LP3987H datasheet(7/9 Pages) POWER

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Preliminary Datasheet

LP3987H

LP3987H – 02 Version 1.2 May.-2013

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Page 7 of 9

Applications Information

Like any low-dropout regulator, the external capacitors

used with the LP3987H must be carefully selected for

regulator stability and performance. Using a capacitor

whose value is > 1µF on the LP3987H input and the

amount of capacitance can be increased without limit.

The input capacitor must be located a distance of not

more than 0.5 inch from the input pin of the IC and

returned to a clean analog ground. There is a special

attention which is the input capacitance should not be

less than output capacitance. Any good quality ceramic or

tantalum can be used for this capacitor. The capacitor

with larger value and lower ESR (equivalent series

resistance) provides better PSRR and line-transient

response. The output capacitor must meet both

requirements for minimum amount of capacitance and

ESR in all LDOs application. The LP3987H is designed

specifically to work with low ESR ceramic output

capacitor in space-saving and performance consideration.

Using a ceramic capacitor whose value is at least 1µF

with ESR is > 25mΩ on the LP3987H output ensures

stability. The LP3987H still works well with output

capacitor of other types due to the wide stable ESR range.

Output capacitor of larger capacitance can reduce noise

and improve load transient response, stability, and PSRR.

The output capacitor should be located not more than 0.5

inch from the VOUT pin of the LP3987H and returned to a

clean analog ground.

Start-up Function Enable Function

The LP3987H features an LDO regulator enable/disable

function. To assure the LDO regulator will switch on, the

EN turn on control level must be greater than 1.4 volts.

The LDO regulator will go into the shutdown mode when

the voltage on the EN pin falls below 0.4 volts. For to

protecting

the

system,

the

LP3987H

have

quick-discharge function. If the enable function is not

needed in a specific application, it may be tied to VIN to

keep the LDO regulator in a continuously on state.

Thermal Considerations

Thermal protection limits power dissipation in LP3987H.

When the operation junction temperature exceeds 165°C,

the OTP circuit starts the thermal shutdown function turn

the pass element off. The pass element turns on again

after the junction temperature cools by 30°C. For continue

operation, do not exceed absolute maximum operation

junction temperature 125°C.

The power dissipation definition in device is :

PD = (VIN−VOUT) x IOUT + VIN x IQ

The maximum power dissipation depends on the thermal

resistance of IC package, PCB layout, the rate of

surroundings airflow and temperature difference between

junction to ambient.

The maximum power dissipation can be calculated by

following formula :

PD(MAX) = ( TJ(MAX) − TA ) /θJA

Where

TJ(MAX) is the maximum operation junction temperature

125°C, TA is the ambient temperature and the θJA is the

junction to ambient thermal resistance. For recommended

operating conditions specification of LP3987H, where

TJ(MAX) is the maximum junction temperature of the die

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