SSL4120T datasheet(24/47 Pages) NXP

24 / 47 page

SSL4120T

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Objective data sheet

Rev. 1 — 21 June 2012

24 of 47

NXP Semiconductors

SSL4120T

Resonant power supply control IC with PFC

The oscillator is controlled by the slope of the half-bridge. The oscillator charge current is

initially set to a low value Iosc(red) (typ. 30

μA). When the start of the half-bridge slope is

detected, the charge current is increased to its normal value. This feature is used in

combination with the adaptive non-overlap time function as described in Section 7.8.4.2

and Figure 11. Since the half-bridge slope normally starts directly after the MOSFET is

switched off, the length of time the oscillator current is low will be negligible under normal

operating conditions.

7.8.6 HBC feedback input (pin SNSFB)

In a typical power supply application, the output voltage is compared and amplified on the

secondary side. The output of the error amplifier is transferred to the primary side via an

opto-coupler. This opto-coupler can be connected directly to the SNSFB pin.

The SNSFB pin supplies the opto-coupler from an internal voltage source Vpu(SNSFB)

(typ. 8.4 V) with a series resistance RO(SNSFB). The series resistance allows spike filtering

via an external capacitor. To ensure sufficient bias current for the opto-coupler, the

feedback input has a threshold current Ifmin(SNSFB) (typ. 0.66 mA) at which the frequency is

at a minimum. The maximum frequency is reached at Ifmax(SNSFB) (typ 2.2 mA). The

maximum frequency that can be reached via the SNSFB pin is lower (typ. 60 %) than the

maximum frequency that can be reached via the SSHBC/EN pin. Figure 14 shows the

relationship between ISNSFB, VSNSFB and VRFMAX.

A: Cfmin =high, Rfmax =high

B: Cfmin =low, Rfmax =low

C: Cfmin = low, Rfmax = too low

Fig 13. Function of Rfmax and Cfmin

flimit(HB)

fmax(B)

Vfmax(fb)(RFMAX) Vfmax(ss)(RFMAX)

VRFMAX

fmax(A)

fmin(B and C)

fmin(A)

fHB

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