LP2951 datasheet(20/47 Pages) TI

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6.3 Feature Description

6.3.1 Output Enable

The SHUTDOWN pin for the device is an active-high pin. The output voltage is enabled when the SHUTDOWN

pin voltage is less than the low-level input voltage of the SHUTDOWN pin. The output voltage is disabled when

the SHUTDOWN pin voltage is greater than the high-level input voltage of the SHUTDOWN pin. If independent

control of the output voltage is not needed, connect the SHUTDOWN pin to the GND of the device.

6.3.2 Dropout Voltage

Dropout voltage (VDO) is defined as VIN – VOUT at the rated output current (IRATED), where the pass transistor

is fully on. VIN is the input voltage, VOUT is the output voltage, and IRATED is the maximum IL listed in the

Recommended Operating Conditions table. At this operating point, the pass transistor is driven fully on. Dropout

voltage indirectly specifies a minimum input voltage greater than the nominal programmed output voltage where

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.

DS(ON)

RATED

(1)

6.3.3 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 6-1 shows a diagram of the current limit.

VOUT(NOM)

0 V

0 mA

VOUT

ICL

IRATED

IOUT

Brickwall

Figure 6-1. Current Limit

LP2951-Q1

SLVSAW6H – JUNE 2011 – REVISED NOVEMBER 2024

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