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參數(shù)資料
| 型號(hào)： | 935271414518 |
| 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(yè)數(shù)： | 10/12頁(yè) |
| 文件大小： | 116K |
| 代理商： | 935271414518 |
Philips Semiconductors
Product data
MAX708R/S/T
3 V microprocessor supervisor circuit with
power fail comparator and manual reset
2002 Feb 13
7
APPLICATION INFORMATION
Operating with a +3 V or +5 V supply
When powered from either +3.0 V or +5.0 V, the MAX708R/S/T
provides voltage monitoring at the reset threshold. The
MAX708R/S/T are ideal for portable applications that are powered
from a +3 V battery or an AC-DC wall adapter that generates +5 V.
Power-fail comparator
An early warning power-fail circuit uses the power-fail input to
monitor the upstream, unregulated DC supply that powers the
+3 V/+3.3 V regulator powering the MAX708R/S/T.
Figure 1, “Simplified system diagram”, shows the resistor divider
network that is connected to PFI. The network resistors R1 and R2
are chosen so that the voltage at PFI will fall below 1.25 V before
the regulator voltage drops out. The PFO signal may be used to
interrupt the microprocessor so it can perform an orderly shutdown.
Any unregulated or regulated supply can be monitored by adjusting
the resistive divider for the desired power supply trip voltage. Table 1
shows examples for nominal trip voltages of 3.7 V and 10.9 V. The
desired trip voltage, VT is calculated by the following equation:
V
T + 1.25 V
(R1
) R2)
R2
Table 1. Power-fail detection circuit of
3.7 V and 10.9 V
R1 1%
R2 1%
VT
5 V
unregulated
power supply
1 M
510 k
3.7 V
12 V
unregulated
power supply
1 M
130 k
10.9 V
Adding hysteresis to the power-fail comparator
Hysteresis adds noise margin to the power-fail comparator and
prevents repeated triggering of PFO when VIN is varying around the
comparator trip point. In Figure 5, “Adding hysteresis to the
power-fail comparator”, R1 and R2 are the divider resistors that set
the desired trip point. R3 is added from the comparator input to the
output to increase hysteresis; it is typically an order of magnitude
greater than R1 and R2. The current through R1 and R2 should be
at least 1 mA to ensure that the 25 nA max PFI input current does
significantly shift the trip point. Capacitor C1 provides noise
rejection.
R1
GND
PFI
R2
C1
R3
VCC
VCC2
MAX708R/S/T
PFO
TO
MICROPROCESSOR
VIN
SL01814
Figure 5. Adding hysteresis to the power-fail comparator.
PFO
VCC2
0 V
VL
VT
VH
VIN
V
T + 1.25 V
(R1
) R2)
R2
The hysteresis of VH and VL voltages are calculated by:
V
H + 1.25 V
1
)
(R3
) R2)
(R2
R3)
R1
V
L + 1.25 V ) R1
1.25
R2
)
1.25
* V
CC
R3
Thus,
V
HYS + VH * VL +
R1
V
CC2
R3
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