STRF6624 datasheet(5/16 Pages) ALLEGRO

Part #	STRF6624

Description	OFF-LINE QUASI-RESONANT FLYBACK SWITCHING REGULATORS

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Manufacturer	ALLEGRO [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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