SSL4120 datasheet(19/48 Pages) NXP

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SSL4120

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Product data sheet

Rev. 2 — 1 November 2012

19 of 48

NXP Semiconductors

SSL4120

Resonant power supply controller IC with PFC for LED lighting

7.7.6 PFC soft-start and soft-stop (SNSCURPFC pin)

The PFC controller features a soft-start function. The function slowly increases the

primary peak current during start-up. The soft-stop function slowly decreases the

transformer peak current before operations are stopped. These functions prevent

transformer rattle during start-up and burst mode operation.

Connecting a resistor Rss(PFC) and capacitor Css(PFC) between the SNSCURPFC pin and

the current sense resistor Rcur(PFC) achieves this. During start-up, an internal current

source Ich(ss)(PFC) charges the capacitor to VSNSCURPFC =Ich(ss)(PFC)  Rss(PFC).

The voltage is limited to the maximum PFC soft-start clamp voltage, Vclamp(ss)PFC. The

additional voltage across the charged capacitor reduces the peak current. After start-up,

the internal current source is switched-off, capacitor Css(PFC) discharges across Rss(PFC)

and the peak current increases.

The start level and the time constant of the rising primary current can be adjusted

externally by changing the values of Rss(PFC) and Css(PFC).

Switching on the internal current source Ich(ss)(PFC) starts a soft-stop. Ich(ss)(PFC) charges

Css(PFC). The increasing capacitor voltage decreases the peak current. The charge current

flows when the voltage on the SNSCURPFC pin is less than the maximum PFC soft-start

voltage = 0.5 V. If VSNSCURPFC exceeds the maximum PFC soft-start voltage, the soft-start

current source starts limiting the charge current. To determine accurately if the capacitor is

charged, the voltage is only measured during the PFC power switch off-time. The PFC is

stopped when VSNSCURPFC > Vstop(ss)(PFC).

7.7.7 PFC overcurrent regulation, OCR-PFC (SNSCURPFC pin)

The maximum peak current is limited cycle-by-cycle by sensing the voltage across an

external sense resistor (Rcur(PFC)) connected to the source of the external MOSFET. The

voltage is measured via the SNSCURPFC pin and is limited to Vocr(PFC).

A voltage peak appears on VSNSCURPFC when the PFC MOSFET is switched on due to the

discharging of the drain capacitance. The leading-edge blanking time tleb(PFC) ensures that

the overcurrent sensing block does not react to this transitory peak.

7.7.8 PFC mains undervoltage protection/brownout protection, UVP mains

(SNSMAINS pin)

The voltage on the SNSMAINS pin is continuously sensed to prevent the PFC trying to

operate at very low mains input voltages. PFC switching stops when

VSNSMAINS <Vuvp(SNSMAINS). Mains undervoltage protection is also called brownout

protection.

VSNSMAINS is clamped to a minimum value of Vpu(SNSMAINS) for fast restart as soon as the

mains input voltage recovers after a mains dropout. The PFC starts or restarts once

VSNSMAINS exceeds the Vstart(SNSMAINS) start level.

Cur PFC

 pk



ocr PFC



ch ss

 PFC



ss PFC







–

cur PFC



---------------------------------------------------------------------------------------------



ss PFC



ss PFC





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