LM76005 datasheet(11/42 Pages) TI1

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VSW

VIN

D = tON/ TSW

tON

tOFF

TSW

-VD

IOUT

ILPK

ûiL

LM76005-Q1

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SNVSBK6 – FEBRUARY 2020

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7.3 Feature Description

7.3.1 Fixed-Frequency, Peak-Current-Mode Control

The following operation description of the LM76005-Q1 refers to the Functional Block Diagram and to the

waveforms in Figure 6. The LM76005-Q1 supplies a regulated output voltage by turning on the internal high-side

(HS) and low0side (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, increases 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 a 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 6. SW Node and Inductor Current Waveforms in

Continuous Conduction Mode

The LM76005-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 LM76005-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 LM76005-Q1 when the inductor current valley reaches zero. The LM76005-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 LM76005-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.

LM76005 Datasheet(PDF) 11 Page - Texas Instruments

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