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參數(shù)資料
| 型號(hào)： | LM9072 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Dual Tracking Low-Dropout System Regulator |
| 中文描述： | 雙跟蹤低壓差穩(wěn)壓器系統(tǒng) |
| 文件頁數(shù)： | 7/15頁 |
| 文件大?。?/td> | 477K |
| 代理商： | LM9072 |
Application Information
(Continued)
RESET OUTPUT (Pin 6)
The Reset Output is an active low logic signal provided to re-
set a system microcontroller on power up and in the event
that the Main Output supply falls out of regulation. This out-
put is guaranteed to provide a logic low level (
<
0.4V) when-
ever the Main Output supply is below 4.45V or is pulled
above 5.75V. This general reset prevents erratic system op-
eration which may occur with out-of-specification supply po-
tentials.
The Reset Output has an active pull down to ground and a
passive pull-up (through a 4 k
resistor) to the Main Output
to ensure voltage compatibility with the system supply. Ca-
pacitive loading on this reset line will directly affect the rise
time of the reset signal. The Reset Output will maintain a
logic low level with a Main Output voltage of only 1V. Below
1V the active pull-down device switches OFF (sink current of
only 500 μA), but with such a low supply potential, system
controllers are generally inoperative.
The Reset Output has a built-in delay time interval which is
programmable by the selection of the delay capacitor.
DELAY CAPACITOR (Pin 7)
The Delay Capacitor (C
) controls a time interval that
the Reset Output remains low after the Main Output has es-
tablished normal operating condition. This feature holds the
system in reset for a time to allow all load circuitry to be prop-
erly biased before executing functions. This interval is ap-
plied at power-up and following any event that may trigger
the system reset function.
Figure 2 illustrates the delayed reset generator. Two com-
parators continually monitor the Main Output supply. Window
comparators C1 and C2 detect if the Main Supply is below
4.6V or exceeds 5.5V typically. If this is true (at power-on, for
example) the control logic turns ON the discharge transistor
and holds C
low (at 0.9V). Comparator C4 then outputs
a logic low system Reset signal within 2 μs after detecting
the out of regulation condition.
The Delay Capacitor remains discharged until the window
comparator senses that the Main Output is within normal op-
erating range (C1 and C2 outputs are both low). When this
condition is met, the discharge transistor is turned OFF and
C
is charged positively by an internal 6 μA current
source. The Reset Output will remain low un the delay ca-
pacitor has reached 4V, at which point it will go high and the
system will begin normal operation. This delay time interval
is controlled by the selection of C
DELAY
and can be deter-
mined from the following equation:
T
DELAY
= (0.5 x 10
6
)
C
DELAY
Typically a 0.1 μF capacitor will produce a delay interval of
50 ms.
To ensure a consistent delay time interval, the discharge
transistor is always latched ON by the window comparators,
and can not be switched OFF to start a new delay interval
until C
has been discharged to less than or equal to
0.9V. This sets a fixed starting voltage (0.9V) and ending
voltage (4V) for the charging of the Delay Capacitor.
ON/OFF INPUT (Pin 1)
The ON/OFF Input enables both the Main and Tracking out-
puts. In a typical application this input is connected to the in-
put supply through a series resistor (nominally 22 k
) and a
switch (Ignition, as an example). When the switch is closed
this input is pulled high and switches ON both regulator out-
puts. This input is internally clamped to a 7V zener diode
through a series 1 k
resistor. The external series resistor
together with an optional 0.1 μF capacitor to ground are op-
tional to provide filtering and current limiting to withstand any
transients that may appear on the input supply in order to
maintain normal operation of the system.
The switching threshold of the ON/OFF comparator has 2V
of hysteresis to ensure noise free control of the system. To
turn the regulators ON this input must be taken above 4V. To
turn the system OFF the ON/OFF Input must be open cir-
cuited or taken below 2V.
DS012906-18
FIGURE 2. Reset Generator
L
www.national.com
7
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