LM76002 datasheet(13/59 Pages) TI2

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VSW

VIN

D = tON/ TSW

tON

tOFF

TSW

-VD

IOUT

ILPK

ûiL

LM76002, LM76003

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SNVSAK0A – OCTOBER 2017 – REVISED OCTOBER 2019

Product Folder Links: LM76002 LM76003

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7.3 Feature Description

7.3.1 Fixed-Frequency, Peak-Current-Mode Control

The following operation description of the LM76002/LM76003 refers to the Functional Block Diagram and to the

waveforms in Figure 10. The LM76002/LM76003 supplies a regulated output voltage by turning on the internal

high side (HS) and low side (LS) NMOS switches with varying duty cycle (D). During high-side switch on-time

tON, the SW pin voltage VSW swings up to approximately VIN, and the inductor current iL increase with linear

slope. The HS switch is off by the control logic. During the HS switch off-time, tOFF, the LS switch is turned on.

Inductor current discharges through the LS switch, which forces the VSW to swing below ground by the voltage

drop across the LS switch. The regulator loop adjusts the duty cycle to maintain a constant output voltage. The

control parameter of buck converter is defined as duty cycle D = tON / tSW. In an ideal buck converter, where

losses are ignored, D is proportional to the output voltage and inversely proportional to the input voltage: D =

VOUT / VIN.

Figure 10. SW Node and Inductor Current Waveforms in

Continuous Conduction Mode

The LM76002/LM76003 synchronous buck converter employs peak current-mode control topology. A voltage-

feedback loop is used to get accurate DC-voltage regulation by adjusting the peak current command based on

voltage offset. The peak inductor current is sensed from the HS switch and compared to the peak current to

control the on-time of the HS switch. The voltage feedback loop is internally compensated, which allows

command for fewer external components, makes it easy to design, and provides stable operation with almost any

combination of output capacitors. The regulator operates with fixed switching frequency in continuous conduction

mode (CCM) and discontinuous conduction mode (DCM). At very light load, the LM76002/LM76003 operates in

PFM to maintain high efficiency, and the switching frequency decreases with reduced load current.

7.3.2 Light Load Operation Modes — PFM and FPWM

DCM operation is employed in the LM76002/LM76003 when the inductor current valley reaches zero. The

LM76002/LM76003 is in DCM when load current is less than half of the peak-to-peak inductor current ripple in

CCM. In DCM, the LS switch is turned off when the inductor current reaches zero. Switching loss is reduced by

turning off the LS FET at zero current, and the conduction loss is lowered by not allowing negative current

conduction. Power conversion efficiency is higher in DCM than CCM under the same conditions.

In DCM, the HS switch on-time reduces with lower load current. When either the minimum HS switch on-time

(tON-MIN) or the minimum peak inductor current (IPEAK-MIN) is reached, the switching frequency decreases to

maintain regulation. At this point, the LM76002/LM76003 operates in PFM. In PFM, switching frequency is

decreased by the control loop when load current reduces to maintain output voltage regulation. Switching loss is

further reduced in PFM operation due to less frequent switching actions.

LM76002 Datasheet(PDF) 13 Page - Texas Instruments

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