LP38853_14 datasheet(15/27 Pages) TI1

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LP38853

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SNVS335C – DECEMBER 2006 – REVISED SEPTEMBER 2011

Figure 32. Typical CSS vs COUT Values

The CSS capacitor must be connected to a clean ground path back to the device ground pin. No components,

other than CSS, should be connected to the SS pin, as there could be adverse effects to VOUT.

If the Soft-Start function is not needed the SS pin should be left open, although some minimal capacitance value

is always recommended.

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

Additional copper area for heat-sinking may be required depending on the maximum device dissipation (PD) and

the maximum anticipated ambient temperature (TA) for the device. 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

(8)

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.

(9)

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.

(10)

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