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参数资料
| 型号： | LM3712YFBPX-308 |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 电源管理 |
| 英文描述： | 2-CHANNEL POWER SUPPLY MANAGEMENT CKT, PBGA9 |
| 封装： | MICRO, BUMP, SMD-9 |
| 文件页数： | 13/14页 |
| 文件大小： | 311K |
| 代理商： | LM3712YFBPX-308 |
Watchdog Timeout Period vs Temperature
(t
WD programmed as 6.2ms)
20011913
Circuit Information
RESET OUTPUT
The Reset input of a P initializes the device into a known
state. The LM3712 microprocessor supervisory circuits assert
a forced reset output to prevent code execution errors during
power-up, power-down, and brownout conditions.
RESET is guaranteed valid for V
CC > 1V. Once VCC exceeds
the reset threshold, an internal timer maintains the output for
the reset timeout period. After this interval, reset goes high.
The LM3712 offers an active-low RESET.
Any time V
CC 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 LM3712 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 Semiconductor for de-
tails.
MANUAL RESET INPUT (MR)
Many P-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
MRT (1.225V) for more than 25 s,
reset is asserted after a typical delay of 12 s. Reset remains
active as long as MR is held low, and releases after the reset
timeout period expires after MR rises above V
MRT. Use MR
with digital logic to assert or to daisy chain supervisory cir-
cuits. 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 volt-
age. The voltage divider generally is chosen so the voltage at
PFI drops below V
PFT several milliseconds before the main
supply voltage drops below the reset threshold, providing ad-
vanced warning of a brownout.
The voltage threshold is set by R
1 and R2 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.
WATCHDOG TIMER INPUT (WDI)
The
watchdog
timer
input
monitors
one
of
the
microprocessor's output lines for activity. Each time a transi-
tion occurs on this monitored line, the watchdog counter is
reset. However, if no transition occurs and the timeout period
is reached, the LM3712 assumes that the microprocessor has
locked up and the watchdog output WDO, is activated.
WDI is a high impedance input. WDO is an active-low totem
pole output.
SPECIAL PRECAUTIONS FOR THE micro SMD
PACKAGE
As with most integrated circuits, the LM3712 is 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 re-
flected 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 per-
formance of the IC. Care should be taken to shield the device
from direct exposure to bright visible or IR light during oper-
ation.
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.
www.national.com
8
LM3712
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| LM3722A-2.32 E WAF | 制造商:Texas Instruments 功能描述: |
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