LP3855-ADJ

SNVS244F – SEPTEMBER 2003 – REVISED APRIL 2013

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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 LP3855-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 LP3855-ADJ can deliver a continuous current of 1.5A 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:

(3)

(4)

The maximum allowable value for junction to ambient Thermal Resistance,

formula:

(5)

The LP3855-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

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

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

package.

The heatsink to be used in the application should have a heatsink to ambient thermal resistance,

(6)

In this equation,

resistance from the junction to the surface of the case.

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 20 shows a curve for the

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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