Active-Semi
Figure 2:
Startup Waveforms
Rev 5, 05-Jun-09
ACT30
pulse-skipped
V
AC
V
DRV1
V
DRVST
5V
V
VDD
I
PRIMARY
V
OUT
Normal Operation
In normal operation, the feedback signal from the
secondary side is transmitted through the
optocoupler as a current signal into V
VDD
pin, which
has dynamic impedance of 9kΩ. The resulting V
VDD
voltage affects the switching of the IC. As seen in
the
Functional Block Diagram,
the Current Limit VC
Generator uses the V
VDD
voltage difference with
4.75V to generate a proportional offset at the
negative input of the Error Comparator.
The drivers turn on at the beginning of each
switching cycle. The current sense resistor current,
which is a fraction of the transformer primary
current, increases with time as the primary current
increases. When the voltage across this current
sense resistor plus the oscillator ramp signal equals
Error Comparator's negative input voltage, the
drivers turn off. Thus, the peak DRV1 current has a
negative voltage coefficient of -0.29A/V and can be
calculated from the following:
3.6Ω (rather than as digital output switches). The
current limit can then be calculated through linear
combination as shown in Figure 3. For TO-92
package, the ACT30A are preprogrammed to
400mA current limit and the ACT30B are
preprogrammed to 800mA current limit, for SOT23-
B package, the ACT30A are preprogrammed to
400mA current limit.
Figure 3:
Driver Output Configurations
I
LIM
=
400 mA
I
DRV 1PEAK
=
0.29 A / V
×
(
4.75V
−
V
VDD
)
for V
VDD
< 4.75V and duty cycle < 50%.
When the output voltage is lower than regulation,
the current into V
VDD
pin is zero and V
VDD
voltage
decreases. At V
VDD
= V
UV
= 3.35V, the peak DRV1
current has maximum value of 400mA.
DRV1
DRV2
⎛
7 .2
Ω
+
R
D
I
LIM
=
400 mA
× ⎜
⎜
3 .6
Ω
+
R
D
⎝
⎞
⎟
⎟
⎠
I
LIM
=
800 mA
Current Limit Adjustment
The IC's proprietary driver arrangement allows the
current limit to be easily adjusted between 400mA
and 1.2A. To understand this, the drivers have to be
utilized as linear resistive devices with typically
Innovative Power
TM
ActiveSwitcher
TM
R
D
⎞
⎛
I
LIM
=
400 mA
× ⎜
2
+
⎟
3 .6
Ω
⎠
⎝
-5-
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