ICE3BRXX65J(Z)(G)
Application Note
10
2010-06-20
6.2
Soft Start
When the IC is turned on after the Startup time, a digital soft start circuit is activated. A gradually increased
soft start voltage is generated by the digital soft start circuit, which in turn releases the duty cycle gradually
from zero. The duty cycle increases to maximum (which is limited by the transformer design) at the end of
the soft start period. When the soft start time ends, IC goes into normal mode and the duty cycle is controlled
by the FB signal. The soft start time is set at 20ms for maximum load. The soft start time is load dependent;
shorter soft start time with lighter load.
Figure 7 shows the soft start behavior at 85Vac input. The primary peak current increases slowly to the
maximum in the soft start period.
Channel 1; C1 : CS Voltage (V
CS
)
Channel 2; C2 : Supply Voltage (V
CC
)
Channel 3; C3 : FB voltage (V
FB
)
Channel 4; C4 : BA Voltage (V
BA
)
Soft start time = 20ms
20ms built in SoftStart time @
Vin=85Vac & 12W load
Figure 7
Soft start at AC line input voltage of 85Vac
6.3
Low standby power - Active Burst Mode
The IC will enter Active Burst Mode function at light load condition which enables the system to achieve the
lowest standby power requirement of less than 50mW. Active Burst Mode means the IC is always in the
active state and can therefore immediately respond to any changes on the FB signal, V
FB
.
6.3.1
Entering Active Burst Mode
Because of the current mode control scheme, the feedback voltage V
FB
actually controls the power delivery
to output. An important relationship between the V
CS
and the V
FB
is expressed in equation (3).
Ramp
Offset
V
CS
FB
V
A
V
V
−
+
⋅
=
(3)
where, V
FB
:feedback voltage, V
CS
:current sense voltage,
A
V
:PWM OP gain, V
Offset-Ramp
:voltage ramp offset
When the output load reduces, the feedback voltage V
FB
drops. If the V
FB
stays below 1.35V for 20ms, the IC
enters into the Active Burst Mode. The threshold power to enter burst mode is expressed in equation (4).
SW
V
sense
Ramp
Offset
FBC
P
SW
sense
CS
P
SW
p
P
enter
BURST
f
A
R
V
V
L
f
R
V
L
f
I
L
P
⋅
⋅
−
⋅
⋅
=
⋅
⋅
⋅
=
⋅
⋅
⋅
=
−
2
5
2
2
_
)
(
2
1
)
(
2
1
2
1
(4)
where,
L
p
: transformer primary inductance
R
sense
: current sense resistance,
f
sw
: switching frequency, V
FBC5
: Feedback level to enter burst mode
Figure 8 shows the waveform with the load change from nominal load to light load. After the 20ms blanking
time IC goes into burst mode.
20ms
1V
V
CS
V
BA
V
CC
V
FB