LP3907-Q1 Datasheet, PDF (18/53 Pages) Texas Instruments – Dual High-Current Step-Down DC-DC And Dual Linear Regulator

LP3907-Q1
SNVSA55A – DECEMBER 2014 – REVISED MAY 2015
www.ti.com
Feature Description (continued)
I_PFM
=
66
mA
+
V_IN
80:
(4)
Once the PMOS power switch is turned off, the NMOS power switch is turned on until the inductor current ramps
to zero. When the NMOS zero-current condition is detected, the NMOS power switch is turned off. If the output
voltage is below the ‘high’ PFM comparator threshold (see Figure 31), the PMOS switch is again turned on and
the cycle is repeated until the output reaches the desired level. Once the output reaches the ‘high’ PFM
threshold, the NMOS switch is turned on briefly to ramp the inductor current to zero and then both output
switches are turned off and the part enters an extremely low power mode. Quiescent supply current during this
‘sleep’ mode is less than 30 µA, which allows the part to achieve high efficiencies under extremely light load
conditions. When the output drops below the ‘low’ PFM threshold, the cycle repeats to restore the output voltage
to approximately 1.6% above the nominal PWM output voltage.
If the load current should increase during PFM mode (see Figure 31) causing the output voltage to fall below the
‘low2’ PFM threshold, the part automatically transitions into fixed-frequency PWM mode.
8.3.2.7 SW1, SW2 Operation
SW1 and SW2 have selectable output voltages ranging from 0.8 V to 3.5 V (typ.). Both SW1 and SW2 in the
LP3907-Q1 are I²C register controlled and are enabled by default through the internal state machine of the
device following a power-on event that moves the operating mode to the Active state. (See Flexible Power
Sequencing of Multiple Power Supplies.) The SW1 and SW2 output voltages revert to default values when the
power-on sequence has been completed. The default output voltage for each buck converter is factory
programmable. (See Application and Implementation.)
8.3.2.8 SW1, SW2 Control Registers
SW1, SW2 can be enabled/disabled through the corresponding control register.
The Modulation mode PWM/PFM is by default automatic and depends on the load as described above in the
functional description. The modulation mode can be overridden by setting I²C bit to a logic 1 in the corresponding
buck control register, forcing the buck to operate in PWM mode regardless of the load condition.
PFM Mode at Light Load
Load current
increases
High PFM Threshold
~1.016 * Vout
Low1 PFM Threshold
~1.008 * Vout
Pfet on
until
Ipfm limit
reached
Nfet on
drains
inductor
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
Low2 PFM
Threshold
Low2 PFM Threshold
Vout
Low2 PFM Threshold,
switch back to PWMmode
PWM Mode at
Moderate to Heavy
Loads
Figure 31. Operation in PFM Mode and Transfer to PWM Mode
18
Submit Documentation Feedback
Product Folder Links: LP3907-Q1
Copyright © 2014–2015, Texas Instruments Incorporated