LM76002 datasheet(13/54 Pages) TI1

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VSW

VIN

D = tON/ TSW

tON

tOFF

TSW

-VD

IOUT

ILPK

ûiL

LM76002-Q1, LM76003-Q1

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SNVSAU3 – DECEMBER 2017

Product Folder Links: LM76002-Q1 LM76003-Q1

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7.3 Feature Description

7.3.1 Fixed-Frequency, Peak-Current-Mode Control

The following operation description of the LM76002-Q1/LM76003-Q1 refers to the Functional Block Diagram and

to the waveforms in Figure 10. The LM76002-Q1/LM76003-Q1 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-Q1/LM76003-Q1 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-

Q1/LM76003-Q1 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-Q1/LM76003-Q1 when the inductor current valley reaches zero. The

LM76002-Q1/LM76003-Q1 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-Q1/LM76003-Q1 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

LM76002 Datasheet(HTML) 13 Page - Texas Instruments

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