MC44603ADWR2 View Datasheet(PDF) - ON Semiconductor
Part Name
Description
MFG CO.
'MC44603ADWR2' PDF : 22 Pages View PDF
MC44603A
Consequently:
Tcharge = CT x DV/Icharge
Tdischarge = CT x DV/Idischarge
where:
Tcharge is the oscillator charge time
DV is the oscillator peak−to−peak value
Icharge is the oscillator charge current
and
Tdischarge is the oscillator discharge time
Idischarge is the oscillator discharge current
So, as fS = 1 /(Tcharge + Tdischarge) when the Regul
arrangement is not activated, the operating frequency can be
obtained from the graph in Figure 2.
NOTE: The output is disabled by the signal VOSC prot when
VCT is lower than 1.0 V (refer to Figure 31).
Synchronization and Demagnetization Blocks
To enable the output, the LOSC latch complementary
output must be low. Reset is activated by the Ldisch output
during the discharge phase. To restart, the LOSC has to be set
(refer to Figure 35). To perform this, the demagnetization
signal and the synchronization must be low.
• Synchronization:
The synchronization block consists of two comparators
that compare the synchronization signal (external) to 0.7 and
3.7 V (typical values). The comparators’ outputs are
connected to the input of an AND gate so that the final output
of the block should be:
− high when 0.7 < SYNC < 3.7 V
− low in the other cases.
As a low level is necessary to enable the output,
synchronized low level pulses have to be generated on the
output of the synchronization block. If synchronization is
not required, the Pin 9 must be connected to the ground.
Oscillator
Output Buffer
3.7 V
Sync
9
0.7 V
Figure 37. Synchronization
• Demagnetization:
In flyback applications, a good means to detect magnetic
saturation of the transformer core, or demagnetization,
consists in using the auxiliary winding voltage. This voltage
is:
− negative during the on−time,
− positive during the off−time,
− equal to zero for the dead−time with generally some
− ringing (refer to Figure 38).
That is why, the MC44603A demagnetization detection
consists of a comparator that can compare the auxiliary
winding voltage to a reference that is typically equal to
65 mV.
VPin 8
Zero Current
Detection
0.75 V
65 mV
On−Time Off−Time Dead−Time
−0.33 V
Figure 38. Demagnetization Detection
A diode D has been incorporated to clamp the positive
applied voltages while an active clamping system limits the
negative voltages to typically −0.33 V. This negative clamp
level is sufficient to avoid the substrate diode switching on.
In addition to the comparator, a latch system has been
incorporated in order to keep the demagnetization block
output level low as soon as a voltage lower than 65 mV is
detected and as long as a new restart is produced (high level
on the output) (refer to Figure 39). This process prevents
ringing on the signal at Pin 8 from disrupting the
demagnetization detection. This results in a very accurate
demagnetization detection.
The demagnetization block output is also directly
connected to the output, disabling it during the
demagnetization phase (refer to Figure 34).
NOTE: The demagnetization detection can be inhibited by
connecting Pin 8 to the ground.
Oscillator
Output
Buffer
RQ
Demag
S
VCC
VDemag Out
Negative Active
Clamping System
8
C Dem
65 mV
D
Figure 39. Demagnetization Block
Standby
• Power Losses in a Classical Flyback Structure
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