LP2954 Datasheet, PDF (14/28 Pages) National Semiconductor (TI) – 5V and Adjustable Micropower Low-Dropout Voltage Regulators

LP2954, LP2954A
SNVS096E – JUNE 1999 – REVISED JULY 2016
www.ti.com
Place a 1-μF capacitor from the IN pin to GND if there is more than 10 inches of wire between the input and the
AC filter capacitor or if a battery input is used.
Programming the output for voltages below 5 V runs the error amplifier at lower gains requiring more output
capacitance for stability. At 3.3-V output, a minimum of 4.7 μF is required. For the worst case condition of 1.23-V
output and 250 mA of load current, a 6.8-μF (or larger) capacitor must be used.
Stray capacitance to the FEEDBACK pin can cause instability. This problem is most likely to appear when using
high value external resistors to set the output voltage. Adding a 100-pF capacitor between the OUT and
FEEDBACK pins and increasing the output capacitance to 6.8 μF (or greater) solves the problem.
8.2.2.2 Minimum Load
When setting the output voltage using an external resistive divider, TI recommends a minimum current of 1 μA
through the resistors to provide a minimum load.
It should be noted that a minimum load current is specified in several of the electrical characteristic test
conditions, so this value must be used to obtain correlation on these tested limits. The part is parametrically
tested down to 100 μA, but is functional with no load.
8.2.2.3 Programming The Output Voltage
The SOIC version of the LP2954 regulator may be pin strapped for 5-V operation using its internal resistive
divider by tying the OUT and SENSE pins together and also tying the FEEDBACK and 5V TAP pins together.
Alternatively, it may be programmed for any voltage between the 1.23-V reference and the 30-V maximum rating
using an external pair of resistors (see Figure 19). The complete equation for the output voltage is:
V_OUT = V_REF × (1 + R1 / R2) + (I_FB × R1)
(2)
where V_REF is the 1.23-V reference and I_FB is the FEEDBACK pin bias current (−20 nA typical). The minimum
recommended load current of 1 μA sets an upper limit of 1.2 MΩ on the value of R2 in cases where the regulator
must work with no load (see Minimum Load). I_FB produces a typical 2% error in V_OUT which can be eliminated at
room temperature by trimming R1. For better accuracy, choosing R2 = 100 kΩ reduces this error to 0.17% while
increasing the resistor program current to 12 μA. Because the typical quiescent current is 120 μA, this added
current is negligible.
*Drive with TTL-low to shutdown
Figure 19. Adjustable Regulator
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