tea1753lt NXP Semiconductors, tea1753lt Datasheet - Page 14

• Manufacturer Part Number: tea1753lt
• Description: Tea1753lt Greenchip Iii Smps Control Ic
• Manufacturer: NXP Semiconductors

NXP Semiconductors
TEA1753LT
Objective data sheet
7.3.2 Valley switching (HV pin)
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
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 is reduced to zero and during this
frequency reduction mode, the primary peak current is kept at an adjustable minimal level
to maintain a high efficiency. As the primary peak current is low in frequency reduction
operation, no audible noise is noticeable at switching frequencies in the audible range.
Valley switching is also active in this mode.
In frequency reduction mode the PFC controller is switched off and the flyback maximum
frequency changes linearly with the control voltage on the FBCTRL pin (see
For stable on and off switching of the PFC, hysteresis has been added. At no load
operation the switching frequency is reduced to (almost) zero.
Refer to
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:
where L
capacitance on the drain node.
f
Fig 9.
=
-------------------------------------------------- -

2

p
Figure

Frequency control of flyback part
is the primary self-inductance of the flyback transformer and C

switching
frequency
f
sw(fb)max
flyback
1

L
All information provided in this document is subject to legal disclaimers.
10. A new cycle starts when the external MOSFET is activated. After the
p

sw(fb)max
C
d


Rev. 01 — 4 March 2011
frequency
reduction
(125 kHz). Again, the external MOSFET is only switched on
PFC off
1.5 V
PFC on
discontinuous
with valley
switching
V
FBCTRL
GreenChip III SMPS control IC
quasi-resonant
TEA1753LT
Section
© NXP B.V. 2011. All rights reserved.
014aaa746
d
is the
7.3.2).
Figure 9
14 of 31
).
(3)