LP3871 Datasheet, PDF (12/14 Pages) National Semiconductor (TI) – 0.8A Fast Ultra Low Dropout Linear Regulators

Application Hints (Continued)
DROPOUT VOLTAGE
The dropout voltage of a regulator is defined as the minimum
input-to-output differential required to stay within 2% of the
nominal output voltage. For CMOS LDOs, the dropout volt-
age is the product of the load current and the Rds(on) of the
internal MOSFET.
REVERSE CURRENT PATH
The internal MOSFET in LP3871 and LP3874 has an inher-
ent parasitic diode. During normal operation, the input volt-
age 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 200mA continuous and 1A
peak.
POWER DISSIPATION/HEATSINKING
LP3871 and LP3874 can deliver a continuous current of
0.8A over the full operating temperature range. A heatsink
may be required depending on the maximum power dissipa-
tion 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
(specified under Electrical Characteristics).
The maximum allowable temperature rise (T_Rmax) depends
on the maximum ambient temperature (T_Amax) of the appli-
cation, and the maximum allowable junction temperature
(T_Jmax):
T_Rmax = T_Jmax− T_Amax
The maximum allowable value for junction to ambient Ther-
mal Resistance, θ_JA, can be calculated using the formula:
θ_JA = T_Rmax / P_D
LP3871 and LP3874 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 resis-
tance 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 de-
pends 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 these packages are soldered to the
copper plane for heat sinking. Figure 3 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.
20063132
FIGURE 3. θ_JA vs Copper (1 Ounce) Area for TO-263
package
As shown in the figure, increasing the copper area beyond 1
square inch produces very little improvement. The minimum
value for θ_JA for the TO-263 package mounted to a PCB is
32˚C/W.
Figure 4 shows the maximum allowable power dissipation
for TO-263 packages for different ambient temperatures,
assuming θ_JA is 35˚C/W and the maximum junction tempera-
ture is 125˚C.
20063133
FIGURE 4. Maximum power dissipation vs ambient
temperature for TO-263 package
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