LP38502-ADJ Datasheet, PDF (12/27 Pages) Texas Instruments – LP3850x-ADJ, LP3850xA-ADJ 1.5-A Flexcap Low-Dropout Linear Regulatorfor 2.7-V to 5.5-V Inputs

LP38500-ADJ, LP38502-ADJ
SNVS539G – NOVEMBER 2007 – REVISED JUNE 2015
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Feature Description (continued)
In cases where extremely fast load changes occur, the output capacitance may have to be increased. For fast
changing loads, the internal parasitics of ESR (equivalent series resistance) and ESL (equivalent series
inductance) degrade the capacitor’s ability to source current quickly to the load. The best capacitor types for
transient performance are (in order):
1. Multilayer Ceramic: with the lowest values of ESR and ESL, they can have ESR values in the range of a few
mΩ. Disadvantage: capacitance values above about 22 µF significantly increase in cost.
2. Low-ESR Aluminum Electrolytics: these are aluminum types (like OSCON) with a special electrolyte which
provides extremely low ESR values, and are the closest to ceramic performance while still providing large
amounts of capacitance. These are cheaper (by capacitance) than ceramic.
3. Solid tantalum: can provide several hundred µF of capacitance, transient performance is slightly worse than
OSCON type capacitors, cheaper than ceramic in large values.
4. General purpose aluminum electrolytics: cheap and provide a lot of capacitance, but give the worst
performance.
In general, managing load transients is done by paralleling ceramic capacitance with a larger bulk capacitance.
In this way, the ceramic can source current during the rapidly changing edge and the bulk capacitor can support
the load current after the first initial spike in current.
7.3.3 Dropout Voltage
The dropout voltage of a regulator is defined as the input-to-output differential required by the regulator to keep
the output voltage within 2% of the nominal value. For CMOS LDOs, the dropout voltage is the product of the
load current and the R_DS(on) of the internal MOSFET pass element.
Since the output voltage is beginning to “drop out” of regulation when it drops by 2%, electrical performance of
the device will be reduced compared to the values listed in the Electrical Characteristics table for some
parameters (line and load regulation and PSRR would be affected).
7.3.4 Reverse Current Path
The internal MOSFET pass element in the LP38500-ADJ and LP38502-ADJ has an inherent parasitic diode.
During normal operation, the input voltage is higher than the output voltage and the parasitic diode is reverse
biased. However, if the output is pulled above the input in an application, then current flows from the output to
the input as the parasitic diode gets forward biased. The output can be pulled above the input as long as the
current in the parasitic diode is limited to 200-mA continuous and 1-A peak. The regulator output pin should not
be taken below ground potential. If the LP38500-ADJ and LP38502-ADJ is used in a dual-supply system where
the regulator load is returned to a negative supply, the output must be diode-clamped to ground.
7.4 Device Functional Modes
7.4.1 Short-Circuit Protection
The LP38500-ADJ and LP38502-ADJ contain internal current limiting which will reduce output current to a safe
value if the output is overloaded or shorted. Depending upon the value of V_IN, thermal limiting may also become
active as the average power dissipated causes the die temperature to increase to the limit value (about 170°C).
The hysteresis of the thermal shutdown circuitry can result in a “cyclic” behavior on the output as the die
temperature heats and cools.
7.4.2 Enable Operation (LP38502-ADJ Only)
The Enable pin (EN) must be actively terminated by either a 10-kΩ pull-up resistor to V_IN, or a driver which
actively pulls high and low (such as a CMOS rail to rail comparator). If active drive is used, the pull-up resistor is
not required. This pin must be tied to V_IN if not used (it must not be left floating).
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