MAX801 Просмотр технического описания (PDF) - Maxim Integrated
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MAX801 Datasheet PDF : 12 Pages
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8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
4.5V to 5.5V
REGULATOR
VCC
LOWLINE
CHOLD
MAX801
MAX808
CHOLD > ILOAD x tSHDN
GND
VLR
TO μP NMI
VCC
CONTROL
CIRCUITRY
MAX801
P MAX808
BATT
P
P
OUT
0.1μF
Figure 3. Using LOWLINE to Provide a Power-Fail Warning to
the µP
Figure 4. VCC and BATT to OUT Switch
calculate the amount of capacitance required to allow
the shutdown routine to complete before reset is
asserted:
CHOLD = (ILOAD x tSHDN) / (VLR(min))
where tSHDN is the time required for the system to com-
plete the shutdown routine (including the VCC to low-
line propagation delay), ILOAD is the current being
drained from the capacitor, and VLR is the low-line to
reset threshold.
Output Supply Voltage
The output supply (OUT) transfers power from VCC or
BATT to the µP, RAM, and other external circuitry. At
the maximum source current of 250mA, VOUT will typi-
cally be 220mV below VCC. Decouple OUT with a 0.1µF
capacitor to ground.
Battery-Backup Mode
Battery-backup mode preserves the contents of RAM in
the event of a brownout or power failure. With a backup
battery installed at BATT, the MAX801/MAX808 automati-
cally switches RAM to backup power when VCC falls.
Two conditions are required for switchover to battery-
backup mode: 1) VCC must be below the reset threshold;
2) VCC must be below VBATT. Table 1 lists the status of
inputs and outputs during battery-backup mode.
BATT is designed to conduct up to 20mA to OUT dur-
ing battery backup. The PMOS switch on-resistance is
approximately 12Ω. Figure 4 shows the two series pass
elements (between the BATT input and OUT) that
facilitate UL recognition. VBATT can exceed VCC during
normal operation without causing a reset.
Table 1. Input and Output Status in
Battery-Backup Mode
PIN
NAME
MAX801 MAX808
STATUS
Battery switchover
1
1
VCC comparator monitors VCC
for active switchover.
2
2 LOWLINE Logic low
3
3
RESET Logic low
4
4
GND
Ground—0V reference for
all signals
5
—
RESET
Logic high; the open-circuit
voltage is equal to VCC.
Logic high. The open-circuit
—
5
CE OUT output voltage is equal to
VBATT (MAX808).
6
—
WDI
WDI is ignored and goes
high impedance.
—
6
CE IN High impedance (MAX808)
7
7
BATT
Supply current is 1µA max for
VBATT ≤ 2.8V.
OUT is connected to BATT
8
8
OUT through two internal PMOS
switches in series.
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