LP2950 datasheet(17/28 Pages) TI

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5 V

1.3 V

Input

Voltage

Output

Voltage

ERROR

4.75 V

OUT

+ VREF

1 )

R2 *

FBR1

LP2950

LP2951

www.ti.com

SLVS582F – APRIL 2006 – REVISED DECEMBER 2009

Figure 1. ERROR Output Timing

Because the ERROR comparator has an open-collector output, an external pullup resistor is required to pull the

output up to VOUT or another supply voltage (up to 30 V). The output of the comparator is rated to sink up to

400

μA. A suitable range of values for the pullup resistor is from 100 kΩ to 1 MΩ. If ERROR is not used, it can

be left open.

Programming Output Voltage (LP2951 Only)

A unique feature of the LP2951 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 2:

Where:

VREF = 1.235 V applied across R2

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

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

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.

(2)

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