AN1049 datasheet(5/24 Pages) STMICROELECTRONICS

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By substituting (9) in (8) it is possible to obtain:

PinSB

= Pinmax ⋅







0.367

− Vo







(10).

The frequency change fosc

⇒ fSB pushes flyback into a deeper DCM operation and causes a sudden in-

crease of the peak primary current (since the input power does not change). As a result, the peak volt-

age on current sense will jump from Vcspk1 to:

Vcspk1

’

= Vo + Rs ⋅

√

⋅ PSB

⋅ fSB

= Vo + (1 − Vo) ⋅

√

PSB

Pinmax

⋅

√fosc

fSB

(11).

This value must be <Vcspk2 not to exceed the hysteresis of the internal comparator, which would cause

the operating frequency to switch back and forth between fosc and fSB. This constraint sets a maximum

limit on the frequency change:

fosc

fSB







0.867

− Vo

0.367

− Vo







(12).

Provided equation (12) is fulfilled, the input power (PinNW) at which the normal operation frequency is re-

sumed (fSB

⇒ fosc) will be:

PinNW

⋅ Lp ⋅ fSB ⋅







0.867

− Vo







(13)

which, considering position (9), can be also expressed in the following terms:

PinNW

= Pinmax ⋅







0.867

− Vo







⋅

fSB

fosc

= PinSB ⋅







0.867

− Vo

0.367

− Vo







⋅

fSB

fosc

(14).

The inspection of equations (8)...(14) shows that

adding an offset Vo lowers the ratios PinSB/Pinmax

and PinNW / Pinmax and raises the limit of fosc / fSB

(with respect to the values with Vo = 0).

This is equivalent to lowering the internal thresholds

VT1 and VT2. The effect will be more pronunciated

on VT1 than on VT2. In practice, the internal thresh-

olds have been fixed at the maximum value able to

allow high enough a frequency jump, with a certain

margin, leaving to an external circuit (like the one

shown in fig. 4) the duty of the adjustment, if neces-

sary.

Referring now to MCM (Mixed Conduction Mode) and

CCM (Continuous Conduction Mode) systems, the

peak voltage on the current sense pin is given by:

Vcspk









+ Rs ⋅ 



⋅ ZE







> PinT

+ Rs ⋅

√

⋅ Pin

> PinT

(15)

where ZE is to be evaluated at fsw = fosc or fsw = fSB, depending on the operating mode. At the transition

CCM

↔ DCM the peak voltage on the current sense pin will be:

VcspkT

= Vo + Rs ⋅

(16).

L5991

Vref

Vo = Vref

R + Rc

Figure 4. Circuit for the adjustment of the

standby thresholds.

AN1049 APPLICATION NOTE

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