LP2951-N_15_15 Datasheet, PDF (22/50 Pages) Texas Instruments – LP295x-N Series of Adjustable Micropower Voltage Regulators

LP2950-N, LP2951-N
SNVS764O – JANUARY 2000 – REVISED DECEMBER 2014
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9.2.1.2.4 Programming the Output Voltage (LP2951-N)
The LP2951-N may be pin-strapped for the nominal fixed output voltage using its internal voltage divider by tying
the output and sense pins together, and also tying the FEEDBACK and VTAP pins together. Alternatively, it may
be programmed for any output voltage between its 1.235-V reference and its 30-V maximum rating. As seen in
Figure 40, an external pair of resistors is required.
The complete equation for the output voltage is
where
• V_REF is the nominal 1.235-V reference voltage and I_FB is the feedback pin bias current, nominally –20 nA (7)
The minimum recommended load current of 1 μA forces an upper limit of 1.2 MΩ on the value of R2, if the
regulator must work with no load (a condition often found in CMOS in standby). I_FB will produce a 2% typical
error in V_OUT which may 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. Since the LP2951-N
typically draws 60 μA at no load with Pin 2 open-circuited, this is a small price to pay.
*Drive with TTL-high to shut down. Ground or leave open if shutdown feature is not to be used.
Note: Pins 2 and 6 are left open.
Figure 40. Adjustable Regulator
Stray capacitance to the LP2951-N Feedback terminal can cause instability. This may especially be a problem
when using high value external resistors to set the output voltage. Adding a 100-pF capacitor between the
Output pin and the Feedback pin, and increasing the output capacitor to at least 3.3 μF, will fix this problem.
9.2.1.2.5 Reducing Output Noise
In reference applications it may be advantageous to reduce the AC noise present at the output. One method is to
reduce the regulator bandwidth by increasing the size of the output capacitor. This is the only way noise can be
reduced on the 3-lead LP2950-N but is relatively inefficient, as increasing the capacitor from 1 μF to 220 μF only
decreases the noise from 430 μV_(RMS) to 160 μV_(RMS) for a 100-kHz bandwidth at 5-V output.
Noise can be reduced fourfold by a bypass capacitor across R1, since it reduces the high frequency gain from 4
to unity. Pick
(8)
or about 0.01 μF. When doing this, the output capacitor must be increased to 3.3 μF to maintain stability. These
changes reduce the output noise from 430 μV to 100 μV rms for a 100-kHz bandwidth at 5-V output. With the
bypass capacitor added, noise no longer scales with output voltage so that improvements are more dramatic at
higher output voltages.
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