L6997

8/23

Eq 6

Where

Figure 4. 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 (input voltage, output current). In L6997 is im-

plemented the voltage feed-forward circuit that allows constant switching frequency during steady-sate

operation with the input voltage variation. There are many factors affecting switching frequency accuracy in

steady-state operation. Some of these are internal as dead times, which depend 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).

1.3 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 section; the on section is the same. In PFM after a turn-on cycle the system turns-

on the low side MOSFET, until the inductor current reaches the zero A value, when the zero-crossing compar-

ator turns off the low side MOSFET. In this way the energy stored in the output capacitor will not flow to ground,

through the low side MOSFET, but it will flow to the load. In PWM mode, after a turn 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 hysteretic controller, in

fact in PFM mode the system reads the output voltage with a comparator and then turns on the high side MOS-

FET when the output voltage goes down a reference value. The device works in discontinuous mode at light

load and in continuous mode at high load. The transition from PFM to PWM occurs when load current is around

C

IN T 2

C

IN T 1

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

V

OUT

∆

V

IN T

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

=

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

+

-

-

+

+

-