LP3971_16 Datasheet, PDF (17/52 Pages) Texas Instruments – Power Management Unit for Advanced Application Processors

LP3971
www.ti.com
SNVS432V – JANUARY 2006 – REVISED MAY 2013
BUCK CONVERTER OPERATION
DEVICE INFORMATION
The LP3971 includes three high efficiency step down DC-DC switching buck converters. Using a voltage mode
architecture with synchronous rectification, the buck converters have the ability to deliver up to 1600 mA
depending on the input voltage, output voltage, ambient temperature and the inductor chosen.
There are three modes of operation depending on the current required - PWM, PFM, and shutdown. The device
operates in PWM mode at load currents of approximately 100 mA or higher, having voltage tolerance of ±3%
with 95% efficiency or better. Lighter load currents cause the device to automatically switch into PFM for reduced
current consumption. Shutdown mode turns off the device, offering the lowest current consumption (I_{Q, SHUTDOWN}
= 0.01 µA typ).
Additional features include soft-start, under voltage protection, current overload protection, and thermal shutdown
protection.
The part uses an internal reference voltage of 0.5V. It is recommended to keep the part in shutdown until the
input voltage is 2.7V or higher.
CIRCUIT OPERATION
The buck converter operates as follows. During the first portion of each switching cycle, the control block turns
on the internal PFET switch. This allows current to flow from the input through the inductor to the output filter
capacitor and load. The inductor limits the current to a ramp with a slope of (V_IN–V_OUT)/L, by storing energy in a
magnetic field.
During the second portion of each cycle, the controller 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.
The output filter stores charge when the inductor current is high, and releases it when inductor current is low,
smoothing the voltage across the load.
The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the
load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and
synchronous rectifier at the SW pin to a low-pass filter formed by the inductor and output filter capacitor. The
output voltage is equal to the average voltage at the SW pin.
PWM OPERATION
During PWM operation the converter operates as a voltage mode controller with input voltage feed forward. This
allows the converter to achieve good load and line regulation. The DC gain of the power stage is proportional to
the input voltage. To eliminate this dependence, feed forward inversely proportional to the input voltage is
introduced.
While in PWM (Pulse Width Modulation) mode, the output voltage is regulated by switching at a constant
frequency and then modulating the energy per cycle to control power to the load. At the beginning of each clock
cycle the PFET switch is turned on and the inductor current ramps up until the comparator trips and the control
logic turns off the switch. The current limit comparator can also turn off the switch in case the current limit of the
PFET is exceeded. Then the NFET switch is turned on and the inductor current ramps down. The next cycle is
initiated by the clock turning off the NFET and turning on the PFET.
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