The military temperature range (−55˚C

≤ T

J

≤+150˚C) parts

are manufactured in ceramic DIP packages which contain a

KOVAR lead frame (unlike the industrial parts, which have a

copper lead frame). The KOVAR material is necessary to

attain 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

θ

the junction-to-ambient thermal resistance, is fixed at

95˚C/W and can not be changed by adding copper foil

patterns to the PC board. This leads to an important fact:

The maximum allowable power dissipation in any application

using the LP2953AMJ is dependent only on the ambient

temperature:

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

θ

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

output 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 alumi-

num 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

Ω or less and a resonant frequency above 500 kHz (the

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

below 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

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

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

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 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

Ω 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

Ω

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.

01112726

FIGURE 3. Power Derating Curve for LP2953AMJ

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