LP38853 Datasheet, PDF (13/18 Pages) Texas Instruments – LP38853 3A Fast-Response High-Accuracy Adjustable LDO Linear Regulator with Enable and Soft-Start

10X difference in leakage between 25°C and 85°C, so the
maximum ambient temperature must be included in the ca-
pacitor selection process.
Typical C_SS values will be in the range of 1 nF to 100 nF,
providing typical Soft-Start times in the range of 70 μs to 7 ms
(5τ). Values less than 1 nF can be used, but the Soft-Start
effect will be minimal. Values larger than 100 nF will provide
soft-start, but may not be fully discharged if V_BIAS falls from
the UVLVO threshold to less than 500 mV in less than 100
µs.
Figure 2 shows the relationship between the C_OUT value and
a typical C_SS value.
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FIGURE 2. Typical C_SS vs C_OUT Values
The C_SS capacitor must be connected to a clean ground path
back to the device ground pin. No components, other than
C_SS, should be connected to the SS pin, as there could be
adverse effects to V_OUT.
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 dis-
able, the regulator output stage. The Enable pin has an
internal pull-up, through a typical 180 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 regu-
lator output on, while pulling the Enable pin below the V_EN
threshold will turn the regulator output off. There is approxi-
mately 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, oth-
erwise 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 de-
pending on the maximum device dissipation (P_D) and the
maximum anticipated ambient temperature (T_A) for the de-
vice. Under all possible conditions, the junction temperature
must be within the range specified under operating condi-
tions.
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)
(9)
 
where I_GND(BIAS) is the portion of the operating ground current
of the device that is related to V_BIAS.
The third part is the power that is dissipated in portions of the
output stage circuitry, and can be determined with the formu-
la:
 
P_D(IN) = V_IN × I_GND(IN)
(10)
 
where I_GND(IN) is the portion of the operating ground current
of the device that is related to V_IN.
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) de-
pends on the maximum anticipated ambient temperature
(T_A) for the application, and the maximum allowable operating
junction temperature (T_J(MAX)) .
(12)
The maximum allowable value for junction to ambient Ther-
mal Resistance, θ_JA, can be calculated using the formula:
(13)
Heat-Sinking The TO-220 Package
The TO220-5 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 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 TO-263 package.
The heat-sink to be used in the application should have a
heat-sink to ambient thermal resistance, θ_HA:
(14)
where θ_JA is the required total thermal resistance from the
junction to the ambient air, θ_CH is the thermal resistance from
the case to the surface of the heart-sink, and θ_JC is the thermal
resistance from the junction to the surface of the case.
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