SSL1750_1

© NXP B.V. 2008. All rights reserved.

Product data sheet

Rev. 01 — 15 September 2008

18 of 29

NXP Semiconductors

SSL1750

SMPS control IC for LED drivers

7.3.7 Maximum on-time

The flyback controller limits the ‘on-time’ of the external MOSFET to 25

µs (typ). When the

‘on-time’ is longer than 25

µs, the IC stops switching and enters the safe restart mode.

7.3.8 Overvoltage protection (pin FBAUX)

An output overvoltage protection is implemented in this IC. This works for the SSL1750 by

sensing the auxiliary voltage via the current flowing into pin FBAUX during the secondary

stroke. The auxiliary winding voltage is a well-defined replica of the output voltage.

Voltage spikes are averaged by an internal filter.

If the output voltage exceeds the OVP trip level, an internal counter starts counting

subsequent OVP events. The counter has been added to prevent incorrect OVP detection

which might occur during ElectroStatic Discharge (ESD) or lightning events. If the output

voltage exceeds the OVP trip level a few times and not again in a subsequent cycle, the

internal counter counts down at twice the speed it uses when counting up. However, when

typically 8 cycles of subsequent OVP events are detected, the IC assumes a true OVP

and the OVP circuit switches the power MOSFET off. As the protection is latched, the

converter only restarts after the internal latch is reset. In a typical application the mains

should be interrupted to reset the internal latch.

(5)

accurate OVP detection possible.

Fig 13. Soft start-up of flyback

014aaa020

SOFT START

CONTROL

OCP

+

10

FBSENSE

0.5 V

Istart(soft)fb

≤ 60 µA

S2

RSS2

CSS2

RSENSE2

V

oovp

()

N

s

N

aux

----------- I

ovp FBAUX

()

R

FBAUX

V

clamp FBAUX

()

+

×

()

=