CoolSET™-F3
Functional Description
Current Limiting is now possible in a very accurate way.
E.g. Ipeak = 0.5A with RSense = 2. Without Propagation
Delay Compensation the current sense threshold is set
to a static voltage level Vcsth=1V. A current ramp of
dI/dt = 0.4A/µs, that means dVSense/dt = 0.8V/µs, and a
3.6
Control Unit
The Control Unit contains the functions for Active Burst
Mode and Auto Restart Mode. The Active Burst Mode
and the Auto Restart Mode are combined with an
Adjustable Blanking Window which is depending on the
external Soft Start capacitor. By means of this
Adjustable Blanking Window, the IC avoids entering
into these two modes accidentally. Furthermore it also
provides a certain time whereby the overload detection
is delayed. This delay is useful for applications which
normally works with a low current and occasionally
require a short duration of high current.
propagation delay time of i.e. tPropagation
=180ns
Delay
leads then to an Ipeak overshoot of 12%. By means of
propagation delay compensation the overshoot is only
about 2% (see Figure 14).
with compensation
without compensation
V
1,3
1,25
1,2
3.6.1
Adjustable Blanking Window
1,15
1,1
SoftS
1,05
1
6.5V
0,95
0,9
RSoftS
5k
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
V
dVSense
dt
µs
4.4V
1
S1
Figure 15
Overcurrent Shutdown
G2
The Propagation Delay Compensation is realized by
means of a dynamic threshold voltage Vcsth (see Figure
15). In case of a steeper slope the switch off of the
driver is earlier to compensate the delay.
C3
VOSC
5.4V
max. Duty Cycle
Auto
&
G5
4.8V
Restart
Mode
C4
off time
Active
Burst
Mode
VSense
Vcsth
t
Propagation Delay
&
FB
G6
C5
1.32V
Signal1
Signal2
Control Unit
t
Figure 16
Dynamic Voltage Threshold Vcsth
Figure 17
Adjustable Blanking Window
VSoftS is clamped at 4.4V by the closed switch S1 after
the SMPS is settled. If overload occurs VFB is
exceeding 4.8V. Auto Restart Mode can’t be entered as
the gate G5 is still blocked by the comparator C3. But
after VFB has exceeded 4.8V the switch S1 is opened
Version 1.3
15
15 Sep 2004
INFINEON [ Infineon ]
