5

Application information

5.1

Theory of operation

The L4985A/B implements a conventional peak current mode control, based on fixed-off-time (FOT) control

technique, with some proprietary circuitries that permit to ideally achieve the same performance of the more

complex/expensive average current mode control.

Referring to Figure 4, the power switch on-time (TON) is programmed by the output voltage control loop

comparing the current sense signal VCS with the internal current reference VCS_REF in order to keep the VOUT

regulation; whereas the power switch off-time (TOFF) is programmed by the “OFF-TIME modulator” circuitry in

order to keep quasi-fixed the switching frequency FSW in all operating conditions (see “Section 5.1 OFF-time

modulator” section for more details).

Figure 4. Control loop connections

The trans-conductance Error Amplifier V/A compares a portion of the output voltage VOUT, brought at its inverting

input externally available on pin FB via the resistor divider R3-R4, with an accurate internal reference VREF (2.5

V typ.) connected to the non-inverting input, and generates an error signal VC proportional to their difference. If

the bandwidth of the error amplifier, essentially determined by the frequency compensation network connected

between pin COMP and ground, is narrow enough – typically below 20 Hz – and a steady-state operation

is assumed, the VC error signal available at pin COMP can be regarded as a DC level, at least as a first

approximation. The VC voltage is then used by the “Vin & multiplier estimator” circuitry that, based also on the FW

and ZCD signals (see Figure 5 and Figure 6for more details), generates a voltage expressed by:

Equation 1

(1)

VGθ =VC  K1  VINθVOUT

where K1 is the circuitry gain (constant term) and

VINθ =Vin,pksinθ(with0≤θ≤π,asaresultofthe

rectification operated by the input bridge) is the instantaneous line input voltage.

Equation 1 shows as the VG(θ) voltage is proportional to the VIN(θ) input voltage and to the VC control voltage like

in a standard current-mode PFC, but without using the standard multiplier block and without the AC line sensing.

L4985

Application information

DS13769 - Rev 3

page 10/27