LP3987 datasheet(6/8 Pages) POWER

6 / 8 page

LP3987

LP3987-datasheet

Oct.-2003

6-6

Applications Information

any

low-dropout

regulator,

the

external

capacitors used with the LP3987 must be carefully

selected for regulator stability and performance.

Using a capacitor whose value is > 1µF on the

LP3987 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. 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

LP3987 is designed specifically to work with low ESR

ceramic output capacitor in space-saving and

performance

consideration.

Using

ceramic

capacitor whose value is at least 1µF with ESR is >

25mΩ on the LP3987 output ensures stability. The

LP3987 still works well with output capacitor of other

types due to the wide stable ESR range. Figure 1

shows the curves of allowable ESR range as a

function of load current for various output capacitor

values. 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 LP3987 and returned to a clean analog ground.

Start-up Function

Enable Function

The

LP3987

features

LDO

regulator

enable/disable function. To assure the LDO regulator

will switch on, the EN turn on control level must be

greater than 1.2 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 LP3987 have a 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.

Bypass Capacitor and Low Noise

Connecting a 22nF between the BP pin and GND pin

significantly reduces noise on the regulator output, it

is critical that the capacitor connection between the

BP pin and GND pin be direct and PCB traces should

be as short as possible. There is a relationship

between the bypass capacitor value and the LDO

regulator turn on time. DC leakage on this pin can

affect the LDO regulator output noise and voltage

regulation performance.

Thermal Considerations

Thermal protection limits power dissipation in

LP3987. When the operation junction temperature

exceeds 165°C, the OTP circuit starts the thermal

shutdown function turn the pass element off. The

pass element turn on again after the junction

temperature cools by 30°C.

For continue operation, do not exceed absolute

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