lm9070t National Semiconductor Corporation, lm9070t Datasheet - Page 9

• Manufacturer Part Number: lm9070t
• Description: Low-dropout System Voltage Regulator With Keep-alive On/off Control
• Manufacturer: National Semiconductor Corporation

Application Information
The LM9070 voltage regulator has been optimized for use in
microprocessor based automotive systems. Several unique
design features have been incorporated to address many
FMEA (Failure Mode Effects Analysis) concerns for fail-safe
system performance.
FAULT TOLERANT FEATURES
While not specifically guaranteed due to production testing
limitations, the LM9070 has been tested and shown to con-
tinue to provide a regulated output and, not generate an
erroneous system reset signal while subjected to high levels
of RF electric field energy (up to 300 V/m signal strength
over a 2 MHz to 400 MHz frequency range). This is very
important in vehicle safety related applications where the
system must continue to operate normally. To maintain this
immunity to RFI the output bypass capacitor is important
(47 µF is recommended).
This regulator is suitable for applications where continuous
connection to the battery is required (Refer to the Typical
Application Circuit ). ON/OFF control of the regulator and
system can be accomplished by switching the ON/OFF input
to the battery or ignition supply V
switch. If this input becomes open circuited, an internal
pull-down resistor ensures that the regulator turns OFF.
When the regulator is switched OFF the current load on the
battery drops to less than 60 µA. With the possibility in many
applications for V
nected in a system through long lengths of wire, the ESD
protection of these pins has been increased to 12 kV with the
addition of small input bypass capacitors.
An output bypass capacitor of at least 10 µF is required for
stability (47 µF is recommended). The ESR of this capacitor
should be less than 3Ω. An input capacitor of 1 µF or larger
is recommended to improve line transient and noise perfor-
mance.
With the Keep-Alive input, a system microprocessor has the
ability to keep the regulator ON (with a logic “0” on Keep-
Alive) after the ON/OFF input has been commanded OFF. A
power-down sequence, when system variables are typically
stored in programmable memory, can be executed and take
as much time as necessary. At the end of the operation the
micro then pulls Keep-Alive high and the regulator and sys-
tem turn OFF and revert to the low quiescent current standby
mode.
For additional system reliability, consideration has been
made for the possibility of a short circuited load at the output
of the regulator. When the regulator is switched ON, conven-
FIGURE 1. Typical Application Circuit
IN
and the ON/OFF input pins to be con-
IN
supply through a SPST
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tional current limiting and thermal shutdown protect the regu-
lator. When the regulator is switched OFF however, a
grounded V
lator output) will force the Keep-Alive input to be low and
thus try to maintain the Keep-Alive mode of operation. With
a shorted load, the drain on the battery could be as high as
1.5A. A separate internal circuit monitors the output voltage
of the regulator. If V
with a shorted load, the Keep-Alive function is logically dis-
abled to ensure that the regulator turns OFF and reverts to
only a 50 µA load on the battery.
Conventional load dump protection is built in to withstand up
to +60V and −50V transients. A 1 kΩ resistor in series with
the ON/OFF and Keep-Alive inputs are recommended to
provide the same level of transient protection for these pins
if required. Protection against reverse polarity battery con-
nections is also built in. With a reversed battery the output of
the LM9070 will not go more negative than one diode drop
below ground. This will prevent damage to any of the 5V load
circuits.
For applications where the control logic is not required the
logic pins should be configured as shown in Figure 2. A
separate device, called the LM9071, can be used. The
LM9071 is available in a 5-lead TO-220 package and does
not provide control logic functions, but still retains all of the
protection features of the LM9070.
RESET FLAG
Excessive loading of the output to the point where the output
voltage drops by 300 mV to 500 mV will signal a reset flag to
the micro. This will warn of a V
unpredictable operation of the system. On power-up and
recovery from a fault condition the delay capacitor is used to
hold the micro in a reset condition for a programmable time
interval to allow the system operating voltages and
clock to stabilize before executing code. The typical delay
time interval can be estimated using the following equation:
INPUT STABILITY
Low dropout voltage regulators which utilize a PNP power
transistor usually exhibit a large increase in current when in
dropout (V
ration characteristics (β reduction) of the PNP transistor. To
significantly minimize this increase in current the LM9070
detects when the PNP enters saturation and reduces the
operating current.
This reduction in input current can create a stability problem
in applications with higher load current (
the input voltage is applied through a long length of wire,
which in effect adds a significant amount of inductance in
IN
FIGURE 2. Control Logic Not Used
CC
<
supply to the micro (due to the shorted regu-
5.5V). This increase is caused by the satu-
OUT
is less than 4V, as would be the case
CC
supply that may produce
>
100 mA) where
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