SSL4120T datasheet(26/47 Pages) NXP

26 / 47 page

SSL4120T

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Objective data sheet

Rev. 1 — 21 June 2012

26 of 47

NXP Semiconductors

SSL4120T

Resonant power supply control IC with PFC

VRFMAX and VSSHBC/EN are of opposite polarity. At initial start-up, VSSHBC/EN is below

Vfmax(SSHBC) (typ. 3.2 V), which corresponds to the maximum frequency. During start-up,

Css(HBC) is charged, VSSHBC/EN rises and the frequency decreases. The contribution of the

soft start function is zero when VSSHBC/EN is above Vfmin(SSHBC) (typ. 7.9 V).

VSSHBC/EN is clamped at a maximum of Vclamp(SSHBC) (typ. 8.4 V) (frequency is at a

minimum) and at a minimum (

≈ 3 V). Below Vfmax(SSHBC) (maximum frequency), the

discharge current is reduced to a maximum-frequency soft start current of typically 5

μA

The voltage is clamped at a minimum of Vpu(EN) (typ. 3 V). Both clamp levels are just

outside the operating area of Vfmax(SSHBC) to Vfmin(SSHBC). The margins avoid frequency

disturbance during normal output voltage regulation, but ensure that overcurrent

regulation can respond quickly.

7.8.8.2

Soft start charge and discharge

At initial start-up, the soft start capacitor Css(HBC) is charged to obtain a decreasing

frequency sweep from maximum to operating frequency. As well as being used to softly

start up the resonant converter, the soft start functionality is also used for regulation

purposes (such as overcurrent regulation). Css(HBC) can therefore be charged or

discharged. In the case of overcurrent regulation, a continuous alternation between

charging and discharging takes place. In this way VSSHBC/EN can be regulated, thereby

overruling the signal from the feedback input.

The (dis-)charge current can have a high value, Iss(hf)(SSHBC) (typ. 160

μA), resulting in a

fast (dis-)charge, or it can have a low value Iss(lf)(SSHBC) (typ. 40

μA), resulting in a slow

(dis-)charge. This two-speed soft start sweep allows for a combination of a short start-up

time for the resonant converter and stable regulation loops (such as overcurrent

regulation).

The fast (dis-)charge speed is used for the upper frequency range where VSSHBC/EN is

below Vss(hf-lf)(SSHBC) (typ. 5.6 V). In the upper frequency range, the currents in the

converter do not react strongly to frequency variations.

Fig 15. Relation between SSHBC/EN voltage and frequency

fHB

VSSHBC

Vfmax,ss(RFMAX)

VRFMAX

fmin

fmax

Vfmax(SSHBC)

Vfmin(SSHBC)

Vclamp(SSHBC)

Vpu(EN)

ISNSFB < Ifmin(SNSFB)

Ifmin(SNSFB) < ISNSFB < Ifmax(SNSFB)

VRFMAX

fHB

Vfmax,fb(RFMAX)

014aaa863

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