Denon DRA-F107 Service Manual page 39

secondary output voltage is insensitive on line
variations. Line variation
waveform slope which controls the duty cycle.
The external R allows an individual adjustment of
Sense
the maximum source current of the integrated
CoolMOS™.
Soft-Start Comparator
FB
0.3V
Oscillator
V
OSC
10k
T
2
C
1
Voltage Ramp
Figure 9 Improved Current Mode
To improve the Current Mode during light load
conditions the amplified current ramp of the PWM-OP
is superimposed on a voltage ramp, which is built by
the switch T , the voltage source V
2
low pass filter composed of R
Figure 10). Every time the oscillator shuts down for
max. duty cycle limitation the switch T2 is closed by
V . When the oscillator triggers the Gate Driver T2 is
OSC
opened so that the voltage ramp can start.
In case of light load the amplified current ramp is to
small to ensure a stable regulation. In that case the
Voltage Ramp is a well defined signal for the
comparison with the FB-signal. The duty cycle is then
controlled by the slope of the Voltage Ramp.
By means of the Comparator C5, the Gate Driver is
switched-off until the voltage ramp exceeds 0.3V. It
allows the duty cycle to be reduced continuously till 0%
by decreasing V below that threshold.
FB
changes the current
PWM Comparator
PWM-Latch
C5
Gate Driver
0.8V
x3.65
R
1
V
1
PWM OP
20pF
and the 1st order
1
and C (see Figure 9,
1 1
DRA-F107 / DRA-F107DAB
V
OSC
max.
Duty Cycle
Voltage Ramp
0.8V
FB
0.3V
Gate Driver
Figure 10 Light Load Conditions
3.2.1 PWM-OP
The input of the PWM-OP is applied over the internal
leading edge blanking to the external sense resistor
R connected to pin Isense. R
Sense
source current into a sense voltage. The sense voltage
is amplified with a gain of 3.65 by PWM OP. The output
of the PWM-OP is connected to the voltage source V1.
The voltage ramp with the superimposed amplified
current signal is fed into the positive inputs of the PWM-
Comparator, C5 and the Soft-Start-Comparator.
3.2.2 PWM-Comparator
The PWM-Comparator compares the sensed current
signal of the integrated CoolMOS
signal V (see Figure 11). V
FB
external optocoupler
combination with the internal pull-up resistor R
provides the load information of the feedback circuitry.
When the amplified current signal of the integrated
CoolMOS™ exceeds the signal V
Comparator switches off the Gate Driver.
39
Functional Description
converts the
Sense
TM
with the feedback
is created by an
FB
or external transistor
the PWM-
FB
t
t
t
in
and
FB