Datasheet

LP3982
SNVS185D –FEBRUARY 2002–REVISED APRIL 2013 www.ti.com
Input Capacitor
The LP3982 requires a minimum input capacitance of about 1μF. The value may be increased indefinitely. The
type is not critical to stability. However, instability may occur with bench set-ups where long supply leads are
used, particularly at near dropout and high current conditions. This is attributed to the lead inductance coupling to
the output through the gate oxide of the pass transistor; thus, forming a pseudo LCR network within the Loop-
gain. A 10 μF tantalum input capacitor remedies this non-situ condition; its larger ESR acts to dampen the
pseudo LCR network. This may only be necessary for some bench setups. 1 μF ceramic input capacitor are fine
for most end-use applications.
If a tantalum input capacitor is intended for the final application, it is important to consider their tendency to fail in
short circuit mode, thus potentially damaging the part.
Noise Bypass Capacitor
The noise bypass capacitor (CC) significantly reduces output noise of the LP3982. It connects between pin 6 and
ground. The optimum value for CC is 33 nF.
Pin 6 directly connects to the high impedance output of the bandgap. The DC leakage of the CC capacitor should
be considered; loading down the reference will reduce the output voltage. NPO and COG ceramic capacitors
typically offer very low leakage. Polypropylene and polycarbonate film carbonate capacitor offer even lower
leakage currents.
CC does not affect the transient response; however, it does affect turn-on time. The smaller the CC value, the
quicker the turn-on time.
Power Dissipation
Power dissipation refers to the part's ability to radiate heat away from the silicon, with packaging being a key
factor. A reasonable analogy is the packaging a human being might wear, a jacket for example. A jacket keeps a
person comfortable on a cold day, but not so comfortable on a hot day. It would be even worse if the person was
exerting power (exercising). This is because the jacket has resistance to heat flow to the outside ambient air, like
the IC package has a thermal resistance from its junctions to the ambient (θ
JA
).
θ
JA
has a unit of temperature per power and can be used to calculate the IC's junction temperature as follows:
T
J
= θ
JA
(PD) + T
A
• T
J
is the junction temperature of the IC
• θ
JA
is the thermal resistance from the junction to the ambient air outside the package
• PD is the power exerted by the IC
• T
A
is the ambient temperature (4)
PD is calculated as follows:
PD = I
OUT
(V
IN
-V
O
)
• θ
JA
for the LP3982 package (VSSOP-8) is 223°C/W with no forced air flow
• 182°C/W with 225 linear feet per minute (LFPM) of air flow
• 163°C/W with 500 LFPM of air flow
• 149°C/W with 900 LFPM of air flow (5)
θ
JA
can also be decreased (improved) by considering the layout of the PC board: heavy traces (particularly at V
IN
and the two V
OUT
pins), large planes, through-holes, etc.
Improvements and absolute measurements of the θ
JA
can be estimated by utilizing the thermal shutdown circuitry
that is internal to the IC. The thermal shutdown turns off the pass transistor of the device when its junction
temperature reaches 160°C (Typical). The pass transistor doesn't turn on again until the junction temperature
drops about 10°C (hysteresis).
Using the thermal shutdown circuit to estimate , θ
JA
can be done as follows: With a low input to output voltage
differential, set the load current to 300 mA. Increase the input voltage until the thermal shutdown begins to cycle
on and off. Then slowly decrease V
IN
(100 mV increments) until the part stays on. Record the resulting voltage
differential (V
D
) and use it in the following equation:
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