7.3 Feature Description

7.3.1 Output Enable

The ON/OFF pin for the device is an active-high pin. The output voltage is enabled when the voltage of the

ON/OFF pin is greater than the high-level input voltage of the ON/OFF pin and disabled with the ON/OFF pin

voltage is less than the low-level input voltage of the ON/OFF pin. If independent control of the output voltage is

not needed, connect the ON/OFF pin to the input of the device.

The device has an internal pulldown circuit that activates when the device is disabled by pulling the ON/OFF pin

voltage lower than the low-level input voltage of the ON/OFF pin to actively discharge the output voltage.

7.3.2 Dropout Voltage

Dropout voltage (VDO) is defined as the input voltage minus the output voltage (VIN – VOUT) at the rated output

current (IRATED), where the pass transistor is fully on. IRATED is the maximum IOUT listed in the Recommended

Operating Conditions table. The pass transistor is in the ohmic or triode region of operation, and acts as a

switch. The dropout voltage indirectly specifies a minimum input voltage greater than the nominal programmed

output voltage at which the output voltage is expected to stay in regulation. If the input voltage falls to less than

the nominal output regulation, then the output voltage falls as well.

For a CMOS regulator, the dropout voltage is determined by the drain-source on-state resistance (RDS(ON)) of the

pass transistor. Therefore, if the linear regulator operates at less than the rated current, the dropout voltage for

that current scales accordingly. The following equation calculates the RDS(ON) of the device.

R

=

DS(ON)

V

DO

I

RATED

(1)

7.3.3 Current Limit

The device has an internal current limit circuit that protects the regulator during transient high-load current faults

or shorting events. The current limit is a brick-wall scheme. In a high-load current fault, the brick-wall scheme

limits the output current to the current limit (ICL). ICL is listed in the Electrical Characteristics table.

The output voltage is not regulated when the device is in current limit. When a current limit event occurs, the

device begins to heat up because of the increase in power dissipation. When the device is in brick-wall current

limit, the pass transistor dissipates power [(VIN – VOUT) × ICL]. If thermal shutdown is triggered, the device

turns off. After the device cools down, the internal thermal shutdown circuit turns the device back on. If the

output current fault condition continues, the device cycles between current limit and thermal shutdown. For more

information on current limits, see the Know Your Limits application note.

Figure 7-3 shows a diagram of the current limit.

LP2985-N

SNVS018Z – MARCH 2000 – REVISED MAY 2023

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