LM2925
Low Dropout Regulator with Delayed Reset
General Description
The LM2925 features a low dropout, high current regulator.
Also included on-chip is a reset function with an externally
set delay time. Upon power up, or after the detection of any
error in the regulated output, the reset pin remains in the active low state for thedurationofthedelay. Typesoferrors detected include any that cause the output to become unregulated: low input voltage, thermal shutdown, short circuit,
input transients, etc. No external pull-up resistor is necessary.The current charging the delay capacitor is very low, allowing long delay times.
Designed primarily for automotive applications, the LM2925
and all regulated circuitry are protected from reverse battery
installations or two-battery jumps. During line transients,
such as a load dump (60V) when the input voltage to the
regulator can momentarily exceed the specified maximum
operating voltage, the 0.75A regulator will automatically shut
down to protect both internal circuits and the load. The
Typical Application Circuit
LM2925 cannot be harmed by temporary mirror-image insertion. Familiar regulator features such as short circuit and
thermal overload protection are also provided.
Features
n 5V, 750 mA output
n Externally set delay for reset
n Input-output differential less than 0.6V at 0.5A
n Reverse battery protection
n 60V load dump protection
n −50V reverse transient protection
n Short circuit protection
n Internal thermal overload protection
n Available in plastic TO-220
n Long delay times available
+
n P
Product Enhancement tested
Connection Diagram
LM2925 Low Dropout Regulator with Delayed Reset
April 1998
*
Required if regulator is located far from power supply filter.
**
C
must be at least 10 µF to maintain stability. May be increased
OUT
without bound to maintain regulation during transients. Locate as close as
possible to 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.
FIGURE 1. Test and Application Circuit
DS005268-1
TO-220 5-Lead
DS005268-2
Order Number LM2925T
See NS Package Number T05A
© 1998 National Semiconductor Corporation DS005268 www.national.com
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Input Voltage
Operating Range 26V
Overvoltage Protection 60V
V
OUT
Electrical Characteristics
=
V
14V, C2=10 µf, I
IN
=
O
500 mA, T
=
25˚C (Note 4) (unless otherwise specified)
J
Internal Power Dissipation (Note 2) Internally Limited
Operating Temperature Range −40˚C to + 125˚C
Maximum Junction Temperature 150˚C
Storage Temperature Range −65˚C to + 150˚C
Lead Temperature
(Soldering, 10 seconds) 260˚C
ESD rating is to be determined
Parameter Conditions Min Typ Max Units
(Note 3)
Output Voltage 6V≤ V
Line Regulation 9V ≤ V
Load Regulation 5 mA ≤ I
Output Impedance 500 mA
≤ 26V, IO≤ 500 mA, V
IN
−40˚C≤ T
6V ≤ V
≤ +125˚C 4.75 5.00 5.25
J
≤ 16V, I
IN
≤ 26V, I
IN
O
DC
=
5 mA 4 25 mV
O
=
5mA 10 50 mV
O
≤ 500 mA 10 50 mV
and 10 mArms, 200 mΩ
100 Hz-10 kHz
Quiescent Current I
≤ 10 mA 3 mA
O
=
I
500 mA 40 100 mA
O
=
I
750 mA 90 mA
O
Output Noise Voltage 10 Hz-100 kHz 100 µVrms
Long Term Stability 20 mV/1000 hr
Ripple Rejection f
Dropout Voltage I
=
120 Hz 66 dB
o
=
500 mA 0.45 0.6 V
O
=
I
750 mA 0.82 V
O
Current Limit 0.75 1.2 A
Maximum Operational 26 31 V
Input Voltage
Maximum Line Transient V
Reverse Polarity Input V
≤ 5.5V 60 70 V
O
≥ − 0.6V, 10Ω Load −15 −30 V
O
Voltage, DC
Reverse Polarity Input 1%Duty Cycle, τ≤100 ms, −50 −80 V
Voltage, Transient 10Ω Load
Reset Output
Electrical Characteristics
=
V
14V, C3=0.1 µF, T
IN
Parameter Conditions Min Typ Max Units
Reset Voltage
Output Low I
Output High I
Reset Internal Pull-up Resistor 30 kΩ
Reset Output Current Limit V
V
Threshold 4.5 V
OUT
Delay Time C
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=
25˚C (Note 4) (unless otherwise specified)
A
=
1.6 mA, V
SINK
SOURCE
RESET
=
.005 µF 12 ms
3
=
C
0.1 µF 150 250 300 ms
3
=
C
4.7 µF tantalum 12 s
3
IN
=
0 4.5 5.0 5.5 V
=
1.2 V 5 mA
(Note 3)
=
35V 0.3 0.6 V
Reset Output
Electrical Characteristics
=
V
14V, C3=0.1 µF, T
IN
Parameter Conditions Min Typ Max Units
Delay Current Pin 4 1.2 1.95 2.5 µA
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device
beyond its rated operating Conditions.
Note 2: Thermal resistance without a heat sink for junction to case temperature is 3˚C/W (TO-220). Thermal resistance for TO-220 case to ambient temperature is
50˚C/W.
Note 3: These parameters are guaranteed and 100%production tested.
Note 4: To ensure constant junction temperature, low duty cycle pulse testing is used.
=
25˚C (Note 4) (unless otherwise specified)
A
(Continued)
(Note 3)
Typical Circuit Waveforms
DS005268-3
FIGURE 2.
Typical Performance Characteristics
Reset Voltage
DS005268-4
Reset Voltage
Delay Time
DS005268-5
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DS005268-6
Typical Performance Characteristics (Continued)
Reset Voltage
on Power-up
Dropout Voltage
Ripple Rejection
DS005268-7
DS005268-10
Delay Time
Line Transient Response
Quiescent Current
DS005268-8
DS005268-11
Reset Pull-up
Resistor R10
DS005268-9
Ripple Rejection
DS005268-12
Output Voltage
DS005268-13
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DS005268-14
DS005268-15
Typical Performance Characteristics (Continued)
Quiescent Current
Peak Output Current
DS005268-19
DS005268-20
Quiescent Current
Maximum Power
Dissipation (TO-220)
DS005268-17
DS005268-21
Load Transient Response
DS005268-18
Output Capacitor ESR
DS005268-22
Definition of Terms
Dropout Voltage: The input-output voltage differential at
which the circuit ceases to regulate against further reduction
in input voltage. Measured when the output voltage has
dropped 100 mV from the nominal value obtained at 14V input, dropout voltage is dependent upon load current and
junction temperature.
Input Voltage:The DC voltage applied to the input terminals
with respect to ground.
Input-Output Differential: The voltage difference between
the unregulated input voltage and the regulated output voltage for which the regulator will operate.
Line Regulation: The change in output voltage for a change
in the input voltage. The measurement is made under conditions of low dissipation or by using pulse techniques such
that the average chip temperature is not significantly affected.
Load Regulation: The change in output voltage for a
change in load current at constant chip temperature.
Long Term Stability: Output voltage stability under accelerated life-test conditions after 1000 hours with maximum
rated voltage and junction temperature.
Output Noise Voltage: The rms AC voltage at the output,
with constant load and no input ripple, measured over a
specified frequency range.
Quiescent Current: The part of the positive input current
that does not contribute to the positive load current. The
regulator ground lead current.
Ripple Rejection: The ratio of the peak-to-peak input ripple
voltage to the peak-to-peak output ripple voltage.
TemperatureStability of V
put voltage for a thermal variation from room temperature to
: The percentage change in ou-
O
either temperature extreme.
Application Hints
EXTERNAL CAPACITORS
The LM2925 output capacitor is required for stability. Without
it, the regulator output will oscillate, sometimes by many
volts. Though the 10 µF shown is the minimum recommended value, actual size and type may vary depending
upon the application load and temperature range. Capacitor
effective series resistance (ESR) also effects the IC stability.
Since ESR varies from one brand to the next, some bench
work may be required to determine the minimum capacitor
value to use in production. Worst-case is usually determined
at the minimum junction and ambient temperature and 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.
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Application Hints (Continued)
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
effective capacitance to zero. To maintain regulator stability
down to −40˚C, capacitors rated at that temperature (such as
tantalums) must be used.
RESET OUTPUT
The range of values for the delay capacitor is limited only by
stray capacitances on the lower extreme and capacitance
leakage on the other. Thus, delay times from microseconds
to seconds are possible. The low charging current, typically
2.0 microamps, allows the use of small, inexpensive disc capacitors for the nominal range of 100 to 500 milliseconds.
This is the time required in many microprocessor systems for
the clock oscillator to stabilize when initially powered up. The
RESET output of the regulator will thus prevent erroneous
data and/or timing functions to occur during this part of operation. The same delay is incorporated after any other fault
condition in the regulator output is corrected.
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Circuit Schematic
DS005268-23
FIGURE 3.
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Physical Dimensions inches (millimeters) unless otherwise noted
5-Lead TO-220 Power Package (T)
Order Number LM2925T
NS Package Number T05A
LM2925 Low Dropout Regulator with Delayed Reset
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