LP2950 datasheet(15/32 Pages) TI

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FEEDBACK

OUT

REF

I R

LP2950, LP2951

www.ti.com

SLVS582I – APRIL 2006 – REVISED NOVEMBER 2014

Feature Description (continued)

7.4.2 Programming Output Voltage (LP2951 Only)

A unique feature of the LP2951 device is its ability to output either a fixed voltage or an adjustable voltage,

depending on the external pin connections. To output the internally programmed fixed voltage, tie the SENSE pin

to the OUTPUT pin and the FEEDBACK pin to the VTAP pin. Alternatively, a user-programmable voltage ranging

from the internal 1.235-V reference to a 30-V max can be set by using an external resistor divider pair. The

resistor divider is tied to VOUT, and the divided-down voltage is tied directly to FEEDBACK for comparison against

the internal 1.235-V reference. To satisfy the steady-state condition in which its two inputs are equal, the error

amplifier drives the output to equal Equation 1:

(1)

Where:

VREF = 1.235 V applied across R2 (see Figure 32)

IFB = FEEDBACK bias current, typically 20 nA

A minimum regulator output current of 1

μA must be maintained. Thus, in an application where a no-load

condition is expected (for example, CMOS circuits in standby), this 1-

μA minimum current must be provided by

the resistor pair, effectively imposing a maximum value of R2 = 1.2 M

Ω (1.235 V/1.2 MΩ ≉ 1 μA).

IFB = 20 nA introduces an error of ≉0.02% in VOUT. This can be offset by trimming R1. Alternatively, increasing

the divider current makes IFB less significant, thus, reducing its error contribution. For instance, using

R2 = 100 k

Ω reduces the error contribution of IFB to 0.17% by increasing the divider current to ≉12 μA. This

increase in the divider current still is small compared to the 600-

μA typical quiescent current of the LP2951 under

no load.

Figure 32. Adjusting the Feedback on the LP2951

7.5 Device Functional Modes

7.5.1 Shutdown Mode

These devices can be placed in shutdown mode with a logic high at the SHUTDOWN pin. Return the logic level

low to restore operation or tie SHUTDOWN to ground if the feature is not being used.

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