LP2956 Datasheet, PDF (11/16 Pages) National Semiconductor (TI) – Dual Micropower Low-Dropout Voltage Regulators

Application Hints (Continued)
ESR may increase by a factor of 20 or 30 as the temperature
is reduced from +25˚C to −30˚C). The value of these capaci-
tors may be increased without limit.
The main output requires less capacitance at lighter load
currents. This capacitor can be reduced to 0.68 µF for cur-
rents below 10 mA or 0.22 µF for currents below 1 mA.
Programming the main output for voltages below 5V requires
more output capacitance for stability. For the worst-case
condition of 1.23V output and 250 mA of load current, a 6.8
µF (or larger) capacitor should be used.
A 1 µF capacitor should be placed from the input pin to
ground if there is more than 10 inches of wire between the in-
put and the AC filter capacitor or if a battery input is used.
Stray capacitance to the Feedback terminal can cause insta-
bility. This problem is most likely to appear when using high
value external resistors to set the output voltage. Adding a
100 pF capacitor between the Output and Feedback pins
and increasing the output capacitance to 6.8 µF (or greater)
will cure the problem.
MINIMUM LOAD ON MAIN OUTPUT
When setting the main output voltage using an external re-
sistive divider, a minimum current of 10 µA is recommended
through the resistors to provide a minimum load.
It should be noted that a minimum load current is specified in
several of the electrical characteristic test conditions, so the
specified value must be used to obtain test limit correlation.
PROGRAMMING THE MAIN OUTPUT VOLTAGE
The main output may be pin-strapped for 5V operation using
its internal resistive divider by tying the Output and Sense
pins together and also tying the Feedback and 5V Tap pins
together.
Alternatively, it may be programmed for any voltage between
the 1.23V reference and the 29V maximum rating using an
external pair of resistors (see Figure 3 ). The complete equa-
tion for the output voltage is:
where V_REF is the 1.23V reference and I_FB is the Feedback
pin bias current (−20 nA typical). The minimum recom-
mended 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).
If I_FB is ignored in the calculation of the output voltage, it will
produce a small error in V_{MAIN OUT}. Choosing R2 = 100 kΩ
will reduce this error to 0.16% (typical) while increasing the
resistor program current to 12 µA. Since the typical quies-
cent current is 130 µA, this added current is negligible.
*See Application Hints
**Drive with high to shut down
FIGURE 3. Adjustable Regulator
DS011339-11
DROPOUT VOLTAGE
The dropout voltage of the regulator is defined as the mini-
mum input-to-output voltage differential required for the out-
put voltage to stay within 100 mV of the output voltage mea-
sured with a 1V differential. The dropout voltage is
independent of the programmed output voltage.
DROPOUT DETECTION COMPARATOR
This comparator produces a logic “LOW” whenever the main
output falls out of regulation by more than about 5%. This fig-
ure results from the comparator’s built-in offset of 60 mV di-
vided by the 1.23V reference (refer to block diagram). The
5% low trip level remains constant regardless of the pro-
grammed output voltage. An out-of-regulation condition can
result from low input voltage, current limiting, or thermal lim-
iting.
Figure 4 gives a timing diagram showing the relationship be-
tween the main output voltage, the ERROR output, and input
voltage as the input voltage is ramped up and down to a
regulator whose main output is programmed for 5V. The ER-
ROR signal becomes low at about 1.3V input. It goes high at
about 5V input, where the main output equals 4.75V. Since
the dropout voltage is load dependent, the input voltage trip
points will vary with load current. The main 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 main output or some other supply voltage.
Using the main output prevents an invalid “HIGH” on the
comparator output which occurs if it is pulled up to an exter-
nal voltage while the regulator input voltage is reduced be-
low 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.
The resistor is not required if the output is unused.
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