LP38851S-ADJ Datasheet, PDF (16/29 Pages) Texas Instruments – LP38851 800 mA Fast-Response High-Accuracy Adjustable LDO Linear Regulator with Enable and Soft-Start

LP38851
SNVS492C – JUNE 2007 – REVISED APRIL 2013
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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 160 kΩ resistor, to V_BIAS.
If the Enable pin is actively driven, pulling the Enable pin above the V_EN threshold of 1.25V (typical) will turn the
regulator output on, while pulling the Enable pin below the V_EN 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 V_BIAS. 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 (r_EN).
POWER DISSIPATION AND HEAT-SINKING
Additional copper area for heat-sinking may be required depending on the maximum device dissipation (P_D) and
the maximum anticipated ambient temperature (T_A) 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:
P_D(PASS) = (V_IN - V_OUT) × I_OUT
(8)
The second part is the power that is dissipated in the bias and control circuitry, and can be determined with the
formula:
P_D(BIAS) = V_BIAS × I_GND(BIAS)
where
• I_GND(BIAS) is the portion of the operating ground current of the device that is related to V_BIAS
(9)
The third part is the power that is dissipated in portions of the output stage circuitry, and can be determined with
the formula:
P_D(IN) = V_IN × I_GND(IN)
where
• I_GND(IN) is the portion of the operating ground current of the device that is related to V_IN
(10)
The total power dissipation is then:
P_D = P_D(PASS) + P_D(BIAS) + P_D(IN)
(11)
The maximum allowable junction temperature rise (ΔT_J) depends on the maximum anticipated ambient
temperature (T_A) for the application, and the maximum allowable operating junction temperature (T_J(MAX)) .
'_J = T_J(MAX) - T_A(MAX)
(12)
The maximum allowable value for junction to ambient Thermal Resistance, θ_JA, can be calculated using the
formula:
T
JA
d
'T_J
P_D
(13)
Heat-Sinking The SFM Package
The SFM package has a θ_JA rating of 60°C/W and a θ_JC rating of 3°C/W. These ratings are for the package only,
no additional heat-sinking, and with no airflow. If the needed θ_JA, as calculated above, is greater than or equal to
60°C/W then no additional heat-sinking is required since the package can safely dissipate the heat and not
exceed the operating T_J(MAX). If the needed θ_JA is less than 60°C/W then additional heat-sinking is needed.
The thermal resistance of a SFM 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 PFM
package.
The heat-sink to be used in the application should have a heat-sink to ambient thermal resistance, θ_HA:
θ_HA ≤ θ_JA - (θ_CH + θ_JC)
16
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