LM2940-5.0 MDE National Semiconductor, LM2940-5.0 MDE Datasheet - Page 12

• Manufacturer Part Number: LM2940-5.0 MDE
• Manufacturer: National Semiconductor

Typical Application
*Required if regulator is located far from power supply filter.
**C
regulator. This capacitor must be rated over the same operating temperature range as the regulator and the ESR is critical; see curve.
Application Hints
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.
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-
OUT
must be at least 22 μF to maintain stability. May be increased without bound to maintain regulation during transients. Locate as close as possible to the
Output Capacitor ESR
FIGURE 1. ESR Limits
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11
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, P
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:
I
P
The next parameter which must be calculated is the maximum
allowable temperature rise, T
using the formula:
where: T
Using the calculated values for T
mum allowable value for the junction-to-ambient thermal re-
sistance, θ
IN
D
= I
= (V
L
÷ I
IN
T
G
− V
A
J
FIGURE 2. Power Dissipation Diagram
(max) is the maximum ambient temperature which
(max) is the maximum allowable junction temper-
OUT
(JA)
20158403
) I
, can now be found:
T
L
R
+ (V
ature.
will be encountered in the application.
(max) = T
D
, must be calculated.
IN
θ
) I
(JA)
G
= T
J
(max) − T
R
R
(max)/P
(max). This is calculated by
R
(max) and P
A
D
(max)
D
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