LM2984T National Semiconductor, LM2984T Datasheet - Page 12

• Manufacturer Part Number: LM2984T
• Description: IC,VOLT REGULATOR,FIXED,+5V,BIPOLAR,ZIP,11PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2984T

• Rohs Compliant: NO

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Application Hints
effective capacitance to zero. To maintain regulator stability
down to −40˚C, capacitors rated at that temperature (such as
tantalums) must be used.
Each output must be terminated by a capacitor, even if it is
not used.
STANDBY OUTPUT
The standby output is intended for use in systems requiring
standby memory circuits. While the high current regulator
outputs are controlled with the ON/OFF pin described later,
the standby output remains on under all conditions as long
as sufficient input voltage is supplied to the IC. Thus,
memory and other circuits powered by this output remain un-
affected by positive line transients, thermal shutdown, etc.
The standby regulator circuit is designed so that the quies-
cent current to the IC is very low (
regulator outputs are off.
The capacitor on the output of this regulator can be in-
creased without bound. This will help maintain the output
voltage during negative input transients and will also help to
reduce the noise on all three outputs. Because the other two
track the standby output: therefore any noise reduction here
will also reduce the other two noise voltages.
BUFFER OUTPUT
The buffer output is designed to drive peripheral sensor cir-
cuitry in a µP system. It will track the standby and main regu-
lator within a few millivolts in normal operation. Therefore, a
peripheral sensor can be powered off this supply and have
the same operating voltage as the µP system. This is impor-
tant if a ratiometric sensor system is being used.
The buffer output can be short circuited while the other two
outputs are in normal operation. This protects the µP system
from disruption of power when a sensor wire, etc. is tempo-
rarily shorted to ground, i.e. only the sensor signal would be
interrupted, while the µP and memory circuits would remain
operational.
The buffer output is similar to the main output in that it is con-
trolled by the ON/OFF switch in order to save power in the
standby mode. It is also fault protected against overvoltage
and thermal overload. If the input voltage rises above ap-
proximately 30V (e.g. load dump), this output will automati-
cally shut down. This protects the internal circuitry and en-
ables the IC to survive higher voltage transients than would
otherwise be expected. Thermal shutdown is necessary
since this output is one of the dominant sources of power
dissipation in the IC.
MAIN OUTPUT
The main output is designed to power relatively large loads,
i.e. approximately 500 mA. It is therefore also protected
against overvoltage and thermal overload.
This output will track the other two within a few millivolts in
normal operation. It can therefore be used as a reference
voltage for any signal derived from circuitry powered off the
standby or buffer outputs. This is important in a ratiometric
sensor system or any system requiring accurate matching of
power supply voltages.
ON/OFF SWITCH
The ON/OFF switch controls the main output and the buffer
output. The threshold voltage is compatible with most logic
families and has about 20 mV of hysteresis to insure “clean”
(Continued)
<
1.5 mA) when the other
12
switching from the standby mode to the active mode and
vice versa. This pin can be tied to the input voltage through
a 10 k
ously.
POWER DOWN OVERRIDE
Another possible approach is to use a diode in series with
the ON/OFF signal and another in series with the main out-
put in order to maintain power for some period of time after
the ON/OFF signal has been removed (see Figure 1 ). When
the ON/OFF switch is initially pulled high through diode D1,
the main output will turn on and supply power through diode
D2 to the ON/OFF switch effectively latching the main out-
put. An open collector transistor Q1 is connected to the ON/
OFF pin along with the two diodes and forces the regulators
off after a period of time determined by the µP. In this way,
the µP can override a power down command and store data,
do housekeeping, etc. before reverting back to the standby
mode.
RESET OUTPUT
This output is an open collector NPN transistor which is
forced low whenever an error condition is present at the
main output or when a µP error is sensed (see µP Monitor
section). If the main output voltage drops by 350 mV or rises
out of regulation by 600 mV typically, the RESET output is
forced low and held low for a period of time set by two exter-
nal components, R
teresis in these two threshold voltages so that the RESET
output has a fast rise and fall time compatible with the re-
quirements of most µP RESET inputs.
DELAYED RESET
Resistor R
RESET output is held low after a main output error condition
has been sensed. The delay is given by the formula:
The delayed RESET will be initiated any time the main out-
put is out of regulation, i.e. during power-up, short circuit, ov-
ervoltage, low line, thermal shutdown or power-down. The
µP is therefore RESET whenever the output voltage is out of
regulation. (It is important to note that a RESET is only initi-
ated when the main output is in error. The buffer and standby
outputs are not directly monitored for error conditions.)
µP MONITOR RESET
There are two distinct and independent error monitoring sys-
tems in the LM2984. The one described above monitors the
main regulator output and initiates a delayed RESET when-
ever this output is in error. The other error monitoring system
is the µP watchdog. These two systems are OR’d together
internally and both force the RESET output low when either
type of error occurs.
resistor if the regulator is to be powered continu-
t
FIGURE 1. Power Down Override
and capacitor C
T
dly
t
and C
= 1.2 R
t
. There is a slight amount of hys-
t
t
set the period of time that the
C
t
(seconds)
DS011252-13