LM76005 datasheet(12/51 Pages) TI2

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7.3 Feature Description

7.3.1 Fixed-Frequency, Peak-Current-Mode Control

The following operation description of the LM76005 refers to Section 7.2 and to the waveforms in Figure 7-1.

The LM76005 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, increases with linear slope. The HS switch is turned

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.

VSW

VIN

D = tON/ TSW

tON

tOFF

TSW

-VD

IOUT

ILPK

ûiL

Figure 7-1. SW Node and Inductor Current Waveforms in Continuous Conduction Mode

The LM76005 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 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 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 when the inductor current valley reaches zero. The LM76005 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 operates in PFM. In PFM, switching frequency is decreased by

the control loop when load current reduces to maintain output voltage regulation. Reference the Section 8.2.3 for

typical steady state switching behavior in PFM. Switching loss is further reduced in PFM operation due to less

frequent switching actions.

In PFM operation, a small positive DC offset is required at the output voltage to activate the PFM detector. The

lower the frequency is in PFM, the more DC offset is needed at VOUT. See Section 6.9 for typical DC offset at

very light load. If the DC offset on VOUT is not acceptable for a given application, TI recommends a static load at

LM76005

SNVSBK5A – FEBRUARY 2020 – REVISED JULY 2020

www.ti.com

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