SSL4120 datasheet(27/48 Pages) NXP

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SSL4120

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Product data sheet

Rev. 2 — 1 November 2012

27 of 48

NXP Semiconductors

SSL4120

Resonant power supply controller IC with PFC for LED lighting

7.8.8.1

Soft-start voltage levels

The relationship between the soft-start voltage at the SSHBC/EN pin and the RFMAX pin

voltage is shown in Figure 15. The relationship is directly related to the frequency.

VRFMAX and VSSHBC/EN are of opposite polarity. VSSHBC/EN <Vfmax(SSHBC) = 3.2 V at initial

start-up which corresponds to the maximum frequency. During start-up, Css(HBC) is

charged, VSSHBC/EN increases and the frequency decreases. The contribution of the

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

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

and at a minimum (

 3 V). Under V

fmax(SSHBC) (maximum frequency), the discharge

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

A The voltage is

clamped at a minimum of Vpu(EN) = 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 the maximum to the operating frequency. As well as being used to

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

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

continuous alternation between charging and discharging occurs during overcurrent

regulation. In this way VSSHBC/EN can be regulated, overruling the signal from the

feedback input.

The charge and discharge current can have a high value, Iss(hf)(SSHBC) = 160 A. This

current results in fast charging and discharging. In addition, it can have a low value

Iss(lf)(SSHBC) =40 A, resulting in a slow charge and discharge. This two-speed soft-start

sweep allows a combination of a short start-up time for the resonant converter and stable

regulation loops (such as overcurrent regulation).

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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