CoolSET®-F3R
ICE3BR4765J
Functional Description
and FB signal is still below 1.35V, the system enters
the Active Burst Mode. This time window prevents a
sudden entering into the Active Burst Mode due to
large load jumps.
VFB
Leaving
Active Burst
Mode
Entering
Active Burst
Mode
After entering Active Burst Mode, a burst flag is set and
the internal bias is switched off in order to reduce the
current consumption of the IC to approx. 450uA.
4.0V
3.5V
3.0V
It needs the application to enforce the VCC voltage
above the Undervoltage Lockout level of 10.5V such
that the Startup Cell will not be switched on
accidentally. Or otherwise the power loss will increase
drastically. The minimum VCC level during Active Burst
Mode depends on the load condition and the
application. The lowest VCC level is reached at no load
condition.
1.35V
Blanking Timer
t
20ms Blanking Time
3.7.2.2
Working in Active Burst Mode
After entering the Active Burst Mode, the FB voltage
rises as VOUT starts to decrease, which is due to the
inactive PWM section. The comparator C6a monitors
the FB signal. If the voltage level is larger than 3.5V, the
internal circuit will be activated; the Internal Bias circuit
resumes and starts to provide switching pulse. In
Active Burst Mode the gate G10 is released and the
current limit is reduced to 0.34V, which can reduce the
conduction loss and the audible noise. If the load at
VOUT is still kept unchanged, the FB signal will drop to
3.0V. At this level the C6b deactivates the internal
circuit again by switching off the internal Bias. The gate
G11 is active again as the burst flag is set after entering
Active Burst Mode. In Active Burst Mode, the FB
voltage is changing like a saw tooth between 3.0V and
3.5V (see figure 24).
VCS
t
t
t
t
t
Current limit level
during Active Burst
Mode
1.03V
0.34V
VVCC
3.7.2.3
Leaving Active Burst Mode
10.5V
The FB voltage will increase immediately if there is a
high load jump. This is observed by the comparator C4.
Since the current limit is app. 34% during Active Burst
Mode, it needs a certain load jump to rise the FB signal
to exceed 4.0V. At that time the comparator C4 resets
the Active Burst Mode control which in turn blocks the
comparator C12 by the gate G10. The maximum
current can then be resumed to stabilize the VOUT.
IVCC
2.5mA
450uA
VOUT
Figure 24
Signals in Active Burst Mode
Version 2.5
16
19 Nov 2012
INFINEON [ Infineon ]
