LP2950_08 Datasheet, PDF (17/25 Pages) Texas Instruments – ADJUSTABLE MICROPOWER VOLTAGE REGULATORS WITH SHUTDOWN

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LP2950, LP2951
ADJUSTABLE MICROPOWER VOLTAGE REGULATORS
WITH SHUTDOWN
SLVS582D – APRIL 2006 – REVISED MARCH 2007
APPLICATION INFORMATION (continued)
Output
Voltage
4.75 V
ERROR
Input
Voltage
5V
1.3 V
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 (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 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:
ǒ Ǔ V_OUT + V_REF
1
)
R1
R2
* I_FBR₁
(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 under no load.
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