LM3940IT-3.3 National Semiconductor, LM3940IT-3.3 Datasheet - Page 6

• Manufacturer Part Number: LM3940IT-3.3
• Description: IC, LDO VOLT REG, 3.3V, 1A, TO-220
• Manufacturer: National Semiconductor

Specifications of LM3940IT-3.3

• Primary Input Voltage: 5V
• Output Voltage Fixed: 3.3V
• Dropout Voltage Vdo: 110mV
• No. Of Pins: 3
• Output Current: 1A
• Operating Temperature Range: -40°C To +125°C
• Termination Type: Through Hole
• Dc: N/A
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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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 33 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-
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.
FIGURE 1. ESR Limits
1208005
6
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:
T
Where: T
Using the calculated values for T
mum allowable value for the junction-to-ambient thermal re-
sistance, θ
θ
IMPORTANT: If the maximum allowable value for θ
found to be
the TO-263 package, or
no heatsink is needed since the package alone will dissipate
enough heat to satisfy these requirements.
If the calculated value for θ
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
section for the TO-263.
IN
D
(JA)
R
= I
= (V
(max) = T
L
= T
+ I
IN
T
G
− V
A
R
FIGURE 2. Power Dissipation Diagram
J
(max)
(max)/P
OUT
(JA)
(max) is the maximum allowable junction tem-
≥
J
) I
, can now be found:
60°C/W for the TO-220 package,
(max) − T
L
+ (V
D
D
perature, which is 125°C for commercial
grade parts.
is the maximum ambient temperature
which will be encountered in the applica-
tion.
, must be calculated.
IN
(JA)
) I
G
≥
will be the same as shown in the next
A
174°C/W for the SOT-223 package,
(max)
R
(JA)
(max). This is calculated by
falls below these limits, a
R
(max) and P
≥
D
, the maxi-
80°C/W for
1208006
(JA)
is