LM2984
Microprocessor Power Supply System
LM2984 Microprocessor Power Supply System
April 1998
General Description
The LM2984 positive voltage regulator features three independent and tracking outputs capable of delivering the
power for logic circuits, peripheral sensors and standby
memory in a typical microprocessor system. The LM2984 includes circuitry which monitors both itsownhigh-currentoutput and also an external µP. If any error conditions are
sensed in either, a reset error flag is set and maintained until
the malfunction terminates. Since these functions are included in the same package with the three regulators, a
great saving in board space can berealizedinthetypicalmicroprocessor system. The LM2984 also features very low
dropout voltages on each of its three regulator outputs (0.6V
at the rated output current). Furthermore, the quiescent current can be reduced to 1 mA in the standby mode.
Designed also for vehicular applications, the LM2984 and all
regulated circuitry are protected from reverse battery installations or 2-battery jumps. Familiar regulator features such
as short circuit and thermal overload protection are also provided. Fixed outputsof5Vare available in the plastic TO-220
power package.
Typical Application Circuit
Features
n Three low dropout tracking regulators
n Output current in excess of 500 mA
n Fully specified for −40˚C to +125˚C operation
n Low quiescent current standby regulator
n Microprocessor malfunction RESET flag
n Delayed RESET on power-up
n Accurate pretrimmed 5V outputs
n Reverse battery protection
n Overvoltage protection
n Reverse transient protection
n Short circuit protection
n Internal thermal overload protection
n ON/OFF switch for high current outputs
+
n P
Product Enhancement tested
C
must be at least 10 µF to maintain stability. May be increased without bound to maintain regulation during transients. Locate as close as possible to
OUT
the regulator. This capacitor must be rated over the same operating temperature range as the regulator. The equivalent series resistance (ESR) of this
capacitor is critical; see curve.
Order Number LM2984T
See NS Package Number TA11B
© 1998 National Semiconductor Corporation DS011252 www.national.com
DS011252-1
Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Input Voltage
Survival Voltage (
Operational Voltage 26V
<
100 ms) 60V
Operating Temperature Range (T
Maximum Junction Temperature
(Note 3) 150˚C
Storage Temperature Range −65˚C to +150˚C
Lead Temperature
(Soldering, 10 sec.) 230˚C
ESD Susceptability (Note 5) 2000V
) −40˚C to +125˚C
A
Internal Power Dissipation Internally Limited
Electrical Characteristics
=
V
14V, I
IN
temperature range, −40˚C ≤ T
(Pin 11)
V
OUT
Output Voltage 5 mA ≤ I
Line Regulation 9V ≤ VIN≤ 16V 2 25/25 mV
Load Regulation 5 mA ≤ I
Output Impedance 250 mAdcand 10 mA
Quiescent Current I
Output Noise Voltage 10 Hz–100 kHz, I
Long Term Stability 20 mV/1000 hr
Ripple Rejection f
Dropout Voltage I
Current Limit 0.92 0.75/0.60 A
Maximum Operational Continuous DC 32 26/26 V
Input Voltage
Maximum Line Transient V
Reverse Polarity V
Input Voltage DC
Reverse Polarity Input T ≤ 100 ms, R
Voltage Transient
OUT
=
5 mA, C
=
10 µF, unless otherwise indicated. Boldface type refers to limits over the entire operating
OUT
≤ +125˚C, all other limits are for T
A
=
A
=
T
25˚C (Note 8) .
j
Parameter Conditions Typical Limit Units
(Note 4)
≤ 500 mA 5.00 4.85/4.75 V
O
6V ≤ VIN≤ 26V 5.15/5.25 V
7V ≤ VIN≤ 26V 5 50/50 mV
≤ 500 mA 12 50/50 mV
OUT
=
f
120 Hz
o
=
500 mA 38 100/100 mA
OUT
=
I
250 mA 14 50/50 mA
OUT
=
120 Hz 70 60/50 dB
o
=
500 mA 0.53 0.80/1.1 V
OUT
=
I
250 mA 0.28 0.50/0.70 V
OUT
≤ 6V, R
OUT
OUT
OUT
≥ −0.6V, R
OUT
,24 mΩ
rms
=
100 mA 100 µV
OUT
=
100Ω,T≤100 ms 65 60/60 V
=
100Ω −30 −15/−15 V
OUT
=
100Ω −55 −35/−35 V
min
max
max
max
max
max
max
min
max
max
min
min
min
min
min
Electrical Characteristics
=
V
IN
perature range, −40˚C ≤ T
V
buffer
Output Voltage 5 mA ≤ I
Line Regulation 9V ≤ VIN≤ 16V 2 25/25 mV
Load Regulation 5 mA ≤ I
Output Impedance 50 mAdcand 10 mA
Quiescent Current I
www.national.com 2
14V, I
buf
=
5 mA, C
=
10 µF, unless otherwise indicated. Boldface type refers to limits over the entire operating tem-
buf
≤ +125˚C, all other limits are for T
A
=
A
=
T
25˚C (Note 8) .
j
Parameter Conditions Typical Limit Units
(Note 4)
(Pin 10)
≤ 100 mA 5.00 4.85/4.75 V
O
6V ≤ VIN≤ 26V 5.15/5.25 V
7V ≤ VIN≤ 26V 5 50/50 mV
≤ 100 mA 15 50/50 mV
buf
=
f
120 Hz
O
=
100 mA 8.0 15/15 mA
buf
, 200 mΩ
rms
min
max
max
max
max
max
Electrical Characteristics (Continued)
=
V
IN
perature range, −40˚C ≤ T
V
buffer
Output Noise Voltage 10 Hz–100 kHz, I
Long Term Stability 20 mV/1000 hr
Ripple Rejection f
Dropout Voltage I
Current Limit 0.23 0.15/0.15 A
Maximum Operational Continuous DC 32 26/26 V
Input Voltage
Maximum Line V
Transient T ≤ 100 ms
Reverse Polarity V
Input Voltage DC
Reverse Polarity Input T ≤ 100 ms, R
Voltage Transient
14V, I
buf
=
5 mA, C
=
10 µF, unless otherwise indicated. Boldface type refers to limits over the entire operating tem-
buf
≤ +125˚C, all other limits are for T
A
=
A
=
T
25˚C (Note 8) .
j
Parameter Conditions Typical Limit Units
(Note 4)
(Pin 10)
=
100 mA 100 µV
OUT
=
120 Hz 70 60/50 dB
o
=
100 mA 0.35 0.50/0.80 V
buf
≤ 6V, R
buf
≥ −0.6V, R
buf
=
100Ω, 65 60/60 V
buf
=
100Ω −30 −15/−15 V
buf
=
100Ω −55 −35/−35 V
buf
min
max
min
min
min
min
min
Electrical Characteristics
=
V
14V, I
IN
perature range, −40˚C ≤ T
V
standby
Output Voltage 1 mA ≤ I
Line Regulation 9V ≤ VIN≤ 16V 2 25/25 mV
Load Regulation 0.5 mA ≤ I
Output Impedance 5 mAdcand1mA
Quiescent Current I
Output Noise Voltage 10 Hz–100 kHz, I
Long Term Stability 20 mV/1000 hr
Ripple Rejection f
Dropout Voltage I
Current Limit 15 12/12 mA
Maximum Operational 4.5V ≤ V
Input Voltage R
Maximum Line V
Transient R
Reverse Polarity V
Input Voltage DC R
Reverse Polarity Input T ≤ 100 ms, R
Voltage Transient
stby
=
1 mA, C
=
10 µF, unless otherwise indicated. Boldface type refers to limits over the entire operating tem-
stby
≤ +125˚C, all other limits are for T
A
=
A
=
T
25˚C (Note 8) .
j
Parameter Conditions Typical Limit Units
(Note 4)
(Pin 9)
≤ 7.5 mA 5.00 4.85/4.75 V
O
6V ≤ VIN≤ 26V 5.15/5.25 V
7V ≤ VIN≤ 26V 5 50/50 mV
≤ 7.5 mA 6 50/50 mV
OUT
=
7.5 mA 1.2 2.0/4.0 mA
stby
=
I
2 mA 0.9 1.5/4.0 mA
stby
=
120 Hz 70 60/50 dB
o
=
1 mA 0.26 0.50/0.60 V
stby
=
I
7.5 mA 0.38 0.60/0.70 V
stby
≤ 6V, 65 60/60 V
stby
=
1000Ω
stby
≤ 6V, T ≤ 100 ms, 65 60/60 V
stby
=
1000Ω
stby
≥ −0.6V, −30 −15/−15 V
stby
=
1000Ω
stby
stby
=
120 Hz 0.9 Ω
rms,fo
=
1 mA 100 µV
stby
=
1000Ω −55 −35/−35
min
max
max
max
max
max
max
min
max
max
min
min
min
V
min
min
3 www.national.com
Electrical Characteristics
=
V
14V, C
IN
tire operating temperature range, −40˚C ≤ T
Parameter Conditions Typical Limit Units
Tracking and Isolation
Tracking I
V
OUT–Vstby
Tracking I
V
buf–Vstby
Tracking I
V
OUT–Vbuf
Isolation (Note 1) R
V
from V
buf
Isolation (Note 1) R
V
from V
stby
Isolation (Note 1) R
V
from V
OUT
Isolation (Note 1) R
V
from V
stby
Note 1: Isolation refers to the ability of the specified output to remain within the tested limits when the other output is shorted to ground.
OUT
OUT
buf
OUT
=
10 µF, C
buf
=
10 µF, C
=
10 µF, unless otherwise indicated. Boldface type refers to limits over the en-
stby
≤ +125˚C, all other limits are for T
A
=
A
=
T
25˚C (Note 8) .
j
(Note 4)
≤ 500 mA, I
OUT
I
≤ 7.5 mA
stby
=
5 mA, I
OUT
I
≤ 7.5 mA
stby
≤ 500 mA, I
OUT
=
I
1mA
stby
=
1Ω,I
OUT
=
1Ω,I
OUT
=
1Ω,I
buf
buf
=
1Ω,I
buf
=
5 mA,
buf
≤ 100 mA,
buf
≤ 100 mA,
buf
≤ 100 mA 5.00 4.50/4.50 V
buf
≤ 7.5 mA 5.00 4.50/4.50 V
stby
≤ 500 mA 5.00 4.50/4.50 V
OUT
≤ 7.5 mA 5.00 4.50/4.50 V
stby
±
30
±
30
±
30
±
100/±100 mV
±
100/±100 mV
±
100/±100 mV
5.50/5.50 V
5.50/5.50 V
5.50/5.50 V
5.50/5.50 V
max
max
max
min
max
min
max
min
max
min
max
Electrical Characteristics
=
V
14V, I
IN
Boldface type refers to limits over the entire operating temperature range, −40˚C ≤ T
=
25˚C (Note 8)
Computer Monitor/Reset Functions
Low V
I
reset
V
Low V
reset
R
t voltage
Power On Reset VµP
Delay (T
∆V
Low (Note 6) −350 −225/−175 mV
OUT
Reset Threshold −500/−550 mV
∆V
High (Note 6) 600 225/175 mV
OUT
Reset Threshold 750/800 mV
Reset Output VµP
Leakage
µP
Input Current (Pin 4) VµP
mon
µP
Input 1.22 0.80/0.80 V
mon
Threshold Voltage 1.22 2.00/2.00 V
µP Monitor Reset VµP
Oscillator Period (T
µP Monitor Reset VµP
Oscillator Pulse Width (RESET
Minimum µP Monitor (Note 7) 2 µs
Input Pulse Width
Reset Fall Time R
OUT
=
5 mA, I
=
5 mA, I
buf
stby
=
5 mA, R
=
130 kΩ,C
t
=
0.33 µF, C
t
=
0.47 µF, unless otherwise indicated,
mon
≤ +125˚C, all other limits are for T
A
A
Parameter Conditions Typical Limit Units
(Note 4)
=
IN
=
IN
4V, V
4V, I
=
0.4V 5 2/0.50 mA
rst
=
1 mA 0.10 0.40/0.40 V
rst
(Pin 2) 1.22 1.15/0.75 V
1.22 1.30/2.00 V
=
5V 50 45/17.0 ms
mon
VµP
rst
dly
mon
mon
mon
mon
window
mon
=
=
10k, V
) 50 55/80.0 ms
1.2 R
tCt
=
=
=
=
=
=
pw
=
5V, V
12V 0.01 1/5.0 µA
rst
2.4V 7.5 25/25 µA
0.4V 0.01 10/15 µA
0V 50 45/30 ms
0.82 R
) 50 55/70 ms
tCmon
0V 1.0 0.7/0.4 ms
=
2000 C
rst
) 1.0 1.3/2.10 ms
mon
=
≤ 10 pF 0.20 1.00/1.00 µs
5V, C
rst
max
max
max
min
min
max
max
max
max
max
max
max
max
max
=
T
J
min
min
min
min
min
min
www.national.com 4
Electrical Characteristics (Continued)
=
V
14V, I
IN
Boldface type refers to limits over the entire operating temperature range, −40˚C ≤ T
=
25˚C (Note 8)
Computer Monitor/Reset Functions
Reset Rise Time R
On/Off Switch Input V
Current (Pin 8) V
On/Off Switch Input 1.22 0.80/0.80 V
Threshold Voltage 1.22 2.00/2.00 V
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating
the device beyond its specified operating ratings.
Note 3: Thermal resistance without a heatsink for junction-to-case temperature is 3˚C/W. Thermal resistance case-to-ambient is 40˚C/W.
Note 4: Tested Limits are guaranteed and 100%production tested.
Note 5: Human body model, 100 pF capacitor discharged through a 1500Ω resistor.
Note 6: Internal comparators detect when the main regulator output (V
High or ∆V
∆V
OUT
to go high again after a delay set by R
Note 7: This parameter is a measure of how short a pulse can be detected at the µP Monitor Input. This parameter is primarily influenced by the value of C
Application Hints Section.)
Note 8: To ensure constant junction temperature, low duty cycle pulse testing is used.
OUT
=
5 mA, I
=
5 mA, I
buf
stby
=
5 mA, R
=
130 kΩ,C
t
=
0.33 µF, C
t
=
0.47 µF, unless otherwise indicated,
mon
≤ +125˚C, all other limits are for T
A
A
Parameter Conditions Typical Limit Units
(Note 4)
=
rst
=
ON
=
ON
Low, and set the Reset Error Flag low. The Reset Error Flag is held low until V
OUT
and Ct. (see application section).
t
10k, V
=
rst
≤ 10 pF 0.60 1.00/1.50 µs
5V, C
rst
2.4V 7.5 25/25 µA
0.4V 0.01 10/10 µA
) changes from the measured output voltage (with V
OUT
returns to regulation. The Reset Error Flag is then allowed
OUT
=
14V) by the specified amount,
IN
mon
Block Diagram
=
max
max
max
min
max
. (See
T
J
DS011252-2
5 www.national.com
Pin Description
Pin No. Pin Name Comments
1V
2R
3C
4µP
5C
6 Ground Regulator ground
7 Reset Reset error flag output
8 ON/OFF Enables/disables high current regulators
9V
10 V
11 V
External Components
IN
t
t
mon
mon
standby
buffer
OUT
Positive supply input voltage
Sets internal timing currents
Sets power-up reset delay timing
Microcomputer monitor input
Sets µC monitor timing
Standby regulator output (7.5 mA)
Buffer regulator output (100 mA)
Main regulator output (500 mA)
Component Typical Value Component
Range
C
IN
R
t
C
t
C
tc
R
tc
C
mon
R
rst
C
stby
C
buf
C
OUT
1 µF 0.47 µF–10 µF Required if device is located far from power supply filter.
130k 24k–510k Sets internal timing currents.
0.33 µF 0.033 µF–3.3 µF Sets power-up reset delay.
0.01 µF 0.001 µF–0.1 µF Establishes time constant of AC coupled computer monitor.
10k 1k–100k Establishes time constant of AC coupled computer monitor. (See
0.47 µF 0.047 µF–4.7 µF Sets time window for computer monitor. Also determines period and pulse
10k 5k–100k Load for open collector reset output. Determined by computer reset input
10 µF 10 µF–no bound A 10 µF is required for stability but larger values can be used to maintain
10 µF 10 µF–no bound A 10 µF is required for stability but larger values can be used to maintain
10 µF 10 µF–no bound A 10 µF is required for stability but larger values can be used to maintain
Comments
applications section.)
width of computer malfunction reset. (See applications section.)
requirements.
regulation during transient conditions.
regulation during transient conditions.
regulation during transient conditions.
www.national.com 6
Typical Circuit Waveforms
Connection Diagram
DS011252-3
DS011252-4
Order Number LM2984T
See NS Package Number TA11B
7 www.national.com
Typical Performance Characteristics
Dropout Voltage (V
Dropout Voltage (V
OUT
OUT
Peak Output Current (V
)
)
OUT
DS011252-16
DS011252-19
)
Dropout Voltage (V
Dropout Voltage (V
buf
buf
Peak Output Current (V
)
DS011252-17
)
DS011252-20
)
buf
Dropout Voltage (V
Dropout Voltage (V
stby
stby
Peak Output Current (V
)
)
stby
DS011252-18
DS011252-21
)
DS011252-22
Quiescent Current (V
OUT
)
DS011252-25
Quiescent Current (V
www.national.com 8
stby
DS011252-24
)
DS011252-27
DS011252-23
)
buf
DS011252-26
Quiescent Current (V
Typical Performance Characteristics (Continued)
Quiescent Current (V
Quiescent Current (V
Output Voltage (V
OUT
OUT
OUT
)
)
DS011252-28
)
DS011252-31
Quiescent Current (V
Quiescent Current (V
Output Voltage (V
buf
)
buf
DS011252-29
)
buf
DS011252-32
)
Quiescent Current (V
Quiescent Current (V
Output Voltage (V
stby
stby
stby
)
)
DS011252-30
)
DS011252-33
Low Voltage Behavior (V
DS011252-34
OUT
DS011252-37
DS011252-35
)
Low Voltage Behavior (V
)
buf
DS011252-38
Low Voltage Behavior (V
DS011252-36
stby
DS011252-39
)
9 www.national.com
Typical Performance Characteristics (Continued)
Line Transient
Response (V
OUT
)
Load Transient
Response (V
OUT
)
Output Impedance (V
OUT
DS011252-40
DS011252-43
)
Line Transient
Response (V
)
buf
Load Transient
Response (V
)
buf
Output Impedance (V
Line Transient
Response (V
DS011252-41
stby
)
DS011252-42
Load Transient
Response (V
DS011252-44
)
buf
Output Impedance (V
stby
)
DS011252-45
)
stby
DS011252-46
www.national.com 10
DS011252-47
DS011252-48
Typical Performance Characteristics (Continued)
Ripple Rejection (V
OUT
Output Voltage
Output Capacitor ESR
(Standby Output, Pin 9)
)
DS011252-49
Ripple Rejection (V
DS011252-8
Output Capacitor ESR
(Buffer Output, Pin 10)
)
buf
DS011252-50
Device Dissipation vs
Ambient Temperature
Ripple Rejection (V
stby
DS011252-9
Output Capacitor ESR
(Main Output, Pin 11)
)
DS011252-51
DS011252-10
Application Hints
OUTPUT CAPACITORS
The LM2984 output capacitors are required for stability.
Without them, the regulator outputs will oscillate, sometimes
by many volts. Though the 10 µF shown are the minimum
recommended values, actual size and type may vary depending upon the application load and temperature range.
Capacitor effective series resistance (ESR) also affects the
IC stability. Since ESR varies from one brand to the next,
some bench work may be required to determine the mini-
DS011252-11
DS011252-12
mum capacitor value to use in production. Worst case is usually determined at the minimum ambient temperature and
the maximum load expected.
Output capacitors can be increased in size to any desired
value above the minimum. One possible purpose of this
would be to maintain the output voltages during brief conditions of negative input transients that might be characteristic
of a particular system.
Capacitors must also be rated at all ambient temperatures
expected in the system. Many aluminum type electrolytics
will freeze at temperatures less than −30˚C, reducing their
11 www.national.com
Application Hints (Continued)
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 unaffected by positive line transients, thermal shutdown, etc.
The standby regulator circuit is designed so that the quiescent current to the IC is very low (
regulator outputs are off.
The capacitor on the output of this regulator can be increased 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 circuitry in a µP system. It will track the standby and main regulator 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 important 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 temporarily 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 controlled 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 approximately 30V (e.g. load dump), this output will automatically shut down. This protects the internal circuitry and enables 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”
<
1.5 mA) when the other
switching from the standby mode to the active mode and
vice versa. This pin can be tied to the input voltage through
a10kΩresistor if the regulator is to be powered continu-
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 output 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 output. 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.
DS011252-13
FIGURE 1. Power Down Override
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 external components, R
teresis in these two threshold voltages so that the RESET
and Ct. There is a slight amount of hys-
t
output has a fast rise and fall time compatible with the requirements of most µP RESET inputs.
DELAYED RESET
Resistor R
RESET output is held low after a main output error condition
and capacitor Ctset the period of time that the
t
has been sensed. The delay is given by the formula:
=
T
dly
1.2 R
(seconds)
tCt
The delayed RESET will be initiated any time the main output is out of regulation, i.e. during power-up, short circuit, overvoltage, 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 initiated 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 systems in the LM2984. The one described above monitors the
main regulator output and initiates a delayed RESET whenever 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.
www.national.com 12
Application Hints (Continued)
This watchdog circuitry continuously monitors a pin on the
µP that generates a positive going pulse during normal operation. The period of this pulse is typically on the order of
milliseconds and the pulse width is typically on the order of
10’s of microseconds. If this pulse ever disappears, the
watchdog circuitry will time out and a RESET low will be sent
to the µP. The time out period is determined by two external
components, R
The width of the RESET pulse is set by C
resistor according to the following:
A square wave signal can also be monitored for errors by filtering the C
signal are detected.
typical circuit used to differentiate the input signal. Resistor
R
and capacitor Ctcpass only the rising edge of the square
tc
wave and create a short positive pulse suitable for the µP
and C
t
T
window
RESET
input such that only the positive edges of the
mon
, according to the formula:
mon
=
0.82 R
=
pw
Figure 2
2000 C
(seconds)
tCmon
(seconds)
mon
is a schematic diagram of a
and an internal
mon
monitor input. If the incoming signal continues in a high state
or in a low state for too long a period of time, a RESET low
will be generated.
DS011252-14
FIGURE 2. Monitoring Square Wave µP Signals
The threshold voltage and input characteristics of this pin are
compatible with nearly all logic families.
There is a limit on the width of a pulse that can be reliably detected by the watchdog circuit. This is due to the output resistance of the transistor which discharges C
state is detected at the input. The minimum detectable pulse
when a high
mon
width can be determined by the following formula:
=
PW
min
20 C
mon
(seconds)
13 www.national.com
Equivalent Schematic Diagram
DS011252-15
www.national.com 14
15
Physical Dimensions inches (millimeters) unless otherwise noted
LM2984 Microprocessor Power Supply System
Molded TO-220 Package (TA)
Order Number LM2984T
NS Package Number TA11B
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