LP3906 datasheet(19/47 Pages) TI

19 / 47 page

VIN

160:

IMODE < 66 mA

)

(Typically

-VOUT

VIN - VOUT

LP3906

www.ti.com

SNVS456M – AUGUST 2006 – REVISED MAY 2013

Additional features include soft-start, under-voltage lock-out, current overload protection, and thermal overload

protection.

CIRCUIT OPERATION DESCRIPTION

A buck converter contains a control block, a switching PFET connected between input and output, a synchronous

rectifying NFET connected between the output and ground (BCKGND pin) and a feedback path. 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

(1)

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

(2)

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 converter to protect itself and external components during overload conditions.

PWM mode implements current limiting using an internal comparator that trips at 2.0 A (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.

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 2000mA (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:

1. The inductor current becomes discontinuous

2. The peak PMOS switch current drops below the IMODE level

(3)

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