UCC386, UCC387, UCC388

LOW DROPOUT 200mA LINEAR REGULATOR

SLUS377B - JULY 1999 - REVISED MAY 2000

5

POST OFFICE BOX 655303

APPLICATION INFORMATION

overcurrent protection

The UCC386/7/8 uses a fixed, absolute, current limit in conjunction with a timed overcurrent function that

significantly reduces power dissipation in the event of a shorted load (see Figure 1). In this diagram, a 100-mA

load is applied to the output of the LDO. At some point, a fault is applied. When the current level exceeds the

overcurrent threshold of about 300 mA, a timer is started. If the current does not fall below the overcurrent

threshold before the timer times out, about 5.6 ms, the LDO declares an overcurrent condition exists and turns

off its output for about 5.6 ms. Note that the output current is internally limited to 600 mA. After the output has

been off for 5.6 ms, it is turned on for about 800

µs and again limited to 600 mA. If the current does not fall below

the overcurrent threshold before the 800-

µs timer expires, the output is again turned off for 5.6 ms. This process

repeats itself until the fault condition is removed from the output of the LDO. The average current supplied to

the faulted load by the LDO is approximately 112 mA. This is well below the maximum rated current of 200 mA

of the LDO. Therefore, for most applications that have adequate thermal dissipation for the LDO to operate at

full rated load, the thermal dissipation will also be adequate in a faulted condition.

UDG-98088

FAULT

APPLIED

0

100 mA

5.6 ms

5.6 ms

800 µs

300 mA

600 mA

FAULT

REMOVED

Figure 1. Current Waveform During a Fault

thermal shutdown

The LDOs have a thermal shutdown circuit that will turn the LDO output off before the die temperature reaches

damaging levels. When the die cools, the LDO will again function. The thermal shutdown circuit has a turn-off

threshold of nominally 140

°C, and a turn-on threshold of 115°C. These temperatures insure that the LDO will

not be damaged due to excessive power dissipation.

maximum load recovery

The LDO will start a load that has a large capacitance and a dc current component. One of the consequences

of the LDOs fault behavior is a maximum output capacitor value and load current that the LDO can restart after

an overcurrent condition has been declared. Figure 2 shows the maximum load that the LDO can re-start from

a faulted condition with a given output filter capacitor. Note that the LDO can start a much higher load than it

can restart after a fault. If the LDO is hiccuping into a load that it cannot re-start, either momentarily

disconnecting the load or a power cycle allows the LDO to start the load.