LP5550 datasheet(22/25 Pages) NSC

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Application Information

PWM/PFM FORCE REGISTER (R9)

By default, the LP5550 automatically transitions between

PFM and PWM to optimize efficiency. The PWM/PFM force

transition and force PFM or PWM operation (see R9 –

PWM/PFM Force Register declaration). Note that if the op-

erating mode of the regulator is forced to be PFM then the

switch current limit is reduced to 100 mA (50 mA average

load current).

EN/RESETN

The LP5550 can be shutdown via the ENABLE or RESETN

pins, or by issuing a shutdown command from PWI. To

disable the LP5550 via hardware (as opposed to the PWI

shutdown command), pull the ENABLE and/or the RESETN

pin(s) low. To enable the LP5550, both the ENABLE and the

RESETN pins must be high. Once enabled, the LP5550

engages the power-up sequence and all voltages return to

their default values.

When using PWI to issue a shutdown command, the PWI will

be disabled along with the regulators in the LP5550. To

re-enable the part, either the ENABLE, RESETN, or both

pins must be toggled (high – low – high). The part will then

enter the power-up sequence and all voltages will return to

their default values. Figure 8 summarizes the ENABLE/

RESETN control.

The ENABLE and RESETN pins provide flexibility for system

control. In larger systems such as a mobile phone, it can be

advantageous to enable/disable a subsystem independently.

For example, the LP5550 may be powering the applications

processor in a mobile phone. The system controller can

power down the applications processor via the ENABLE pin,

but leave on other subsystems. When the phone is turned off

or in a fault condition, the system controller can have a

global reset command that is connected to all the sub-

systems (RESETN for the LP5550). However, if this type of

control is not needed, the ENABLE and RESETN pins can

be tied together and used as a single enable/disable pin.

INDUCTOR

A 10uH or 4.7uH inductor should be used with the LP5550.

The inductor should be rated to handle the peak load current

plus the ripple current:

CURRENT LIMIT

The switching converter in the LP5550 detects the peak

inductor current and limits it for protection (see Electrical

Characteristics table and/or Typical Performance section).

To determine the average current limit from the peak current

limit, the inductor size, input and output voltage, and switch-

ing frequency must be known. The LP5550 is designed to

work with a 4.7uH or 10uH inductor, so:

INPUT CAPACITOR

The input capacitor to the switching converter supplies the

AC switching current drawn from the switching action of the

internal power FETs. The input current of a buck converter is

discontinuous, so the ripple current supplied by the input

capacitor is large. The input capacitor must be rated to

handle this current:

The power dissipated in the input capacitor is given by:

The input capacitor must be rated to handle both the RMS

current and the dissipated power. A 10 µF ceramic capacitor

is recommended for the LP5550.

20154551

FIGURE 8. ENABLE and RESETN operation

LP5550 Datasheet(PDF) 22 Page - National Semiconductor (TI)

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