LP38841 Datasheet, PDF (7/18 Pages) National Semiconductor (TI) – 0.8A Ultra Low Dropout Linear Regulators Stable with Ceramic Output Capacitors

LP38841
www.ti.com
SNVS289B – DECEMBER 2004 – REVISED SEPTEMBER 2006
UNDER VOLTAGE LOCKOUT
The bias voltage is monitored by a circuit which prevents the regulator output from turning on if the bias voltage
is below approximately 4V.
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
V_BIAS 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
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
The maximum allowable value for junction to ambient Thermal Resistance, θ_JA, can be calculated using the
formula:
θ_JA = T_Rmax / P_D
These parts are available in TO-220 and 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 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 TO220 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 TO263
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 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 TO-263 PACKAGE
The TO-263 package uses the copper plane on the PCB as a heatsink. The tab of this package is soldered to
the copper plane for heat sinking. The graph below shows a curve for the θ_JA of 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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