LP38798_15 Datasheet, PDF (15/27 Pages) Texas Instruments – LP38798 High PSRR, Ultra-Low-Noise, 800-mA Linear Voltage Regulator for RF/Analog Circuits

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LP38798
SNOSCT6B – MARCH 2013 – REVISED DECEMBER 2014
Feature Description (continued)
7.3.2 Enable Input Operation
The Enable pin (EN) is pulled high internally by a 2 μA (typical) current source from the IN pin, and internally
clamped at 5 V (typical) by a zener. Pulling the EN pin low, by sinking the I_EN current to ground, will turn the
output off.
If the EN function is not needed the EN pin should be left open (floating). Do not connect the EN pin directly to
V_IN if there is any possibility that V_IN might exceed 5.5 V (that is, EN pin AbsMax). If external pull-up is required,
the external current into the EN pin should be limited to no more than 10 μA.
R_PULL-UP > ( V_PULL-UP - 5V) / 10 μA )
(1)
7.3.3 Undervoltage Lock-Out (UVLO)
The LP38798 incorporates UVLO. The UVLO circuit monitors the input voltage and keeps the LP38798 disabled
while a rising V_IN is less than 2.65 V (typical). The rising UVLO threshold is approximately 350 mV below the
recommended minimum operating V_IN of 3 V.
7.3.4 Output Current Limiting
The LP38798 incorporates active output current limiting. The threshold for the output current limiting is set well
above the ensured output operating current such that it does not interfere with normal operation.
Note that output current limiting is provided as a safety feature and is outside the recommended operating
conditions. Operation at the current limit is not recommended as the device junction temperature (T_J) will rise
rapidly and operation will likely cross into thermal shutdown behavior .
7.3.5 Thermal Shutdown
The LP38798 includes thermal protection that will shut-off the output current when activated by excessive device
dissipation. Thermal shutdown (T_SD) will occur when the junction temperature has risen to 170°C. The junction
temperature must fall typically 12°C from the shutdown temperature for the output current to be restored.
Junction temperature is calculated from the formula in Equation 2:
T_J = (T_A + ( P_D × R_θJA))
(2)
Where the power being dissipated, P_D, is defined as:
P_D = ((V_IN – V_OUT) × I_OUT)
(3)
NOTE
Thermal shutdown is provided as a safety feature and is outside the specified Operating
Ratings temperature range. Operation with a junction temperature (T_J) above 125°C is not
recommended as the device behavior is not specified.
7.4 Device Functional Modes
The LP38798 has two functional modes:
1. Enabled: When the EN pin voltage is above the V_EN(ON) threshold, and V_IN is above the UVLO threshold, the
device is enabled.
2. Disabled: When the EN pin voltage is below the (V_EN(ON) + ΔV_EN) threshold, or V_IN is below the UVLO
threshold, the device is disabled.
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