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LM2925 Low Dropout Regulator with Delayed Reset

April 1998

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 the duration of the delay. Types of errors 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 LM2925 cannot be harmed by temporary mirror-image insertion. Familiar regulator features such as short circuit and thermal overload protection are also provided.

n n n n n n n n n n n 5V, 750 mA output Externally set delay for reset Input-output differential less than 0.6V at 0.5A Reverse battery protection 60V load dump protection -50V reverse transient protection Short circuit protection Internal thermal overload protection Available in plastic TO-220 Long delay times available P+ Product Enhancement tested

Typical Application Circuit Connection Diagram
TO-220 5-Lead


Order Number LM2925T See NS Package Number T05A


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.

*Required if regulator is located far from power supply filter. **COUT must be at least 10 F to maintain stability. May be increased

FIGURE 1. Test and Application Circuit

1998 National Semiconductor Corporation



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 Overvoltage Protection 26V 60V

Internal Power Dissipation (Note 2) Operating Temperature Range Maximum Junction Temperature Storage Temperature Range Lead Temperature (Soldering, 10 seconds) ESD rating is to be determined

Internally Limited -40C to + 125C 150C -65C to + 150C 260C

VOUT Electrical Characteristics
VIN = 14V, C2 = 10 f, IO = 500 mA, TJ = 25C (Note 4) (unless otherwise specified) Parameter Output Voltage Line Regulation Load Regulation Output Impedance Quiescent Current Conditions 6V VIN 26V, IO 500 mA, -40C TJ +125C 9V VIN 16V, IO = 5 mA 6V VIN 26V, IO = 5 mA 5 mA IO 500 mA 500 mADC and 10 mArms, 100 Hz-10 kHz IO 10 mA IO = 500 mA IO = 750 mA Output Noise Voltage Long Term Stability Ripple Rejection Dropout Voltage Current Limit Maximum Operational Input Voltage Maximum Line Transient Reverse Polarity Input Voltage, DC Reverse Polarity Input Voltage, Transient 1% Duty Cycle, 100 ms, 10 Load -50 -80 V VO 5.5V VO - 0.6V, 10 Load 60 -15 70 -30 V V fo = 120 Hz IO = 500 mA IO = 750 mA 0.75 26 10 Hz-100 kHz 3 40 90 100 20 66 0.45 0.82 1.2 31 0.6 100 mA mA mA Vrms mV/1000 hr dB V V A V 4.75 5.00 4 10 10 200 5.25 25 50 50 mV mV mV m Min Typ (Note 3) V Max Units

Reset Output Electrical Characteristics
VIN = 14V, C3 = 0.1 F, TA = 25C (Note 4) (unless otherwise specified) Parameter Reset Voltage Output Low Output High Reset Internal Pull-up Resistor Reset Output Current Limit VOUT Threshold Delay Time C3 = .005 F C3 = 0.1 F C3 = 4.7 F tantalum 150 VRESET = 1.2 V ISINK = 1.6 mA, VIN = 35V ISOURCE = 0 0.3 4.5 5.0 30 5 4.5 12 250 12 300 0.6 5.5 V V k mA V ms ms s Conditions Min Typ (Note 3) Max Units



Reset Output Electrical Characteristics
Parameter Delay Current


VIN = 14V, C3 = 0.1 F, TA = 25C (Note 4) (unless otherwise specified) Conditions Pin 4 Min 1.2 Typ (Note 3) 1.95 2.5 A
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 3C/W (TO-220). Thermal resistance for TO-220 case to ambient temperature is 50C/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.



Typical Circuit Waveforms



Typical Performance Characteristics
Reset Voltage Reset Voltage Delay Time






Typical Performance Characteristics
Reset Voltage on Power-up Delay Time


Reset Pull-up Resistor R10

DS005268-8 DS005268-7 DS005268-9

Dropout Voltage

Line Transient Response

Ripple Rejection

DS005268-11 DS005268-10


Ripple Rejection

Quiescent Current

Output Voltage

DS005268-13 DS005268-14




Typical Performance Characteristics
Quiescent Current

(Continued) Load Transient Response

Quiescent Current

DS005268-18 DS005268-19 DS005268-17

Peak Output Current

Maximum Power Dissipation (TO-220)

Output Capacitor ESR

DS005268-20 DS005268-21 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. Temperature Stability of VO: The percentage change in ouput voltage for a thermal variation from room temperature to 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 F 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.



Application Hints


Capacitors must also be rated at all ambient temperatures expected in the system. Many aluminum type electrolytics will freeze at temperatures less than -30C, reducing their effective capacitance to zero. To maintain regulator stability down to -40C, 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.



Circuit Schematic



LM2925 Low Dropout Regulator with Delayed Reset

Physical Dimensions

inches (millimeters) unless otherwise noted

5-Lead TO-220 Power Package (T) Order Number LM2925T NS Package Number T05A

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