LP5550 Datasheet, PDF (20/28 Pages) National Semiconductor (TI) – PowerWise Technology Compliant Energy Management Unit

LP5550
SNVS378G – OCTOBER 2005 – REVISED APRIL 2013
PFM Mode at Light Load
Z
PWM Mode at
-
Moderate to Heavy
A
Loads
x
High PFM
i
s
Threshold
Load current
increases
Low PFM
Threshold
Pfet on
until
Ipfm limit
reached
Nfet on
drains
conductor
current
until
I inductor=0
High PFM
Voltage
Threshold
reached,
go into
sleep mode
Low PFM
Threshold,
turn on
PFET
Current load
increases,
draws Vout
towards
PFM/PWM
Threshold
PFM/PWM Threshold
Figure 28. Operation in PFM Mode and Transfer to PWM Mode
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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 register (R9) provides the option to override the automatic transition and force PFM or PWM operation (see
R9 - PFM/PWM Force Register). Note that if the operating 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 29
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 subsystems
(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.
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