ssl1750 NXP Semiconductors, ssl1750 Datasheet - Page 14

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

NXP Semiconductors
SSL1750_1
Product data sheet
7.3.2 Valley switching (pin HV)
In frequency reduction mode the PFC controller is switched to burst mode operation and
the flyback maximum frequency changes linearly with the control voltage on pin FBCTRL
(see
50 mV (typ) hysteresis. At no load operation the switching frequency of the flyback can be
reduced to (almost) zero.
See
on-time (determined by the FBSENSE voltage and the FBCTRL voltage), the MOSFET is
switched off and the secondary stroke starts. After the secondary stroke, the drain voltage
shows an oscillation with a frequency of approximately, see
where L
capacitance on the drain node.
As soon as the internal oscillator voltage is high again and the secondary stroke has
ended, the circuit waits for the lowest drain voltage before starting a new primary stroke.
Figure 10
oscillator signal.
Valley switching allows high frequency operation as capacitive switching losses are
reduced, see
magnetics possible.
--------------------------------------------------- -
2
P
Fig 9.
=
switching frequency
Figure
Figure
1
-- -
2
p
C
Frequency control of flyback part
1
is the primary self-inductance of the flyback transformer and C
shows the drain voltage, valley signal, secondary stroke signal, and the internal
d
f
L
sw(fb)max
10. A new cycle starts when the external MOSFET is activated. After the
9). For stable on-off switching of the PFC burst mode pin FBCTRL has a
p
Equation
V
C
2
d
f
Rev. 01 — 15 September 2008
PFC burst mode
frequency
reduction
3. High frequency operation makes small and cost-effective
1.5 V
PFC on
discontinuous
with valley
switching
V
FBCTRL
SMPS control IC for LED drivers
Equation 2
quasi resonant
014aaa026
SSL1750
© NXP B.V. 2008. All rights reserved.
d
is the
14 of 29
(2)
(3)