參數(shù)資料
型號(hào)： MC44604P
廠商： ON SEMICONDUCTOR
元件分類： 穩(wěn)壓器
英文描述： RV Series - Econoline Unregulated DC-DC Converters; Input Voltage (Vdc): 3.3V; Output Voltage (Vdc): 3.3V; Power: 2W; High Isolation 2W Converter; Approved for Medical Applications; EN and UL Safety Certificates; 6kVDC Isolation; Skinny DIP24 Package; Optional Continuous Short Circuit Protected; Fully Encapsulated; Very Low Isolation Capacitance
中文描述： 0.75 A SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDIP16
封裝： PDIP-16
文件頁(yè)數(shù)： 15/24頁(yè)
文件大?。?/td> 341K
代理商： MC44604P
MC44604
http://onsemi.com
15
The Sawtooth Generation
In the steady state, the oscillator voltage varies between
about 1.6 V and 3.6 V.
Indeed, the sawtooth is obtained by charging and
discharging an external capacitor CT (Pin 10), using two
distinct current sources = Icharge and Idischarge. In fact, CT
is permanently connected to the charging current source
(0.4 Iref) and so, the discharge current source has to be
higher than the charge one to be able to decrease the CT
voltage. This condition is performed, its value being
(2 Iref).
Two comparators are used to generate the sawtooth. They
compare the CT voltage to the oscillator valley and peak
values. The comparison to the low value enables to detect the
end of the discharge phase while the comparison to the high
value determines when the charge cycle must be stopped. A
latch (LDISCH) memorizes the oscillator state.
Figure 30. Oscillator
10
CT
1 V
Vref
0.4 IREF
CVOS PROT
COSC HIGH
COSC LOW
0
1
IREGUL
1.6 V
Q
MC44604
COSC REGUL
Vosc prot
Vosc
R
S
LOSC
3.6 V
Q
S
R
DISCH
0
1
Vdemag out
CT< 1.6 V
DISCHARGE
IDISCHARGE
Now, in addition to the charge and discharge cycles, a
third state can exist. This phase can be produced when at the
end of the discharge phase, the oscillator has to wait for a
demagnetization pulse before re–starting. During this
delay, the CT voltage must remain equal to the oscillator
valley value (
1.6 V). So, a third regulated current source
IREGUL controlled by COSC REGUL, s connected o CT n
order to perfectly compensate the (0.4 Iref) current source
that permanently supplies CT.
On–time is only allowed during the oscillator capacitor
charge. So, the maximum duty cycle is 80%. (Note 1)
The demagnetization condition is taken into account by a
second latch (Losc). (Refer to demagnetization § for further
details.)
Oscillator Frequency
The oscillator frequency can be deducted using the
following equations:
Tcharge
CT
V
.
Icharge
Tdischarge
CT
V Idischarge
where:
Tcharge is the oscillator charge time
V 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:
fosc = 1 /(Tcharge + Tdischarge) if the REGUL
arrangement is not activated, the following equation can
be obtained:
fosc
0·395
Rref
CT
Demagnetization Block
(Note 2)
To enable the output, the Losc latch complementary output
must be low. Now, this latch reset is activated by the LDISCH
output during the discharge phase. So, to restart, the Losc has
to be set (refer to Figure 30). To perform this, the
demagnetization signal must be low.
In a fly–back, a good means to detect the demagnetization
consists in using the VCC winding voltage. Indeed this
voltage is:
— negative during the on–time,
— positive during the off–time,
— equal to zero for the dead–time with generally a
ringing (refer to Figure 31).
That is why, the MC44604 demagnetization detection
consists of a comparator that can compare the VCC winding
voltage to a reference that is typically equal to 65 mV.
Note 1. The output is disabled by the signal Vosc prot when VCT
is lower than 1 V. (Refer to Figure 29 and Figure 30.)
Note 2. The demagnetization detection can be inhibited by
connecting pin 8 to the ground.
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