LP2960 Datasheet, PDF (11/17 Pages) National Semiconductor (TI) – Adjustable Micropower 0.5A Low-Dropout Regulators

Application Hints
EXTERNAL CAPACITORS
Bypass capacitors on the input and output of the LP2960 are
required: without these capacitors, the part will oscillate.
A capacitor (whose value is at least 4.7 µF) must be con-
nected from the V_IN pin to ground. If the input capacitor is lo-
cated more than one inch away from the LP2960, the capaci-
tor may have to be increased to 22 µF to assure stability. A
capacitor is also required between V_OUT and Ground, and
the minimum amount of capacitance required here depends
on output voltage.
If the output voltage of the LP2960 is set to 5V, a minimum of
10 µF is needed in output capacitance. At 3.3V output, at
least 22 µF is required to assure stability.
ESR LIMIT: The ESR of the capacitor used on the LP2960
must be less than 0.7Ω throughout the entire operating tem-
perature range to assure stability.
The ESR of an aluminum eIectroIytic capacitor is typically
only specified at 25˚C, and does not reflect the maximum
ESR that can be expected to occur over the entire tempera-
ture range of the capacitor.
Aluminum electrolytics show a marked increase in ESR at
low temperatures (ESR can increase by a factor of 30 or
more when going from 25˚C to −30˚C) which could lead to
oscillation probIems in applications with very low ambient
temperatures. Solid tantalum capacitors are recommended
for use in such cases.
Regulator instability can be caused by stray (board layout)
capacitance appearing at the Feedback terminal. Oscilla-
tions from this effect are most Iikely to occur when very high
value resistors are used to set the output voltage.
Adding a 100 pF capacitor between the Output and Feed-
back pins and increasing the output capacitor to at least
22 µF will stop the osciIIations.
MINIMUM LOAD
The internal resistive divider in the LP2960 provides suffi-
cient output loading for proper regulation. If externaI resis-
tors are used to set the LP2960 output voltage, a minimum
current of 5 µA through the externaI resistive divider is rec-
ommended.
It should be noted that a minimum load current is specified in
several of the test conditions listed under Electrical Charac-
teristics, and this value of load current must be used to get
correlation on these test limits.
PROGRAMMING THE OUTPUT VOLTAGE
The LP2960 regulator may be pin-strapped for operation at
the nominal output voltage using its internal resistive divider
by tying the Output and Sense pins together and also tying
the Feedback and V_TAP pins together.
Alternatively, it may be programmed for any voltage between
the 1.23V reference and the 30V maximum rating using an
external pair of resistors (see Basic Application Circuit).
The complete equation for the output voltage is:
V_OUT = V_REF x (1 + R1/R2) + (I_FB x R1)
The term V_REF is the 1 .23V reference and I_FB is the Feed-
back pin bias current (−20 nA typical). The minimum recom-
mended load current of 5 µA sets an upper limit of 240 kΩ on
the value of R2 in cases where the regulator must work with
no load (see Minimum Load).
For best output accuracy, choosing R2 = 100 kΩ will reduce
the error resulting from I_FB to 0.17% while increasing the re-
sistive divider current to 12 µA. Since the typicaI quiescent
current of the LP2960 is 450 µA, this added current through
R2 is negligible.
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.
OUTPUT ISOLATION
If the LP2960 output is connected to an active voltage
source (such as a battery) the regulator input should not be
shorted to ground, as this will cause a large current to flow
from the battery into the LP2960 output lead.
If the LP2960 input is left floating with the output connected
to a battery, a small current (a few mA) will flow into the out-
put lead.
The “reverse” current flowing from the battery into the
LP2960 output can be prevented by using a blocking diode
between the output and the battery.
REDUCING OUTPUT NOISE
In reference applications it may be desirabIe to reduce the
AC noise present on the output. One method is to reduce
regulator bandwidth by increasing output capacitance. This
is relatively inefficient, since large increases in capacitance
are required to get significant improvement.
Noise can be reduced more effectively by a bypass capacitor
placed across R1 (refer to Basic Application Circuit).
A 0.1 µF capacitor connected across R1 will reduce the high
frequency gain of the circuit to unity, lowering the RMS out-
put noise voltage from 210 µV to 130 µV (typical) using a 10
Hz–100 kHz bandwidth test measurement.
Also, output noise is no longer proportional to the output volt-
age, so improvements are more pronounced at higher output
voltages.
IMPORTANT: Since the 0.1 µF capacitor reduces the AC
gain of the LP2960 to unity, the output capacitance must be
increased to at least 33 µF to assure regulator stability.
DROPOUT DETECTION COMPARATOR
The dropout detection comparator produces a logic “LOW”
on the Error output whenever the LP2960 output drops 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 (refer to block diagram).
The “5% below nominal” trip level remains constant regard-
less of the programmed output voltage. An out-of-regulation
condition can result from low input voltage, current limiting,
or thermal limiting.
The figure below gives a timing diagram showing the rela-
tionship between the output voltage, the Error output, and in-
put voltage as the input voltage is ramped up and down to a
regulator programmed for 5V output.
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