LP3963ES-2.5 datasheet(13/17 Pages) NSC

13 / 17 page

Application Hints (Continued)

Once the power pass element shuts down, the control 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 LP3963/LP3966 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 1

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 10k

Ω 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.

SENSE PIN

In applications where the regulator output is not very close to

the load, LP3966 can provide better remote load regulation

using the SENSE pin. Figure 2 depicts the advantage of the

SENSE option. LP3963 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-

tance. For example, in the case of a 3.3V output, if the trace

resistance is 100m

Ω, the voltage at the remote load will be

3V with 3A of load current, I

LOAD. The LP3966 regulates 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 2. If the sense option pin is not required, the

sense pin must be connected to the V

OUT pin.

10126707

FIGURE 1. Error Flag Operation

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