LP2954IS Datasheet, PDF (12/24 Pages) Texas Instruments – LP2954/LP2954A 5V and Adjustable Micropower Low-Dropout Voltage Regulators

LP2954, LP2954A
SNVS096D – JUNE 1999 – REVISED MARCH 2013
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where V_REF is the 1.23V reference and I_FB is the Feedback pin bias current (−20 nA typical). The minimum
recommended load current of 1 μA sets an upper limit of 1.2 MΩ on the value of R2 in cases where the regulator
must work with no load (see MINIMUM LOAD). I_FB will produce a typical 2% error in V_OUT which can be
eliminated at room temperature by trimming R1. For better accuracy, choosing R2 = 100 kΩ will reduce this error
to 0.17% while increasing the resistor program current to 12 μA. Since the typical quiescent current is 120 μA,
this added current is negligible.
* See Application Hints
** Drive with TTL-low to shut down
Figure 24. Adjustable Regulator
DROPOUT DETECTION COMPARATOR
This comparator produces a logic “LOW” whenever the output falls out of regulation by more than about 5%. This
figure results from the comparator's built-in offset of 60 mV divided by the 1.23V reference. The 5% low trip level
remains constant regardless of the programmed output voltage. An out-of-regulation condition can result from
low input voltage, current limiting, or thermal limiting.
Figure 25 gives a timing diagram showing the relationship between the output voltage, the ERROR output, and
input voltage as the input voltage is ramped up and down to a regulator programmed for 5V output. The ERROR
signal becomes low at about 1.3V input. It goes high at about 5V input, where the output equals 4.75V. Since the
dropout voltage is load dependent, the input voltage trip points will vary with load current. The output voltage
trip point does not vary.
The comparator has an open-collector output which requires an external pull-up resistor. This resistor may be
connected to the regulator output or some other supply voltage. Using the regulator output prevents an invalid
“HIGH” on the comparator output which occurs if it is pulled up to an external voltage while the regulator input
voltage is reduced below 1.3V. In selecting a value for the pull-up resistor, note that while the output can sink
400 μA, this current adds to battery drain. Suggested values range from 100 kΩ to 1 MΩ. This resistor is not
required if the output is unused.
When V_IN ≤ 1.3V, the error flag pin becomes a high impedance, allowing the error flag voltage to rise to its pull-
up voltage. Using V_OUT as the pull-up voltage (rather than an external 5V source) will keep the error flag voltage
below 1.2V (typical) in this condition. The user may wish to divide down the error flag voltage using equal-value
resistors (10 kΩ suggested) to ensure a low-level logic signal during any fault condition, while still allowing a valid
high logic level during normal operation.
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