LP3971 datasheet(14/42 Pages) NSC

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Buck Converter Operation

DEVICE INFORMATION

The LP3971 includes three high efficiency step down DC-DC

switching buck converters. Using a voltage mode architec-

ture 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 induc-

tor 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 approxi-

mately 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 pro-

tection, 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 2.8V 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

IN–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 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 in-

versely 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.

Internal Synchronous Rectification

While in PWM mode, the converters 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 converters to protect itself

and external components during overload conditions. PWM

mode implements current limiting using an internal compara-

tor that trips at 2.1A (typ). If the output is shorted to ground

the device enters a timed current limit mode where the NFET

is turned on for a longer duration until the inductor current

falls below a low threshold, ensuring inductor current has

more time to decay, thereby preventing runaway.

PFM OPERATION

At very light loads, the converter enters PFM mode and

operates with reduced switching frequency and supply cur-

rent 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:

A: The inductor current becomes discontinuous.

B: The peak PMOS switch current drops below the I

MODE

level, (Typically I

MODE < 30 mA + VIN/42

Ω).

20180711

FIGURE 3. Typical PWM Operation

LP3971 数据表(PDF) 14 Page - National Semiconductor (TI)

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