SSL1750 datasheet(6/29 Pages) NXP

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SSL1750_1

Product data sheet

Rev. 01 — 15 September 2008

6 of 29

NXP Semiconductors

SSL1750

SMPS control IC for LED drivers

7.1.1 Start-up and undervoltage lockout

Initially the capacitor on pin VCC is charged from the high voltage mains via pin HV.

As long as VCC is below Vtrip, the charge current is low. This protects the IC in case pin

VCC is shorted to ground. For a short start-up time the charge current above Vtrip is

increased until VCC reaches Vth(UVLO). If VCC is between Vth(UVLO) and Vstartup, the charge

current is low again, ensuring a low duty cycle during fault conditions.

The control logic activates the internal circuitry and switches off the charge current when

the voltage on pin VCC passes the Vstartup level. First, the output on pin LATCH is activated

and the soft-start capacitors on the pins PFCSENSE and FBSENSE are charged. When

the voltage on pin LATCH exceeds the Ven(LATCH) voltage and the soft-start capacitor on

pin PFCSENSE is charged, the PFC circuit is activated. The supply current from the pin

HV is then switched on again and the PFC circuit charges the Cbus capacitor. When the

voltage on pin VOSENSE reaches the Vstart(fb) level, the charge current is switched off and

the ﬂyback converter is activated (providing the soft-start capacitor on pin FBSENSE is

charged). The output voltage of the ﬂyback converter is then regulated to its nominal

output voltage. The IC supply is taken over by the auxiliary winding of the ﬂyback

converter. See Figure 5.

When the PFC is started, there is initially no supply takeover from the auxiliary winding. To

make a small VCC capacitor possible, the VCC voltage is regulated to the Vstartup level, as

long as the ﬂyback converter has not yet started. Regulation is done by hysteretic control

with a limited (high level) charge current. The hysteresis is typically 300 mV.

If during start-up pin LATCH does not reach the Ven(LATCH) level before VCC reaches

Vth(UVLO), its output is deactivated and the charge current is switched on again.

Fig 4.

Typical conﬁguration of SSL1750

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SSL1750T

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