Datasheet

LM2941, LM2941C
SNVS770G –JUNE 1999–REVISED APRIL 2013 www.ti.com
Definition of Terms
Dropout Voltage: The input-voltage differential at which the circuit ceases to regulate against further reduction
in input voltage. Measured when the output voltage has dropped 100mV from the nominal value obtained at
(V
OUT
+ 5V) 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: That 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 V
O
: The percentage change in output voltage for a thermal variation from room
temperature to either temperature extreme.
APPLICATION HINTS
Output Capacitor
A Tantalum capacitor with a minimum capacitance value of 22 μF, and ESR in the range of 0.01Ω to 5Ω, is
required at the output pin for loop stability. It must be located less than 1 cm from the device. There is no
limitation on any additional capacitance.
Alternately, a high quality X5R/X7R 22 μF ceramic capacitor may be used for the output capacitor only if an
appropriate value of series resistance is added to simulate the ESR requirement. The ceramic capacitor selection
must include an appropriate voltage de-rating of the capacitance value due to the applied output voltage. The
series resistor (for ESR simulation) should be in the range of 0.1Ω to 1.0Ω.
Setting the Output Voltage
The output voltage range is 5V to 20V and is set by the two external resistors, R1 and R2. See the Typical
Applications. The output voltage is given by the formula:
V
OUT
= V
REF
x ((R1+R2) / R1) (1)
where V
REF
is typically 1.275V.
Using 1.00 kΩ for R1 will ensure that the bias current error of the adjust pin will be negligible. Using a R1 value
higher than 10 kΩ may cause the output voltage to shift across temperature due to variations in the adjust pin
bias current.
Calculating the upper resistor (R2) value of the pair when the lower resistor (R1) value is known is accomplished
with the following formula:
R2 = R1 x ((V
OUT
/ V
REF
) - 1) (2)
The resistors used for R1 and R2 should be high quality, tight tolerance, and with matching temperature
coefficients. It is important to remember that, although the value of V
REF
is ensured, the final value of V
OUT
is not.
The use of low quality resistors for R1 and R2 can easily produce a V
OUT
value that is unacceptable.
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