LM2984 [NSC]
Microprocessor Power Supply System; 微处理器电源系统
| 型号： | LM2984 |
| 厂家： | 
National Semiconductor |
| 描述： | Microprocessor Power Supply System |
| 文件： | 总16页 (文件大小：402K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
April 1998
LM2984
Microprocessor Power Supply System
General Description
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
The LM2984 positive voltage regulator features three inde-
pendent and tracking outputs capable of delivering the
power for logic circuits, peripheral sensors and standby
memory in a typical microprocessor system. The LM2984 in-
cludes circuitry which monitors both its own high-current out-
put 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 in-
cluded in the same package with the three regulators, a
great saving in board space can be realized in the typical mi-
croprocessor 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 cur-
rent can be reduced to 1 mA in the standby mode.
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
Designed also for vehicular applications, the LM2984 and all
regulated circuitry are protected from reverse battery instal-
lations or 2-battery jumps. Familiar regulator features such
as short circuit and thermal overload protection are also pro-
vided. Fixed outputs of 5V are available in the plastic TO-220
power package.
Typical Application Circuit
DS011252-1
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
Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Operating Temperature Range (TA)
Maximum Junction Temperature
(Note 3)
−40˚C to +125˚C
150˚C
Storage Temperature Range
Lead Temperature
−65˚C to +150˚C
Input Voltage
(Soldering, 10 sec.)
230˚C
2000V
<
Survival Voltage ( 100 ms)
60V
26V
ESD Susceptability (Note 5)
Operational Voltage
Internal Power Dissipation
Internally Limited
Electrical Characteristics
=
=
=
VIN 14V, IOUT 5 mA, COUT 10 µF, unless otherwise indicated. Boldface type refers to limits over the entire operating
=
=
temperature range, −40˚C ≤ TA ≤ +125˚C, all other limits are for TA Tj 25˚C (Note 8) .
Parameter
Conditions
Typical
Limit
Units
(Note 4)
VOUT (Pin 11)
Output Voltage
Line Regulation
5 mA ≤ IO ≤ 500 mA
5.00
4.85/4.75
5.15/5.25
25/25
Vmin
Vmax
6V ≤ VIN ≤ 26V
9V ≤ VIN ≤ 16V
2
5
mVmax
mVmax
mVmax
mΩ
7V ≤ VIN ≤ 26V
50/50
Load Regulation
5 mA ≤ IOUT ≤ 500 mA
250 mAdc and 10 mArms
12
24
50/50
Output Impedance
,
=
fo 120 Hz
=
Quiescent Current
IOUT 500 mA
38
14
100/100
50/50
mAmax
mAmax
µV
=
IOUT 250 mA
=
Output Noise Voltage
Long Term Stability
Ripple Rejection
10 Hz–100 kHz, IOUT 100 mA
100
20
mV/1000 hr
dBmin
Vmax
=
fo 120 Hz
70
60/50
0.80/1.1
0.50/0.70
0.75/0.60
26/26
=
Dropout Voltage
IOUT 500 mA
0.53
0.28
0.92
32
=
IOUT 250 mA
Vmax
Current Limit
Amin
Maximum Operational
Input Voltage
Continuous DC
Vmin
=
Maximum Line Transient
Reverse Polarity
VOUT ≤ 6V, ROUT 100Ω, T ≤ 100 ms
65
60/60
Vmin
Vmin
=
VOUT ≥ −0.6V, ROUT 100Ω
−30
−15/−15
Input Voltage DC
Reverse Polarity Input
Voltage Transient
=
T ≤ 100 ms, ROUT 100Ω
−55
−35/−35
Vmin
Electrical Characteristics
=
=
=
VIN 14V, Ibuf 5 mA, Cbuf 10 µF, unless otherwise indicated. Boldface type refers to limits over the entire operating tem-
=
=
perature range, −40˚C ≤ TA ≤ +125˚C, all other limits are for TA Tj 25˚C (Note 8) .
Parameter
Conditions
Typical
Limit
Units
(Note 4)
Vbuffer (Pin 10)
Output Voltage
Line Regulation
5 mA ≤ IO ≤ 100 mA
5.00
4.85/4.75
5.15/5.25
25/25
Vmin
Vmax
6V ≤ VIN ≤ 26V
9V ≤ VIN ≤ 16V
2
5
mVmax
mVmax
mVmax
mΩ
7V ≤ VIN ≤ 26V
50/50
Load Regulation
5 mA ≤ Ibuf ≤ 100 mA
50 mAdc and 10 mArms
15
200
50/50
Output Impedance
,
=
fO 120 Hz
=
Quiescent Current
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Ibuf 100 mA
8.0
15/15
mAmax
2
Electrical Characteristics (Continued)
=
=
=
VIN 14V, Ibuf 5 mA, Cbuf 10 µF, unless otherwise indicated. Boldface type refers to limits over the entire operating tem-
=
=
perature range, −40˚C ≤ TA ≤ +125˚C, all other limits are for TA Tj 25˚C (Note 8) .
Parameter
Conditions
Typical
Limit
Units
(Note 4)
Vbuffer (Pin 10)
=
Output Noise Voltage
Long Term Stability
Ripple Rejection
Dropout Voltage
Current Limit
10 Hz–100 kHz, IOUT 100 mA
100
20
µV
mV/1000 hr
dBmin
=
fo 120 Hz
70
60/50
0.50/0.80
0.15/0.15
26/26
=
Ibuf 100 mA
0.35
0.23
32
Vmax
Amin
Maximum Operational
Input Voltage
Continuous DC
Vmin
=
Maximum Line
Vbuf ≤ 6V, Rbuf 100Ω,
65
60/60
Vmin
Vmin
Vmin
Transient
T ≤ 100 ms
=
Reverse Polarity
Input Voltage DC
Reverse Polarity Input
Voltage Transient
Vbuf ≥ −0.6V, Rbuf 100Ω
−30
−55
−15/−15
−35/−35
=
T ≤ 100 ms, Rbuf 100Ω
Electrical Characteristics
=
=
=
VIN 14V, Istby 1 mA, Cstby 10 µF, unless otherwise indicated. Boldface type refers to limits over the entire operating tem-
=
=
perature range, −40˚C ≤ TA ≤ +125˚C, all other limits are for TA Tj 25˚C (Note 8) .
Parameter
Conditions
Typical
Limit
Units
(Note 4)
Vstandby (Pin 9)
Output Voltage
Line Regulation
1 mA ≤ IO ≤ 7.5 mA
5.00
4.85/4.75
5.15/5.25
25/25
Vmin
Vmax
6V ≤ VIN ≤ 26V
9V ≤ VIN ≤ 16V
2
5
mVmax
mVmax
mVmax
Ω
7V ≤ VIN ≤ 26V
50/50
Load Regulation
Output Impedance
Quiescent Current
0.5 mA ≤ IOUT ≤ 7.5 mA
6
50/50
=
5 mAdc and 1 mArms, fo 120 Hz
0.9
1.2
0.9
100
20
=
Istby 7.5 mA
2.0/4.0
1.5/4.0
mAmax
mAmax
µV
=
Istby 2 mA
10 Hz–100 kHz, Istby 1 mA
=
Output Noise Voltage
Long Term Stability
Ripple Rejection
mV/1000 hr
dBmin
Vmax
=
fo 120 Hz
70
60/50
0.50/0.60
0.60/0.70
12/12
=
Dropout Voltage
Istby 1 mA
0.26
0.38
15
=
Istby 7.5 mA
Vmax
Current Limit
mAmin
Vmin
Maximum Operational
Input Voltage
4.5V ≤ Vstby ≤ 6V,
65
60/60
=
Rstby 1000Ω
Maximum Line
Vstby ≤ 6V, T ≤ 100 ms,
65
60/60
Vmin
=
Transient
Rstby 1000Ω
Reverse Polarity
Input Voltage DC
Reverse Polarity Input
Voltage Transient
Vstby ≥ −0.6V,
−30
−55
−15/−15
−35/−35
Vmin
=
Rstby 1000Ω
=
T ≤ 100 ms, Rstby 1000Ω
Vmin
3
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Electrical Characteristics
=
=
=
=
VIN 14V, COUT 10 µF, Cbuf 10 µF, Cstby 10 µF, unless otherwise indicated. Boldface type refers to limits over the en-
=
=
tire operating temperature range, −40˚C ≤ TA ≤ +125˚C, all other limits are for TA Tj 25˚C (Note 8) .
Parameter
Conditions
Typical
Limit
Units
(Note 4)
Tracking and Isolation
Tracking
=
±
±
±
±
±
±
±
IOUT ≤ 500 mA, Ibuf 5 mA,
30
30
30
100/ 100
mVmax
mVmax
mVmax
VOUT–Vstby
Istby ≤ 7.5 mA
=
±
100/ 100
Tracking
IOUT 5 mA, Ibuf ≤ 100 mA,
Vbuf–Vstby
Istby ≤ 7.5 mA
±
100/ 100
Tracking
IOUT ≤ 500 mA, Ibuf ≤ 100 mA,
=
Istby 1 mA
VOUT–Vbuf
=
Isolation (Note 1)
Vbuf from VOUT
Isolation (Note 1)
Vstby from VOUT
Isolation (Note 1)
VOUT from Vbuf
Isolation (Note 1)
Vstby from Vbuf
ROUT 1Ω, Ibuf ≤ 100 mA
5.00
5.00
5.00
5.00
4.50/4.50
5.50/5.50
4.50/4.50
5.50/5.50
4.50/4.50
5.50/5.50
4.50/4.50
5.50/5.50
Vmin
Vmax
Vmin
Vmax
Vmin
Vmax
Vmin
Vmax
=
ROUT 1Ω, Istby ≤ 7.5 mA
=
Rbuf 1Ω, IOUT ≤ 500 mA
=
Rbuf 1Ω, Istby ≤ 7.5 mA
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.
Electrical Characteristics
=
=
=
=
=
=
=
VIN 14V, IOUT 5 mA, Ibuf 5 mA, Istby 5 mA, Rt 130 kΩ, Ct 0.33 µF, Cmon 0.47 µF, unless otherwise indicated,
=
Boldface type refers to limits over the entire operating temperature range, −40˚C ≤ TA ≤ +125˚C, all other limits are for TA TJ
=
25˚C (Note 8)
Parameter
Conditions
Typical
Limit
Units
(Note 4)
Computer Monitor/Reset Functions
=
=
Ireset Low
Vreset Low
Rt voltage
VIN 4V, Vrst 0.4V
5
2/0.50
0.40/0.40
1.15/0.75
1.30/2.00
45/17.0
mAmin
Vmax
=
=
VIN 4V, Irst 1 mA
0.10
1.22
1.22
50
(Pin 2)
Vmin
Vmax
=
Power On Reset
Delay
VµPmon 5V
msmin
msmax
mVmin
mVmax
mVmin
mVmax
µAmax
=
(Tdly 1.2 Rt Ct)
50
55/80.0
∆VOUT Low
(Note 6)
−350
−225/−175
−500/−550
225/175
750/800
1/5.0
Reset Threshold
∆VOUT High
(Note 6)
600
Reset Threshold
Reset Output
Leakage
=
=
VµPmon 5V, Vrst 12V
0.01
=
µPmon Input Current (Pin 4)
VµPmon 2.4V
7.5
0.01
1.22
1.22
50
25/25
10/15
µAmax
µAmax
Vmin
=
VµPmon 0.4V
µPmon Input
0.80/0.80
2.00/2.00
45/30
Threshold Voltage
µP Monitor Reset
Oscillator Period
µP Monitor Reset
Oscillator Pulse Width
Minimum µP Monitor
Input Pulse Width
Reset Fall Time
Vmax
=
VµPmon 0V
msmin
msmax
msmin
msmax
µs
=
(Twindow 0.82 RtCmon
)
50
55/70
=
VµPmon 0V
1.0
1.0
2
0.7/0.4
1.3/2.10
=
(RESETpw 2000 Cmon
)
(Note 7)
=
=
Rrst 10k, Vrst 5V, Crst ≤ 10 pF
0.20
1.00/1.00
µsmax
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4
Electrical Characteristics (Continued)
=
=
=
=
=
=
=
VIN 14V, IOUT 5 mA, Ibuf 5 mA, Istby 5 mA, Rt 130 kΩ, Ct 0.33 µF, Cmon 0.47 µF, unless otherwise indicated,
=
Boldface type refers to limits over the entire operating temperature range, −40˚C ≤ TA ≤ +125˚C, all other limits are for TA TJ
25˚C (Note 8)
=
Parameter
Conditions
Typical
Limit
Units
(Note 4)
Computer Monitor/Reset Functions
=
=
Reset Rise Time
On/Off Switch Input
Current (Pin 8)
Rrst 10k, Vrst 5V, Crst ≤ 10 pF
0.60
7.5
1.00/1.50
25/25
µsmax
µAmax
µAmax
Vmin
=
VON 2.4V
=
VON 0.4V
0.01
1.22
1.22
10/10
On/Off Switch Input
Threshold Voltage
0.80/0.80
2.00/2.00
Vmax
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
OUT
) changes from the measured output voltage (with V
14V) by the specified amount,
returns to regulation. The Reset Error Flag is then allowed
IN
∆V
High or ∆V
Low, and set the Reset Error Flag low. The Reset Error Flag is held low until V
OUT OUT
OUT
to go high again after a delay set by R and C . (see application section).
t
t
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
mon
. (See
Application Hints Section.)
Note 8: To ensure constant junction temperature, low duty cycle pulse testing is used.
Block Diagram
DS011252-2
5
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Pin Description
Pin No.
Pin Name
VIN
Comments
Positive supply input voltage
Sets internal timing currents
Sets power-up reset delay timing
Microcomputer monitor input
Sets µC monitor timing
1
2
Rt
3
Ct
4
µPmon
Cmon
5
6
Ground
Reset
ON/OFF
Vstandby
Vbuffer
VOUT
Regulator ground
7
Reset error flag output
8
Enables/disables high current regulators
Standby regulator output (7.5 mA)
Buffer regulator output (100 mA)
Main regulator output (500 mA)
9
10
11
External Components
Component
Typical Value
Component
Range
Comments
CIN
Rt
1 µF
130k
0.47 µF–10 µF
24k–510k
Required if device is located far from power supply filter.
Sets internal timing currents.
Ct
0.33 µF
0.01 µF
10k
0.033 µF–3.3 µF
0.001 µF–0.1 µF
1k–100k
Sets power-up reset delay.
Ctc
Rtc
Establishes time constant of AC coupled computer monitor.
Establishes time constant of AC coupled computer monitor. (See
applications section.)
Cmon
Rrst
Cstby
Cbuf
0.47 µF
10k
0.047 µF–4.7 µF
5k–100k
Sets time window for computer monitor. Also determines period and pulse
width of computer malfunction reset. (See applications section.)
Load for open collector reset output. Determined by computer reset input
requirements.
10 µF
10 µF
10 µF
10 µF–no bound
10 µF–no bound
10 µF–no bound
A 10 µF is required for stability but larger values can be used to maintain
regulation during transient conditions.
A 10 µF is required for stability but larger values can be used to maintain
regulation during transient conditions.
COUT
A 10 µF is required for stability but larger values can be used to maintain
regulation during transient conditions.
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6
Typical Circuit Waveforms
DS011252-3
Connection Diagram
DS011252-4
Order Number LM2984T
See NS Package Number TA11B
7
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Typical Performance Characteristics
Dropout Voltage (VOUT
)
Dropout Voltage (Vbuf
)
Dropout Voltage (Vstby)
DS011252-18
DS011252-16
DS011252-17
Dropout Voltage (VOUT
)
Dropout Voltage (Vbuf
)
Dropout Voltage (Vstby)
DS011252-19
DS011252-20
DS011252-21
Peak Output Current (VOUT
)
Peak Output Current (Vbuf
)
Peak Output Current (Vstby)
DS011252-24
DS011252-22
DS011252-23
Quiescent Current (VOUT
)
Quiescent Current (Vbuf
)
Quiescent Current (Vstby)
DS011252-25
DS011252-26
DS011252-27
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8
Typical Performance Characteristics (Continued)
Quiescent Current (VOUT
)
Quiescent Current (Vbuf
)
Quiescent Current (Vstby)
DS011252-28
DS011252-31
DS011252-34
DS011252-29
DS011252-32
DS011252-35
DS011252-30
DS011252-33
DS011252-36
Quiescent Current (VOUT
)
Quiescent Current (Vbuf
)
Quiescent Current (Vstby)
Output Voltage (VOUT
)
Output Voltage (Vbuf
)
Output Voltage (Vstby)
Low Voltage Behavior (VOUT
)
Low Voltage Behavior (Vbuf
)
Low Voltage Behavior (Vstby)
DS011252-37
DS011252-38
DS011252-39
9
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Typical Performance Characteristics (Continued)
Line Transient
Response (VOUT
Line Transient
Response (Vbuf
Line Transient
Response (Vstby
)
)
)
DS011252-40
DS011252-41
DS011252-42
Load Transient
Response (VOUT
Load Transient
Response (Vbuf
Load Transient
Response (Vstby
)
)
)
DS011252-43
DS011252-44
DS011252-45
Output Impedance (VOUT
)
Output Impedance (Vbuf
)
Output Impedance (Vstby)
DS011252-46
DS011252-47
DS011252-48
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10
Typical Performance Characteristics (Continued)
Ripple Rejection (VOUT
)
Ripple Rejection (Vbuf
)
Ripple Rejection (Vstby)
DS011252-49
DS011252-50
DS011252-51
Output Voltage
Device Dissipation vs
Ambient Temperature
DS011252-8
DS011252-9
Output Capacitor ESR
(Standby Output, Pin 9)
Output Capacitor ESR
(Buffer Output, Pin 10)
Output Capacitor ESR
(Main Output, Pin 11)
DS011252-10
DS011252-11
DS011252-12
mum capacitor value to use in production. Worst case is usu-
ally determined at the minimum ambient temperature and
the maximum load expected.
Application Hints
OUTPUT CAPACITORS
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 condi-
tions of negative input transients that might be characteristic
of a particular system.
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 de-
pending 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-
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
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switching from the standby mode to the active mode and
vice versa. This pin can be tied to the input voltage through
a 10 kΩ resistor if the regulator is to be powered continu-
ously.
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.
POWER DOWN OVERRIDE
Each output must be terminated by a capacitor, even if it is
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.
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 ( 1.5 mA) when the other
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
DS011252-13
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.
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 exter-
nal components, Rt and Ct. There is a slight amount of hys-
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.
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.
DELAYED RESET
Resistor Rt and capacitor Ct set the period of time that the
RESET output is held low after a main output error condition
has been sensed. The delay is given by the formula:
=
Tdly 1.2 RtCt (seconds)
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.)
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.
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.
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”
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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.
Application Hints (Continued)
This watchdog circuitry continuously monitors a pin on the
µP that generates a positive going pulse during normal op-
eration. 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, Rt and Cmon, according to the formula:
=
Twindow 0.82 RtCmon (seconds)
The width of the RESET pulse is set by Cmon and an internal
resistor according to the following:
DS011252-14
FIGURE 2. Monitoring Square Wave µP Signals
=
RESETpw 2000 Cmon (seconds)
The threshold voltage and input characteristics of this pin are
compatible with nearly all logic families.
A square wave signal can also be monitored for errors by fil-
tering the Cmon input such that only the positive edges of the
signal are detected. Figure 2 is a schematic diagram of a
typical circuit used to differentiate the input signal. Resistor
There is a limit on the width of a pulse that can be reliably de-
tected by the watchdog circuit. This is due to the output re-
sistance of the transistor which discharges Cmon when a high
state is detected at the input. The minimum detectable pulse
width can be determined by the following formula:
R
tc and capacitor Ctc pass only the rising edge of the square
wave and create a short positive pulse suitable for the µP
=
PWmin 20 Cmon (seconds)
13
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Equivalent Schematic Diagram
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14
15
Physical Dimensions inches (millimeters) unless otherwise noted
Molded TO-220 Package (TA)
Order Number LM2984T
NS Package Number TA11B
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