LP38500-ADJ, LP38502-ADJ

SNVS539H – NOVEMBER 2007 – REVISED SEPTEMBER 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

m

Ω. 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 RDS(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 VIN, 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-k

Ω pull-up resistor to VIN, 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 VIN if not used (it must not be left floating).

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