LP3855ES-2.5 National Semiconductor, LP3855ES-2.5 Datasheet - Page 14

• Manufacturer Part Number: LP3855ES-2.5
• Description: IC,VOLT REGULATOR,FIXED,+2.5V,CMOS,SIP,5PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LP3855ES-2.5

• Rohs Compliant: NO

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Application Hints
The primary source of noise in low-dropout regulators is the
internal reference. In CMOS regulators, noise has a low
frequency component and a high frequency component,
which depend strongly on the silicon area and quiescent
current. Noise can be reduced in two ways: by increasing the
transistor area or by increasing the current drawn by the
internal reference. Increasing the area will decrease the
chance of fitting the die into a smaller package. Increasing
the current drawn by the internal reference increases the
total supply current (ground pin current). Using an optimized
trade-off of ground pin current and die size, LP3852/LP3855
achieves low noise performance and low quiescent current
operation.
The total output noise specification for LP3852/LP3855 is
presented in the Electrical Characteristics table. The Output
noise density at different frequencies is represented by a
curve under typical performance characteristics.
SHORT-CIRCUIT PROTECTION
The LP3852 and LP3855 are short circuit protected and in
the event of a peak over-current condition, the short-circuit
control loop will rapidly drive the output PMOS pass element
off. Once the power pass element shuts down, the control
SENSE PIN
In applications where the regulator output is not very close to
the load, LP3855 can provide better remote load regulation
using the SENSE pin. Figure 4 depicts the advantage of the
SENSE option. LP3852 regulates the voltage at the output
pin. Hence, the voltage at the remote load will be the regu-
lator output voltage minus the drop across the trace resis-
(Continued)
FIGURE 3. Error Flag Operation
14
loop will rapidly cycle the output on and off until the average
power dissipation causes the thermal shutdown circuit to
respond to servo the on/off cycling to a lower frequency.
Please refer to the section on thermal information for power
dissipation calculations.
ERROR FLAG OPERATION
The LP3852/LP3855 produces a logic low signal at the Error
Flag pin when the output drops out of regulation due to low
input voltage, current limiting, or thermal limiting. This flag
has a built in hysteresis. The timing diagram in Figure 3
shows the relationship between the ERROR flag and the
output voltage. In this example, the input voltage is changed
to demonstrate the functionality of the Error Flag.
The internal Error flag comparator has an open drain output
stage. Hence, the ERROR pin should be pulled high through
a pull up resistor. Although the ERROR flag pin can sink
current of 1mA, this current is energy drain from the input
supply. Hence, the value of the pull up resistor should be in
the range of 10kΩ to 1MΩ. The ERROR pin must be
connected to ground if this function is not used. It should
also be noted that when the shutdown pin is pulled low, the
ERROR pin is forced to be invalid for reasons of saving
power in shutdown mode.
tance. For example, in the case of a 3.3V output, if the trace
resistance is 100mΩ, the voltage at the remote load will be
3.15V with 1.5A of load current, I
the voltage at the sense pin. Connecting the sense pin to the
remote load will provide regulation at the remote load, as
shown in Figure 4. If the sense option pin is not required, the
sense pin must be connected to the V
20031007
LOAD
. The LP3855 regulates
OUT
pin.