SSL4120 datasheet(25/48 Pages) NXP

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SSL4120

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Product data sheet

Rev. 2 — 1 November 2012

25 of 48

NXP Semiconductors

SSL4120

Resonant power supply controller IC with PFC for LED lighting

The half-bridge slope controls the oscillator. The oscillator charge current is initially set to

a low value Iosc(red) =30 A. 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.

The length of time the oscillator current is low is negligible under normal operating

conditions because the half-bridge slope normally starts directly after the MOSFET is

switched off.

7.8.6 HBC feedback input (SNSFB pin)

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

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

optocoupler. This optocoupler can be connected directly to the SNSFB pin.

The SNSFB pin supplies the optocoupler from an internal voltage source

Vpu(SNSFB) = 8.4 V with the RO(SNSFB) series resistance. The series resistance allows spike

filtering using an external capacitor. To ensure sufficient optocoupler bias current, the

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

a minimum.

The maximum frequency is reached at Ifmax(SNSFB) = 2.2 mA. The maximum frequency

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

that can be reached using 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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