LP3891ES-1.2 Datasheet, PDF (9/19 Pages) Texas Instruments – 1.5% Output Accuracy (25°C), 60 nA Typical Quiescent Current in Shutdown

LP3891
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SNVS235D – 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.
(4)
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 TO220 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. Figure 19 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.
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