L6997S

8/30

(6)

Where

Figure 7. Integrator loop block diagram

Respect to a traditional PWM controller, that has an internal oscillator setting the switching frequency, in a hys-

teretic system the frequency can change with some parameters. For example, while in a standard fixed switch-

ing frequency topology, the increase of the losses (increasing the output current, for example) generates a

variation in the On Time and Off Time, in a fixed On Time topology , the increase of the losses generates only

a variation on the Off Time, changing the switching frequency. In the device is implemented the voltage feed-

forward circuit that allows constant switching frequency during steady-sate operation and withinthe input range

variation. Any way there are many factors affecting switching frequency accuracy in steady-state operation.

Some of these are internal as dead times, which depends on high side MOSFET driver. Others related to the

external components as high side MOSFET gate charge and gate resistance, voltage drops on supply and

ground rails, low side and high side RDSON and inductor parasitic resistance.

During a positive load transient, (the output current increases), the converter switches at its maximum frequency

(the period is TON+TOFFmin) to recover the output voltage drop. During a negative load transient, (the output

current decreases), the device stops to switch (high side MOSFET remains off).

4.3 Transition from PWM to PFM/PSK

To achieve high efficiency at light load conditions, PFM mode is provided. The PFM mode differs from the PWM

mode essentially for the off phase; the on phase is the same. In PFM after a On cycle the system turns-on the

low side MOSFET until the inductor current goes down zero, when the zero-crossing comparator turns off the

low side MOSFET. In PWM mode, after On cycle, the system keeps the low side MOSFET on until the next turn-

on cycle, so the energy stored in the output capacitor will flow through the low side MOSFET to ground. The

PFM mode is naturally implemented in an hysteretic controller enabling the zero current comparator by en-

abling, in fact in PFM mode the system reads the output voltage with a comparator and then turns on the high

side MOSFET when the output voltage goes down to reference value. The device works in discontinuous mode

C

INT2

C

INT1

----------------

V

OUT

∆

∆V

INT

------------------

=

PCB TRACES

LOAD

From Vsense

Q

Cint1

R1

INT

Vref

FB

DS

R

S

Vout

Vin

HGATE

LGATE

Q

OSC

Vref

One-shot generator

FFSR

PWM comparator

Vsense

Gndsense

R2

Cint2

HS

LS

Integrator amplifier

+

-

-

+

+

-