SSL4120T datasheet(23/47 Pages) NXP

23 / 47 page

SSL4120T

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Objective data sheet

Rev. 1 — 21 June 2012

23 of 47

NXP Semiconductors

SSL4120T

Resonant power supply control IC with PFC

Two external components determine the frequency range:

• Capacitor Cfmin connected between pin CFMIN and ground sets the minimum

frequency in combination with an internally trimmed current source Iosc(min).

• Resistor Rfmax connected between pin RFMAX and ground sets the frequency range

and thus the maximum frequency.

The oscillator frequency depends on the charge and discharge currents of Cfmin. The

(dis-)charge current contains a fixed component, Iosc(min), that determines the minimum

frequency, and a variable component that is 4.9 times greater than the current in pin

RFMAX. IRFMAX is determined by the value of Rfmax and the voltage on pin RFMAX:

• The voltage on pin RFMAX is Vfmin(RFMAX) (typ. 0 V) at the minimum frequency.

• The voltage on pin RFMAX is Vfmax(fb)(RFMAX) (typ. 1.5 V) at the maximum feedback

frequency.

• The voltage on pin RFMAX is Vfmax(ss)(RFMAX) (typ. 2.5 V) at the maximum soft start

frequency.

The maximum frequency of the oscillator is limited internally. The HB frequency is limited

to flimit(HB) (min. 500 kHz). Figure 13 illustrates the relationship between VRFMAX, Rfmax,

Cfmin and fHB.

Fig 12. Determination of frequency

VOLTAGE PIN SSHBC

POLARITY INVERSION

(max 2.5 V)

VOLTAGE PIN RFMAX

CONVERSION TO CURRENT

via Rfmax

FEEDBACK CURRENT

PIN SNSFB

FIXED fmin CURRENT

CONVERSION TO

VOLTAGE (max 1.5 V)

(DIS-)CHARGE CURRENT

PIN CFMIN

CONVERSION TO

FRQUENCY via Cfmin

014aaa860

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