SSL1750 datasheet(17/29 Pages) NXP

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SSL1750_1

Product data sheet

Rev. 01 — 15 September 2008

17 of 29

NXP Semiconductors

SSL1750

SMPS control IC for LED drivers

7.3.6 Soft start-up (pin FBSENSE)

To prevent audible transformer noise during start-up, the transformer peak current, IDM is

slowly increased by the soft-start function. This can be achieved by inserting a resistor

and a capacitor between pin FBSENSE and the current sense resistor.

An internal current source charges the capacitor to V = Istart(soft)(fb) × RSS2, with a

maximum of approximately 0.5 V.

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

adjusted externally by changing the values of RSS2 and CSS2. See Equation 4.

(4)

The soft start current Istart(soft)(fb) is switched on as soon as VCC reaches Vstartup. When the

voltage on pin VOSENSE reaches the Vstart(fb) level and the voltage on pin FBSENSE has

reached 0.5 V, the ﬂyback converter starts switching.

The soft-start current ﬂows as long as the voltage on pin FBSENSE is below

approximately 0.5 V. If the voltage on pin FBSENSE exceeds 0.5 V, the soft-start current

source starts limiting the current. After the ﬂyback converter has started, the soft-start

current source is switched off.

a. Circuit diagram

b. Timing diagram

Fig 12. Time-out protection

014aaa049

FBCTRL

2.5 V

4.5 V

µA

3 k

Ω

3.5 V

TIMEOUT

014aaa050

4.5 V

2.5 V

VFBCTRL

output

voltage

intended output

voltage not

reached within

time-out time.

intended output voltage

reached within time-out

time.

restart

SoftStart

SS2

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