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參數(shù)資料
| 型號： | LM3704YDMM-643 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Microprocessor Supervisory Circuits with Power Fail Input, Low Line Output and Manual Reset |
| 中文描述： | 微處理器監(jiān)控電路，帶有電源故障輸入，低壓線路輸出和手動復(fù)位 |
| 文件頁數(shù)： | 9/17頁 |
| 文件大?。?/td> | 319K |
| 代理商： | LM3704YDMM-643 |
Circuit Information
Reset Output
The Reset input of a μP initializes the device into a known
state. The LM3704/LM3705 microprocessor supervisory cir-
cuits assert a forced reset output to prevent code execution
errors during power-up, power-down, and brownout condi-
tions.
RESET is guaranteed valid for V
>
1V. Once V
ex-
ceeds the reset threshold, an internal timer maintains the
output for the reset timeout period. After this interval, reset
goes high. The LM3704 offers an active-low RESET; The
LM3705 offers an active-high RESET.
Any time V
drops below the reset threshold (such as
during a brownout), the reset activates. When V
CC
again
rises above the reset threshold, the internal timer starts.
Reset holds until V
CC
exceeds the reset threshold for longer
than the reset timeout period. After this time, reset releases.
The Manual Reset input (MR) will initiate a forced reset also.
See the
Manual Reset Input
section.
Reset Threshold
The LM3704/LM3705 family is available with a reset voltage
of 3.08V. Other reset thresholds in the 2.20V to 5.0V range,
in steps of 10 mV, are available; contact National Semicon-
ductor for details.
Manual Reset Input (MR)
Many μP-based products require a manual reset capability,
allowing the operator to initiate a reset. The MR input is fully
debounced and provides an internal 56 k
pull-up. When the
MR input is pulled below V
(1.225V) for more than 25 μs,
reset is asserted after a typical delay of 12 μs. Reset remains
active as long as MR is held low, and releases after the reset
timeout period expires after MR rises above V
. Use MR
with digital logic to assert or to daisy chain supervisory
circuits. It may be used as another low-line comparator by
adding a buffer.
Power-Fail Comparator (PFI/PFO)
The PFI is compared to a 1.225V internal reference, V
PFT
. If
PFI is less than V
PFT
, the Power Fail Output PFO drops low.
The power-fail comparator signals a falling power supply,
and is driven typically by an external voltage divider that
senses either the unregulated supply or another system
supply voltage. The voltage divider generally is chosen so
the voltage at PFI drops below V
several milliseconds
before the main supply voltage drops below the reset thresh-
old, providing advanced warning of a brownout.
The voltage threshold is set by R
1
and R
2
and is calculated
as follows:
Note this comparator is completely separate from the rest of
the circuitry, and may be employed for other functions as
needed.
Low-Line Output (LLO)
The low-line output comparator is typically used to provide a
non-maskable interrupt to a μP when V
CC
begins falling. LLO
monitors V
CC
and goes low when V
CC
falls below V
LLOT
(typically 1.02
V
RST
) with hysteresis of 0.0032
V
RST
.
Special Precautions for the micro SMD Package
As with most integrated circuits, the LM3704 and LM3705
are sensitive to exposure from visible and infrared (IR) light
radiation. Unlike a plastic encapsulated IC, the micro SMD
package has very limited shielding from light, and some
sensitivity to light reflected from the surface of the PC board
or long wavelength IR entering the die from the side may be
experienced. This light could have an unpredictable affect on
the electrical performance of the IC. Care should be taken to
shield the device from direct exposure to bright visible or IR
light during operation.
Micro SMD Mounting
The micro SMD package requires specific mounting tech-
niques which are detailed in National Semiconductor Appli-
cation Note AN-1112. Referring to the section
Surface
Mount Technology (SMT) Assembly Considerations
, it
should be noted that the pad style which must be used with
the 9-pin package is the NSMD (non-solder mask defined)
type.
For best results during assembly, alignment ordinals on the
PC board may be used to facilitate placement of the micro
SMD device.
L
www.national.com
9
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