LP3913 datasheet(51/70 Pages) TI1

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VIN

80:

IPFM = 66 mA +

VIN

160:

)

(Typically IMODE < 66 mA +

-VOUT

VIN - VOUT

LP3913

www.ti.com

SNVS489I – NOVEMBER 2006 – REVISED SEPTEMBER 2013

(2)

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

(3)

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 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 1000 mA (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:

The inductor current becomes discontinuous orThe peak PMOS switch current drops below the IMODE level

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During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage

during PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy

load. The PFM comparators sense the output voltage via the feedback pin and control the switching of the output

FETs such that the output voltage ramps between 0.8% and 1.6% (typical) above the nominal PWM output

voltage. If the output voltage is below the ‘high’ PFM comparator threshold, the PMOS power switch is turned on.

It remains on until the output voltage exceeds the ‘high’ PFM threshold or the peak current exceeds the IPFM level

set for PFM mode. The typical peak current in PFM mode is:

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