LP2951-33-Q1 Datasheet, PDF (15/30 Pages) Texas Instruments – Adjustable Micropower Voltage Regulators With Shutdown

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LP2951-33-Q1, LP2951-50-Q1
SLVSAW6E – JUNE 2011 – REVISED NOVEMBER 2014
Feature Description (continued)
7.3.2 Programming Output Voltage
A unique feature of the LP2951-xx-Q1 devices are their 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 V_TAP 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 V_OUT, 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:
V
OUT
=
V
REF
´ ^æçè1+
R1
R2
ö
÷ø
-
IR
FB 1
(1)
Where:
V_REF = 1.235 V applied across R2 (see Figure 33)
I_FB = 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 MΩ (1.235 V/1.2 MΩ ≉ 1 μA).
I_FB = 20 nA introduces an error of ≉0.02% in V_OUT. This can be offset by trimming R1. Alternatively, increasing
the divider current makes I_FB less significant, thus, reducing its error contribution. For instance, using
R2 = 100 kΩ reduces the error contribution of I_FB 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-xx-
Q1 devices under no load.
V_OUT
R1
FEEDBACK
R2
Figure 33. Adjusting the Feedback on the LP2951-xx-Q1
7.4 Device Functional Modes
7.4.1 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.
Copyright © 2011–2014, Texas Instruments Incorporated
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Product Folder Links: LP2951-33-Q1 LP2951-50-Q1