Datasheet
V
OUT
+ V
REF
ǒ
1 )
R1
R2
Ǔ
* I
FB
R
1
5 V
1.3 V
Input
Voltage
Output
Voltage
ERROR
4.75 V
LP2951-33-Q1
LP2951-50-Q1
www.ti.com
SLVSAW6D –JUNE 2011–REVISED APRIL 2013
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 V
OUT
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
A unique feature of the LP2951-xx-Q1 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 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 2:
(2)
Where:
V
REF
= 1.235 V applied across R2
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 under no load.
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