Datasheet

LP38501-ADJ, LP38503-ADJ
SNVS522H –AUGUST 2007–REVISED APRIL 2013 www.ti.com
REVERSE CURRENT PATH
The internal MOSFET pass element in the LP38501/3-ADJ has an inherent parasitic diode. During normal
operation, the input voltage is higher than the output voltage and the parasitic diode is reverse biased. However,
if the output is pulled above the input in an application, then current flows from the output to the input as the
parasitic diode gets forward biased. The output can be pulled above the input as long as the current in the
parasitic diode is limited to 200 mA continuous and 1A peak. The regulator output pin should not be taken below
ground potential. If the LP38501/3-ADJ is used in a dual-supply system where the regulator load is returned to a
negative supply, the output must be diode-clamped to ground.
POWER DISSIPATION/HEATSINKING
The maximum power dissipation (P
D(MAX)
) of the LP38501/3-ADJ is limited by the maximum junction temperature
of 125°C, along with the maximum ambient temperature (T
A(MAX)
) of the application, and the thermal resistance
(θ
JA
) of the package. Under all possible conditions, the junction temperature (T
J
) must be within the range
specified in the Operating Ratings. The total power dissipation of the device is given by:
P
D
= ( (V
IN
− V
OUT
) x I
OUT
) + (V
IN
x I
GND
)
where
• I
GND
is the operating ground current of the device (specified under Electrical Characteristics) (3)
The maximum allowable junction temperature rise (ΔT
J
) depends on the maximum expected ambient
temperature (T
A(MAX)
) of the application, and the maximum allowable junction temperature (T
J(MAX)
):
ΔT
J
= T
J(MAX)
− T
A(MAX)
(4)
The maximum allowable value for junction to ambient Thermal Resistance, θ
JA
, can be calculated using the
formula:
θ
JA
= ΔT
J
/ P
D(MAX)
(5)
The LP38501/3-ADJ is available in the TO-263 package. The thermal resistance depends on the amount of
copper area allocated to heat transfer.
HEATSINKING TO-263, PFM THIN PACKAGES
The TO-263 package and TO-263 THIN package use the copper plane on the PCB as a heatsink. The DAP of
the package is soldered to the copper plane for heat sinking. Figure 26 shows a typical curve for the θ
JA
of the
TO-263 package for different copper area sizes (the thermal performance of both the TO-263 and TO-263 THIN
are the same). The tests were done using a PCB with 1 ounce copper on top side only which were square in
shape.
Figure 26. θ
JA
vs Copper Area for TO-263 Package
As shown in the figure, increasing the copper area beyond 1.5 square inch produces very little improvement.
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