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STRF6676 Datasheet(PDF) 5 Page - Allegro MicroSystems

Part No. STRF6676
Description  OFF-LINE QUASI-RESONANT FLYBACK SWITCHING REGULATORS
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Maker  ALLEGRO [Allegro MicroSystems]
Homepage  http://www.allegromicro.com
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STRF6676 Datasheet(HTML) 5 Page - Allegro MicroSystems

 
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Series STR-F6600
OFF-LINE
QUASI-RESONANT FLYBACK
SWITCHING REGULATORS
5
The voltage on the V
IN terminal (pin 4) controls startup
and shutdown of the Series STR-F6600 devices.
Figure 1 shows a typical start up circuit. The V
IN
terminal voltage during startup is shown in figure 2.
Functional Description and Operation
continued, next page...
Figure 1 – Start-Up Circuit
VIN
TIME
DRIVE WINDING
VOLTAGE
16 V
(TYP.)
UNDER-VOLTAGE LOCKOUT (VINQ)
11 V
(MAX.)
ON-STATE VOLTAGE (VINT)
STARTUP
DELAY
Figure 2 – Waveform of V
IN Terminal Voltage
at Startup
IIN
VIN
11 V
(MAX.)
30 mA
(MAX.)
100
µA
(MAX.)
14.4 V
(MIN.)
IIN(ON)
IIN(OFF)
Figure 3 – Supply Terminal Current, I
IN
At startup, C2 is charged through the startup resistor R
S.
When the V
IN terminal voltage reaches 16 V (typ.), the
control circuit enables regulator operation. Once the
regulator starts, it draws up to 30 mA from C2 causing the
voltage on C2 to fall momentarily. Once the regulator
output voltage is established, the drive winding D starts to
charge C2 via D2. The voltage on C2 thus recovers to the
nominal drive voltage (18 V).
As shown in figure 3, the input current is below 100
µA
(at T
M = 25°C) prior to control circuit turn on. The latch
circuit holding current is 400
µA (max.). To ensure latch
operation, the current in R
S at the lowest ac input voltage
should be at least 500
µA.
The value of R
S thus determines the charge time of C2
and thus the startup delay. R
S is typicaly 68 kΩ for wide
operation (90 V ac to 265 V ac) and 100 k
Ω for 220 volt
ac operation.
The choice of C2 is a compromise between an accept-
able startup delay (in conjunction with R
S) and a hold-up
time sufficient to keep pin 4 above its under-voltage
shutdown threshold of 11 V. Typically C2 is in the range
of 47
µF to 100 µF.


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