LP3856-ADJ_15 Datasheet, PDF (13/22 Pages) Texas Instruments – 3A Fast Response Ultra Low Dropout Linear Regulators

LP3856-ADJ
www.ti.com
SNVS243E – SEPTEMBER 2003 – REVISED APRIL 2013
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 voltage is the product of the load current and
the Rds(on) of the internal MOSFET.
REVERSE CURRENT PATH
The internal MOSFET in LP3856-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 200mA
continuous and 1A peak.
POWER DISSIPATION/HEATSINKING
The LP3856-ADJ can deliver a continuous current of 3A over the full operating temperature range. 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
(3)
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
application, and the maximum allowable junction temperature (T_Jmax):
T_Rmax = T_Jmax− T_Amax
(4)
The maximum allowable value for junction to ambient Thermal Resistance, θ_JA, can be calculated using the
formula:
θ_JA = T_Rmax / P_D
(5)
LP3856-ADJ is 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.
(6)
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. Figure 27 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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