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9.
P2
f
MAX
switching
frequency
V
IN_MIN
f
MIN
P
OUT_MIN
QR frequency characteristics at different input voltages
Figure 9-11
Point P1 is the minimum frequency f
specified minimum input voltage and maximum output power
required by the application. Of course the minimum
frequency has to be chosen above the audible limit (>20
kHz).
Start-up Sequence
When the rectified AC voltage V
connected to pin 8) reaches the Mains dependent operation
level (Mlevel: between 60 and 100 V), the internal 'Mlevel
switch' will be opened and the start-up current source is
enabled to charge capacitor C2521 at the V
below.
The 'soft start' switch is closed when the V
of 7 V and the 'soft start' capacitor C
5 and the sense resistor R3526), is charged to 0.5 V.
Once the V
capacitor is charged to the start-up voltage
CC
V
(11 V), the IC starts driving the MOSFET. Both
CC-start
internal current sources are switched 'off' after reaching this
start-up voltage. Resistor R
start' capacitor, such that the peak current will slowly
increase. This to prevent 'transformer rattle'.
During start-up, the V
CC
moment that the primary auxiliary winding takes over this
voltage.
I
in(Vcc)
V
CC
1
C
Vcc
Mlevel
0.5V
V
CC
V
=11V
(start)
C
charged
Vcc
≈7V
by current
V
SENSE
I
L
V
OUT
V
GATE
Figure 9-12
L01.1A
Circuit Description
V
IN_MAX
P1
P
OUT_MAX
power
CL 16532020_077.eps
100401
that occurs at the
MIN
(via the centre tap
IN
pin as shown
CC
reaches a level
CC
(C2522, between pin
SS
(3524) will discharge the 'soft
SS
capacitor will be discharged until the
V
IN
8
I
SS
soft start
V
R
SENSE
SS
I
L
5
C
SS
+
V
OCP
R
SENSE
-
2
Start-up sequence
Charging of V
capacitor
CC
taken over by the winding
τ = R
⋅C
SS
SS
CL 16532020_078.eps
110401
The moment that the voltage on pin 1 drops below the 'under
voltage lock out' level (UVLO = ± 9 V), the IC will stop
switching and will enter a safe restart from the rectified mains
voltage.
Operation
The supply can run in three different modes depending on
the output power:
•
Quasi-Resonant mode (QR) The QR mode, described
above, is used during normal operation. This will give a
high efficiency.
•
Frequency Reduction mode (FR) The FR mode (also
called VCO mode) is implemented to decrease the
switching losses at low output loads. In this way the
efficiency at low output powers is increased, which
enables power consumption smaller than 3 W during
stand-by. The voltage at the pin 3 (Ctrl) determines
where the frequency reduction starts. An external Ctrl
voltage of 1.425 V corresponds with an internal VCO
level of 75 mV. This fixed VCO level is called V
The frequency will be reduced in relation to the VCO
voltage between 75 mV and 50 mV (at levels larger than
75 mV, Ctrl voltage < 1.425V, the oscillator will run on
maximum frequency f
oscH
(V
) the frequency is reduced to the minimum level
VCO,max
of 6 kHz. Valley switching is still active in this mode.
•
Minimum Frequency mode (MinF) At VCO levels below
50 mV, the minimum frequency will remain on 6 kHz,
which is called the MinF mode. Because of this low
frequency, it is possible to run at very low loads without
having any output regulation problems.
f
= 175 kHz
oscH
kH
MinF
FR
switching
frequency
V
VCO ,max
f
= 6 kHz
oscL
P
OUT_MIN
Multi mode operation
Figure 9-13
Safe-Restart Mode
This mode is introduced to prevent the components from
being destroyed during eventual system fault conditions. It is
also used for the Burst mode. The Safe-Restart mode will be
entered if it is triggered by one of the following functions:
•
Over voltage protection,
•
Short winding protection,
•
Maximum 'on time' protection,
•
V
reaching UVLO level (fold back during overload),
CC
•
Detecting a pulse for Burst mode,
•
Over temperature protection.
When entering the Safe-Restart mode, the output driver is
immediately disabled and latched. The V
charge the V
capacitor anymore and the V
CC
drop until UVLO is reached. To recharge the V
the internal current source (I
(restart)(VCC)
to initiate a new start-up sequence as described before. This
Safe-Restart mode will persist until the controller detects no
faults or burst triggers.
Standby
The set goes to Standby in the following cases:
•
After pressing the 'standby' key on the remote control.
.
VCO,start
= 175 kHz typically). At 50 mV
frequency limit
QR
V
VCO,start
P
OUT_MAX
power
CL 16532020_080.eps
100401
winding will not
CC
voltage will
CC
capacitor,
CC
) will be switched 'on'
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