LP3925 Datasheet, PDF (22/51 Pages) Texas Instruments – High Performance Power Management Unit for Handset Applications

Buck Information
The LP3925 has integrated three high efficiency step-down
DC-DC switching buck converters that deliver a constant volt-
age from a single cell battery to portable devices. Using
voltage mode architecture with synchronous rectification, the
buck has the ability to deliver up to 800 mA depending on the
input voltage and output voltage, ambient temperature, and
the inductor chosen.
There are two modes of operation depending on the current
required - PWM (Pulse Width Modulation), ECO (ECOnomy)
mode. The device operates in PWM mode at load currents of
approximately 50 mA (typ.) or higher. Lighter output current
loads cause the device to automatically switch into ECO
mode for reduced current consumption and a longer battery
life. 2 buck regulators are capable of DVS control. Additional
features include soft-start, under voltage protection, current
overload protection, and thermal shutdown protection. Only
three external power components are required for implemen-
tation.
BUCK CIRCUIT OPERATION
The switching buck converter operates as follows. During the
first portion of each switching cycle, the control block in the
LP3925 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 (VIN–VOUT)/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 in-
ductor draws current from ground through the NFET to the
output filter capacitor and load, which ramps the inductor cur-
rent down with a slope of –V_OUT/L.
The output filter stores charge when the inductor current is
high, and releases it when 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 rec-
tifier 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 excellent 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
mode, the output voltage is regulated by switching at a con-
stant 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.
FIGURE 10. Typical PWM Operation
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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 pro-
vides 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 to protect itself and external
components during overload conditions. PWM mode imple-
ments current limit using an internal comparator that trips at
1100 mA (typ.). If the output is shorted to ground and output
voltage becomes lower than 0.3V (typ.), the device enters a
timed current limit mode where the switching frequency will
be one fourth, and NFET synchronous rectifier is disabled,
thereby preventing excess current and thermal runaway.
ECO MODE OPERATION
The buck switches from ECO state to PWM state based on
output load current. At light loads (less than 50mA), the con-
verter enters ECO mode. In this mode the part operates with
low Iq. During ECO operation, the converter positions the
output voltage slightly higher (+30mV typ.) than the nominal
output voltage in PWM operation. The more complete under-
standing of an ECO mode operation can be derived from
diagram in Figure 11.
FIGURE 11. Typical ECO Operation
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Power FETs are controlled by “SW Control” which is a com-
bination of ‘comp’ and ‘pwm’ signals, dependent on ‘PWM
threshold’ level. “ECO Comparator” is a simple comparator
with hysteresis. “Err Amp” and “PWM” are error amplifier with
a ramp generator. ‘PWM threshold’ is current sensing at
PFET, that lets control logic know which input has to be used.
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