LP2953QML Datasheet, PDF (14/24 Pages) Texas Instruments – Adjustable Micropower Low-Dropout Voltage Regulators

Application Hints
HEATSINK REQUIREMENTS
The maximum allowable power dissipation for the LP2953 is
limited by the maximum junction temperature (+150°C) and
the two parameters that determine how quickly heat flows
away from the die: the ambient temperature and the junction-
to-ambient thermal resistance of the part.
The military parts which are manufactured in ceramic DIP
packages 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.
The power dissipation calculations are done using a fixed
value for θ_(J–A), the junction-to-ambient thermal resistance, of
134°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 LP2953 is dependent only on the ambient tempera-
ture:
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 aluminum electrolyt-
ics contain electrolytes which freeze at −30°C, which requires
the use of solid tantalums below −25°C. The important pa-
rameters of the capacitor are an ESR of about 5Ω 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 lim-
its.
20161126
FIGURE 1. Power Derating Curve for LP2953
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 2). 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 Minimim Load ). I_FB will produce a typical 2% error in
V_O which can be eliminated at room temperature by trimming
R1. For better accuracy, choosing R2 = 100 kΩ will reduce
this error to 0.17% while increasing the resistor program cur-
rent to 12 μA. Since the typical quiescent current is 120 μA,
this added current is negligible.
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