LP3987
Applications Information
Like 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 a 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.
Enable Function
The
LP3987
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 1.2 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.
Start-up Function
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
LP3987-datasheet
Oct.-2003
6-6
POWER [ LOWPOWER SEMICONDUCTOR INC ]
