Output Status in Battery-Backup Mode
SIGNAL
STATUS
VCC
Disconnected from VOUT
VOUT
Connected to VBATT through an internal 80Ω
PMOS switch
VBATT
PFI
PFO
RESET
RESET
WDI
Connected to VOUT. Current drawn from the
battery is less than 1µA, as long as
VCC < VBATT - 1V.
Power-fail comparator is disabled.
Logic low
Logic low
Logic high (MAX805L only)
Watchdog timer is disabled
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