Quasi-resonant CoolSET design guide
ICE2QRxx65/80x
Design Guide
12
8 August 2011
The feedback voltage is lower than the threshold of V
EB
(1.25V). Accordingly, the peak voltage
across the shunt resistor is 0.17V;
The up/down counter has its maximal value of 7;
The two above conditions have to been fulfilled for a certain blanking time duration t
BEB
(30ms)
Once all of these conditions are fulfilled, the active burst mode flip-flop is set and the CoolSET enters burst
mode operation and the gate will be turned off until V
FB
increase to on threshold V
BH
. The total blanking time
to enter the active burst mode depends on the up counting time and the 30ms extra blanking time.
ms
ms
value
counter
T
ing
totalblank
30
48
_
7
For example. If before the load change, current up/down counter value is 3, then the total blanking time will
be:
(7-3) x 48ms + 30ms = 222ms.
This multi-conditional determination for entering active burst mode operation prevents mistriggering of
entering active burst mode operation, so that the CoolSET enters active burst mode operation only when the
output power is really low during the preset blanking time.
4.4.2
During Burst Mode Operation
After entering the Active Burst Mode the feedback voltage rises as V
O
starts to decrease due to the inactive
PWM section. One comparator observes the feedback signal if the voltage level V
BH
(3.6V) is exceeded. In
that case the internal circuit is again activated by the internal bias to start with switching.
Turn-on of the power MOSFET is triggered by the timer. The PWM generator for burst mode operation
composes of a timer with a fixed frequency of 52 kHz, typically, and an analog comparator. Turn-off is
resulted by comparison of the voltage signal V
1
with an internal threshold, by which the voltage across the
shunt resistor V
csB
is 0.34V, accordingly. A turn-off can also be triggered by the maximal duty ratio CoolSET
which sets the maximal duty ratio to 50%. In operation, the output flip-flop will be reset by one of these
signals which come first.
V
FBEB
V
FBBOn
V
FBLB
V
FB
t
V
CSB
1.0V
V
CS
V
VCCoff
V
VCC
t
t
V
O
t
V
FBBOff
time to 7
th
zero and
blanking Window (t
BEB
)
Current limit level
during Active Burst
Mode
Leaving
Active Burst
Mode
Entering
Active Burst
Mode
Max. Ripple < 1%
Figure 7 Signals in active burst mode
