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參數(shù)資料
| 型號： | 935271416518 |
| 廠商： | NXP SEMICONDUCTORS |
| 元件分類： | 電源管理 |
| 英文描述： | 2-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO8 |
| 封裝： | 3.90 MM, PLASTIC, MS-012, SO-8 |
| 文件頁數(shù)： | 11/12頁 |
| 文件大?。?/td> | 116K |
| 代理商： | 935271416518 |
Philips Semiconductors
Product data
MAX708R/S/T
3 V microprocessor supervisor circuit with
power fail comparator and manual reset
2002 Feb 13
8
Monitoring a negative voltage
The power-fail comparator can be used to monitor a negative
voltage using the circuit shown in Figure 6.
When the negative supply is operating in tolerance, PFO is LOW.
However, when the negative supply drops (goes positive) PFO goes
HIGH.
R1
GND
PFI
R2
VCC
MAX708R/S/T
PFO
SL01815
V–
+3 V to +3.3 V
TO
P
INTERRUPT
Figure 6. Circuit for monitoring a negative voltage.
When monitoring a negative supply, the trip voltage, VT is a
negative voltage. Use the following ratio to determine the divider
resistors:
V
CC * 1.25
R1
+
1.25
* V
T
R2
Solving for VT:
V
T + 1.25 *
R2
R1
V
CC * 1.25
VIN
0 V
VT
0 V
+3 V
to
+3.3 V
PFO
Ensuring a valid reset down to VCC = 0 V
When VCC falls below 1 V, the MAX708R/S/T RESET no longer
sinks current (i.e., it becomes open circuit). A high impedance
CMOS logic input connected to RESET can drift to undetermined
voltages. In most applications in which the microprocessor circuitry
is inoperative below 1 V this will not present a problem. However, in
applications in which RESET must be valid down to 0 V, use a
relatively large pull-down resistor from RESET to ground (Figure 7).
100 k
is small enough to provide a path for any stray leakage
currents to flow to ground (holding RESET LOW), while it is large
enough not to load RESET.
SL01816
VCC
RESET
MAX708R/S/T
GND
100 k
VCC
MICROPROCESSOR
GND
RESET
Figure 7. RESET valid to VCC = Ground circuit.
Interfacing to a microprocessor with bi-directional
reset pins
Microprocessors with bi-directional reset pins, such as the Motorola
68HC11 series, can be connected to the MAX708R/S/T RESET
output. The bi-directional reset of the microprocessor functions both
as a driven reset input and as an active reset driver.
To ensure a correct output on the MAX708R/S/T even when the
microprocessor reset pin is in the opposite state, connect a 4.7 k
resistor between the reset pins as shown in Figure 8. This allows the
microprocessor to issue commands to the system regardless of the
state of the RESET pin.
SL01817
VCC
RESET
MAX708R/S/T
GND
4.7 k
VCC
MOTOROLA
68HCXX
GND
RESET
Figure 8. Interfacing to a microprocessor with bi-directional
Reset I/O
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