Datasheet

LP3892
www.ti.com SNVS236D –SEPTEMBER 2003–REVISED APRIL 2013
SHUTDOWN OPERATION
Pulling down the shutdown (S/D) pin will turn-off the regulator. Pin S/D must be actively terminated through a
pull-up resistor (10 kΩ to 100 kΩ) for a proper operation. If this pin is driven from a source that actively pulls high
and low (such as a CMOS rail to rail comparator), the pull-up resistor is not required. This pin must be tied to Vin
if not used.
POWER DISSIPATION/HEATSINKING
A heatsink may be required depending on the maximum power dissipation and maximum ambient temperature of
the application. Under all possible conditions, the junction temperature must be within the range specified under
operating conditions. The total power dissipation of the device is given by:
P
D
= (V
IN
−V
OUT
)I
OUT
+ (V
IN
)I
GND
(1)
where I
GND
is the operating ground current of the device.
The maximum allowable temperature rise (T
Rmax
) depends on the maximum ambient temperature (T
Amax
) of the
application, and the maximum allowable junction temperature (T
Jmax
):
T
Rmax
= T
Jmax
− T
Amax
(2)
The maximum allowable value for junction to ambient Thermal Resistance, θ
JA
, can be calculated using the
formula:
θ
JA
= T
Rmax
/ P
D
(3)
These parts are available in TO-220 and DDPAK/TO-263 packages. The thermal resistance depends on amount
of copper area or heat sink, and on air flow. If the maximum allowable value of θ
JA
calculated above is ≥ 60 °C/W
for TO-220 package and ≥ 60 °C/W for DDPAK/TO-263 package no heatsink is needed since the package can
dissipate enough heat to satisfy these requirements. If the value for allowable θ
JA
falls below these limits, a heat
sink is required.
HEATSINKING TO-220 PACKAGE
The thermal resistance of a TO-220 package can be reduced by attaching it to a heat sink or a copper plane on
a PC board. If a copper plane is to be used, the values of θ
JA
will be same as shown in next section for
DDPAK/TO-263 package.
The heatsink to be used in the application should have a heatsink to ambient thermal resistance, θ
HA
≤ θ
JA
− θ
CH
− θ
JC
.
In this equation, θ
CH
is the thermal resistance from the case to the surface of the heat sink and θ
JC
is the thermal
resistance from the junction to the surface of the case. θ
JC
is about 3°C/W for a TO-220 package. The value for
θ
CH
depends on method of attachment, insulator, etc. θ
CH
varies between 1.5°C/W to 2.5°C/W. If the exact value
is unknown, 2°C/W can be assumed.
HEATSINKING DDPAK/TO-263 PACKAGE
The DDPAK/TO-263 package uses the copper plane on the PCB as a heatsink. The tab of these packages are
soldered to the copper plane for heat sinking. The graph below shows a curve for the θ
JA
of DDPAK/TO-263
package for different copper area sizes, using a typical PCB with 1 ounce copper and no solder mask over the
copper area for heat sinking.
Figure 20. θ
JA
vs Copper (1 Ounce) Area for DDPAK/TO-263 Package
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