ssl1750 NXP Semiconductors, ssl1750 Datasheet - Page 13

• Manufacturer Part Number: ssl1750
• Description: Smps Control Ic For Led Drivers
• Manufacturer: NXP Semiconductors

NXP Semiconductors
SSL1750_1
Product data sheet
7.2.11 Driver (pin PFCDRIVER)
7.3.1 Multimode operation
7.3 Flyback controller
The driver circuit to the gate of the power MOSFET has a current sourcing capability of
typically 500 mA and a current sink capability of typically 1.2 A. This permits fast turn-on
and turn-off of the power MOSFET for efficient operation.
The SSL1750 includes a controller for a flyback converter. The flyback converter operates
in quasi-resonant or discontinuous conduction mode with valley switching. The auxiliary
winding of the flyback transformer provides demagnetization detection and powers the IC
after start-up.
The SSL1750 flyback controller can operate in various modes, see
At high output power the converter switches to quasi-resonant mode. The next converter
stroke is started after demagnetization of the transformer current. In quasi-resonant mode
switching losses are minimized as the converter only switches on when the voltage across
the external MOSFET is at its minimum (valley switching, see also
To prevent high frequency operation at lower loads, the quasi-resonant operation changes
to discontinuous mode operation with valley skipping in which the switching frequency is
limited for EMI to f
on when the voltage across the MOSFET is at its minimum.
At very low power and standby levels the frequency is controlled down by a Voltage
Controlled Oscillator (VCO). The minimum frequency can be reduced to zero. During
frequency reduction mode, the primary peak current is kept at a minimal level of Ipkmax/4
to maintain a high efficiency. (Ipkmax is the maximum primary peak current set by the
sense resistor and the maximum sense voltage.) As the primary peak current is low in
frequency reduction mode operation (Ipk = Ipkmax/4), no audible noise is noticeable at
switching frequencies in the audible range. Valley switching is also active in this mode.
Fig 8.
switching frequency
Multimode operation flyback
f
sw(fb)max
sw(fb)(max)
Rev. 01 — 15 September 2008
PFC burst mode
frequency
reduction
(125 kHz typ). Again, the external MOSFET is only switched
PFC on
discontinuous
with valley
switching
output power
SMPS control IC for LED drivers
quasi resonant
Section
Figure
SSL1750
© NXP B.V. 2008. All rights reserved.
014aaa025
8.
7.3.2).
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