LP2956A Datasheet, PDF (13/20 Pages) National Semiconductor (TI) – Dual Micropower Low-Dropout Voltage Regulators

The main output requires less capacitance at lighter load cur-
rents. This capacitor can be reduced to 0.68 μF for currents
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 con-
dition 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 input 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 to-
gether.
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 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).
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 re-
sistor program current to 12 μA. Since the typical quiescent
current is 130 μA, this added current is negligible.
*See Application Hints
**Drive with high to shut down
FIGURE 3. Adjustable Regulator
1133911
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
measured with a 1V differential. The dropout voltage is inde-
pendent 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
divided 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 reg-
ulator 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 com-
parator 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. The
resistor is not required if the output is unused.
13
11339 Version 8 Revision 4 Print Date/Time: 2009/12/07 17:53:15
www.national.com