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LM2940CT-9.0 データシートの表示(PDF) - National ->Texas Instruments

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LM2940CT-9.0 1A Low Dropout Regulator National-Semiconductor
National ->Texas Instruments National-Semiconductor
LM2940CT-9.0 Datasheet PDF : 20 Pages
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Application Information
EXTERNAL CAPACITORS
The output capacitor is critical to maintaining regulator stabil-
ity, and must meet the required conditions for both ESR
(Equivalent Series Resistance) and minimum amount of ca-
pacitance.
MINIMUM CAPACITANCE:
The minimum output capacitance required to maintain stabil-
ity is 22 μF (this value may be increased without limit). Larger
values of output capacitance will give improved transient re-
sponse.
ESR LIMITS:
The ESR of the output capacitor will cause loop instability if it
is too high or too low. The acceptable range of ESR plotted
versus load current is shown in the graph below. It is essen-
tial that the output capacitor meet these requirements, or
oscillations can result.
Output Capacitor ESR
FIGURE 1. ESR Limits
882206
It is important to note that for most capacitors, ESR is speci-
fied only at room temperature. However, the designer must
ensure that the ESR will stay inside the limits shown over the
entire operating temperature range for the design.
For aluminum electrolytic capacitors, ESR will increase by
about 30X as the temperature is reduced from 25°C to −40°
C. This type of capacitor is not well-suited for low temperature
operation.
Solid tantalum capacitors have a more stable ESR over tem-
perature, but are more expensive than aluminum electrolyt-
ics. A cost-effective approach sometimes used is to parallel
an aluminum electrolytic with a solid Tantalum, with the total
capacitance split about 75/25% with the Aluminum being the
larger value.
If two capacitors are paralleled, the effective ESR is the par-
allel of the two individual values. The “flatter” ESR of the
Tantalum will keep the effective ESR from rising as quickly at
low temperatures.
HEATSINKING
A heatsink may be required depending on the maximum pow-
er dissipation and maximum ambient temperature of the ap-
plication. Under all possible operating conditions, the junction
temperature must be within the range specified under Abso-
lute Maximum Ratings.
To determine if a heatsink is required, the power dissipated
by the regulator, PD, must be calculated.
The figure below shows the voltages and currents which are
present in the circuit, as well as the formula for calculating the
power dissipated in the regulator:
IIN = IL + IG
PD = (VIN − VOUT) IL + (VIN) IG
882237
FIGURE 2. Power Dissipation Diagram
The next parameter which must be calculated is the maximum
allowable temperature rise, TR(MAX). This is calculated by us-
ing the formula:
TR(MAX) = TJ(MAX) − TA(MAX)
where: TJ(MAX) is the maximum allowable junction tempera-
ture, which is 125°C for commercial grade
parts.
TA(MAX) is the maximum ambient temperature which
will be encountered in the application.
Using the calculated values for TR(MAX) and PD, the maximum
allowable value for the junction-to-ambient thermal resis-
tance, θ(JA), can now be found:
θ(JA) = TR(MAX) / PD
IMPORTANT: If the maximum allowable value for θ(JA) is
found to be 53°C/W for the TO-220 package, 80°C/W for
the TO-263 package, or 174°C/W for the SOT-223 pack-
age, no heatsink is needed since the package alone will
dissipate enough heat to satisfy these requirements.
If the calculated value for θ(JA)falls below these limits, a
heatsink is required.
HEATSINKING TO-220 PACKAGE PARTS
The TO-220 can be attached to a typical heatsink, or secured
to a copper plane on a PC board. If a copper plane is to be
used, the values of θ(JA) will be the same as shown in the next
section for the TO-263.
If a manufactured heatsink is to be selected, the value of
heatsink-to-ambient thermal resistance, θ(H−A), must first be
calculated:
θ(H−A) = θ(JA) − θ(C−H) − θ(J−C)
Where: θ(J−C) is defined as the thermal resistance from the
junction to the surface of the case. A value of
3°C/W can be assumed for θ(J−C) for this cal-
culation.
13
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