LP3905 Datasheet, PDF (8/15 Pages) National Semiconductor (TI) – Power Management Unit For Low Power Handheld Applications

Functional Description (Continued)
DC/DC BUCK REGULATORS
The LP3905 Buck regulators are high efficiency step down
DC-DC switching converters used for delivering a constant
voltage from either a single Li-Ion or three cell NiMH/NiCd
battery to portable devices such as cell phones and PDAs.
Using a voltage mode architecture with synchronous rectifi-
cation, the Buck Regulators have the ability to deliver up to
600 mA depending on the input voltage, output voltage,
ambient temperature and the inductor chosen.
There are three modes of operation depending on the cur-
rent required - PWM, PFM, and shutdown. The standard
device operates in PWM mode at load currents of approxi-
mately 80 mA or higher, having voltage tolerance of ±4%
with 90% efficiency or better. Lighter load currents cause the
device to automatically switch into PFM for reduced current
BUCK CONVERTER BLOCK DIAGRAM
consumption and a longer battery life. Shutdown mode turns
off the device, offering the lowest current consumption . A
fixed mode device is also available which is fixed in PWM
mode for both low and high load currents.
An adjustable voltage version is also avalable for which the
output voltage can be selected by using two external resis-
tors at each of the two buck outputs.
Additional features include soft-start, under voltage protec-
tion, current overload protection, and thermal shutdown pro-
tection.
The part uses an internal reference voltage of 0.5V. It is
recommended to keep the part in shutdown until the input
voltage is 3V or higher.
FIGURE 7. Simplified Functional Diagram
20152918
CIRCUIT OPERATION
The LP3905 Buck regulators operate as follows. During the
first portion of each switching cycle, the control block in the
LP3905 turns on the internal PFET switch. This allows cur-
rent 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
www.national.com
8