L7983 datasheet(12/43 Pages) STMICROELECTRONICS

12 / 43 page

5.2.1

LNM/LCM selection and synchronization

Depending on the low-side power MOSFET management, the inductor current can be allowed to reverse or not.

The choice can be performed during device operation by acting on the LNM/LCM pin.

When the low-noise mode (LNM) operation is selected, by forcing high pin LNM/LCM, the inductor current can

reverse. In this way a constant switching frequency is achieved, so limiting the output voltage ripple and providing

a prompt transient response.

Figure 7. LNM selected, no load

If the LNM/LCM pin is forced low, the low-consumption mode (LCM) is activated, with the aim of maximizing the

light load efficiency. When LCM is selected, the high-side MOSFET is turned on as soon as the FB pin is sensed

lower than VREFLCM, i.e. VREF reference voltage increased by 2% typ.:

VREFLCM=VREF∙1,02typ.

(2)

In LCM mode the HS MOS is turned on until ISKIP current is reached, then it is turned-off. The low-side MOSFET

is then turned on until one of the following conditions occurs:

•

The sensed inductor current drops to zero

•

The switching period (programmed through the FSW pin) has expired and the FB pin is still lower than the

voltage reference. In this case a new switching cycle is performed.

In LCM working mode the regulator switching frequency is load-dependent (i.e. LX pulses can be skipped) with

greater advantage to power conversion efficiency at light load.

The following waveforms are showing the switching activity in LCM working mode and different load.

L7983

Control loop and voltage programming

DS13354 - Rev 1

page 12/43

L7983 Datasheet(PDF) 12 Page - STMicroelectronics