Application information

5 Application information

L4984D

5.1 Theory of operation

The L4984D implements conventional “peak” current mode control, where the on-time TON

of the external power switch is determined by the peak inductor current reaching the

programmed value. The off-time TOFF, instead, is determined by a special fixed-off-time

(FOT) modulator in such a way that the resulting switching period is constant as long as the

boost converter is operated in CCM (i.e. the current in the boost inductor remains greater

than zero in a switching cycle).

To understand how TOFF needs to be modulated to achieve a fixed switching frequency

independent of the instantaneous line voltage and the load, it is useful to consider the V·s

balance equation for the boost inductor under the assumption of CCM operation:

Equation 1

TON Vpk sinθ= TOFF ( Vout − Vpk sinθ)

where Vpk is the peak line voltage, Vout the regulated output voltage and θ the

instantaneous phase angle of the line voltage. Solving for TON, we get:

Equation 2

TON

=

⎜⎜⎝⎛

Vout

Vpk sinθ

−

1⎟⎟⎠⎞

TOFF

then, the switching period TSW is:

Equation 3

Tsw

=

TON

+ TOFF

= ⎜⎜⎝⎛

Vout

Vpk sin θ

− 1⎟⎟⎠⎞ TOFF

+ TOFF

=

Vout

Vpk sin

θ

TOFF

In the end, if TOFF is changed proportionally to the instantaneous line voltage, i.e. if:

Equation 4

TOFF = K t Vpk sinθ

then TSW is equal to Kt·Vout and, since Vout is regulated by the voltage loop, also TSW (and

fSW = 1/TSW) is fixed. This result is based on the sole assumption that the instantaneous line

voltage and the output load are such that the boost inductor operates in CCM.

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