lm9072 National Semiconductor Corporation, lm9072 Datasheet - Page 7

• Manufacturer Part Number: lm9072
• Description: Dual Tracking Low-dropout System Regulator
• Manufacturer: National Semiconductor Corporation

Application Information
RESET OUTPUT (Pin 6)
The Reset Output is an active low logic signal provided to
reset a system microcontroller on power up and in the event
that the Main Output supply falls out of regulation. This
output is guaranteed to provide a logic low level (
whenever the Main Output supply is below 4.45V or is pulled
above 5.75V. This general reset prevents erratic system
operation which may occur with out-of-specification supply
potentials.
The Reset Output has an active pull down to ground and a
passive pull-up (through a 4 k
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
the Reset Output remains low after the Main Output has
established normal operating condition. This feature holds
the system in reset for a time to allow all load circuitry to be
properly biased before executing functions. This interval is
applied 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
a logic low system Reset signal within 2 µs after detecting
the out of regulation condition.
DELAY
low (at 0.9V). Comparator C4 then outputs
DELAY
) controls a time interval that
resistor) to the Main Output
(Continued)
FIGURE 2. Reset Generator
<
0.4V)
7
The Delay Capacitor remains discharged until the window
comparator senses that the Main Output is within normal
operating range (C1 and C2 outputs are both low). When this
condition is met, the discharge transistor is turned OFF and
C
source. The Reset Output will remain low un the delay
capacitor 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
determined from the following equation:
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
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
outputs. In a typical application this input is connected to the
input 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
outputs. This input is internally clamped to a 7V zener diode
through a series 1 k
together with an optional 0.1 µF capacitor to ground are
optional 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.
DELAY
DELAY
is charged positively by an internal 6 µA current
has been discharged to less than or equal to
T
DELAY
= (0.5 x 10
resistor. The external series resistor
6
) • C
DELAY
DS012906-18
DELAY
and can be
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