LP3913 Datasheet, PDF (51/70 Pages) Texas Instruments – Power Management IC for Flash Memory-Based Portable Media Players

LP3913
www.ti.com
SNVS489I – NOVEMBER 2006 – REVISED SEPTEMBER 2013
V_IN - V_OUT
L
(2)
by storing energy in a magnetic field. During the second portion of each cycle, the control block turns the PFET
switch off, blocking current flow from the input, and then turns the NFET synchronous rectifier on. The inductor
draws current from ground through the NFET to the output filter capacitor and load, which ramps the inductor
current down with a slope of
-V_OUT
L
(3)
The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage
across the load.
PWM Operation
During PWM operation the converter operates as a voltage-mode controller with input voltage feed forward. This
allows the converter to achieve excellent load and line regulation. The DC gain of the power stage is proportional
to the input voltage. To eliminate this dependence, feed forward voltage inversely proportional to the input
voltage is introduced.
Internal Synchronous Rectification
While in PWM mode, the buck uses an internal NFET as a synchronous rectifier to reduce rectifier forward
voltage drop and associated power loss. Synchronous rectification provides a significant improvement in
efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier
diode.
Current Limiting
A current limit feature allows the buck to protect itself and external components during overload conditions PWM
mode implements cycle-by-cycle current limiting using an internal comparator that trips at 1000 mA (typical).
PFM Operation
At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply
current to maintain high efficiency.
The part will automatically transition into PFM mode when either of two conditions occurs for a duration of 32 or
more clock cycles:
The inductor current becomes discontinuous orThe peak PMOS switch current drops below the I_MODE level
V_IN
(Typically I_MODE < 66 mA + _160: )
(4)
During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage
during PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy
load. The PFM comparators sense the output voltage via the feedback pin and control the switching of the output
FETs such that the output voltage ramps between 0.8% and 1.6% (typical) above the nominal PWM output
voltage. If the output voltage is below the ‘high’ PFM comparator threshold, the PMOS power switch is turned on.
It remains on until the output voltage exceeds the ‘high’ PFM threshold or the peak current exceeds the I_PFM level
set for PFM mode. The typical peak current in PFM mode is:
V_IN
I_PFM = 66 mA + _80:
(5)
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