LP38855 datasheet(11/20 Pages) TI

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TJA

'TJ = TJ(MAX) - TA(MAX)

LP38855

www.ti.com

SNVS461D – OCTOBER 2006 – REVISED APRIL 2013

ENABLE OPERATION

The Enable pin (EN) provides a mechanism to enable, or disable, the regulator output stage. The Enable pin has

an internal pull-up, through a typical 180 k

Ω resistor, to VBIAS.

If the Enable pin is actively driven, pulling the Enable pin above the VEN threshold of 1.25V (typical) will turn the

regulator output on, while pulling the Enable pin below the VEN threshold will turn the regulator output off. There

is approximately 100 mV of hysteresis built into the Enable threshold provide noise immunity.

If the Enable function is not needed this pin should be left open, or connected directly to VBIAS. If the Enable pin

is left open, stray capacitance on this pin must be minimized, otherwise the output turn-on will be delayed while

the stray capacitance is charged through the internal resistance (rEN).

POWER DISSIPATION AND HEAT-SINKING

A heat-sink 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 the sum of three different points of dissipation in the device.

The first part is the power that is dissipated in the NMOS pass element, and can be determined with the formula:

PD(PASS) = (VIN - VOUT) × IOUT

(1)

The second part is the power that is dissipated in the bias and control circuitry, and can be determined with the

formula:

PD(BIAS) = VBIAS × IGND(BIAS)

where

•

IGND(BIAS) is the portion of the operating ground current of the device that is related to VBIAS

(2)

The third part is the power that is dissipated in portions of the output stage circuitry, and can be determined with

the formula:

PD(IN) = VIN × IGND(IN)

where

•

IGND(IN) is the portion of the operating ground current of the device that is related to VIN

(3)

The total power dissipation is then:

PD = PD(PASS) + PD(BIAS) + PD(IN)

(4)

The maximum allowable junction temperature rise (

ΔTJ) depends on the maximum anticipated ambient

temperature (TA(MAX)) for the application, and the maximum allowable operating junction temperature (TJ(MAX)):

(5)

The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

(6)

Heat-Sinking the TO-220 Package

The TO-220 package has a

θJA rating of 60°C/W, and a θJC rating of 3°C/W. These ratings are for the package

only, no additional heat-sinking, and with no airflow.

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

θJA will be same as shown in next section for

DDPAK/TO-263 package.

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