LP2952 National Semiconductor, LP2952 Datasheet - Page 11

• Manufacturer Part Number: LP2952
• Description: Adjustable Micropower Low-Dropout Voltage Regulators
• Manufacturer: National Semiconductor

Application Hints
KOVAR lead frame (unlike the industrial parts, which have a
copper lead frame). The KOVAR material is necessary to at-
tain the hermetic seal required in military applications.
The KOVAR lead frame does not conduct heat as well as
copper, which means that the PC board copper can not be
used to significantly reduce the overall junction-to-ambient
thermal resistance in applications using the LP2953AMJ
part.
The power dissipation calculations for military applications
are done exactly the same as was detailed in the previous
section, with one important exception: the value for
the junction-to-ambient thermal resistance, is fixed at
95˚C/W and can not be changed by adding copper foil pat-
terns to the PC board. This leads to an important fact: The
maximum allowable power dissipation in any application us-
ing the LP2953AMJ is dependent only on the ambient tem-
perature:
Figure 3 shows a graph of maximum allowable power dissi-
pation vs. ambient temperature for the LP2953AMJ, made
using the 95˚C/W value for
junction temperature of 150˚C (caution: the maximum ambi-
ent temperature which will be reached in a given application
must always be used to calculate maximum allowable power
dissipation).
EXTERNAL CAPACITORS
A 2.2 µF (or greater) capacitor is required between the out-
put pin and ground to assure stability when the output is set
to 5V. Without this capacitor, the part will oscillate. Most type
of tantalum or aluminum electrolytics will work here. Film
types will work, but are more expensive. Many aluminum
electrolytics contain electrolytes which freeze at −30˚C,
which requires the use of solid tantalums below −25˚C. The
important parameters of the capacitor are an ESR of about
5
ESR may increase by a factor of 20 or 30 as the temperature
is reduced from 25˚C to −30˚C). The value of this capacitor
may be increased without limit.
At lower values of output current, less output capacitance is
required for stability. The capacitor can be reduced to
0.68 µF for currents below 10 mA or 0.22 µF for currents be-
low 1 mA.
Programming the output for voltages below 5V runs the error
amplifier at lower gains requiring more output capacitance
for stability. At 3.3V output, a minimum of 4.7 µF is required.
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.
or less and a resonant frequency above 500 kHz (the
(J–A)
(Continued)
and assuming a maximum
(J–A)
,
11
MINIMUM LOAD
When setting the output voltage using an external resistive
divider, a minimum current of 1 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 this
value must be used to obtain correlation on these tested
limits.
PROGRAMMING THE OUTPUT VOLTAGE
The regulator 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 30V maximum rating using an
external pair of resistors (see Figure 4 ). The complete equa-
tion for the output voltage is:
where V
pin bias current (−20 nA typical). The minimum recom-
mended load current of 1 µA sets an upper limit of 1.2 M on
the value of R2 in cases where the regulator must work with
no load (see MINIMUM LOAD ). I
error in V
by trimming R1. For better accuracy, choosing R2 = 100 k
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.
FIGURE 3. Power Derating Curve for LP2953AMJ
REF
OUT
is the 1.23V reference and I
which can be eliminated at room temperature
FB
will produce a typical 2%
FB
DS011127-26
is the Feedback
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