MC44603 Datasheet, PDF (18/24 Pages) ON Semiconductor – MIXED FREQUENCY MODE GREENLINE PWM CONTROLLER

MC44603
Undervoltage Lockout Section
Figure 47. VCC Management
RF _Stby
Rref
Vref _enable
Pin 15
Pin 16
VCC
Cstartup
1
10
Reference Block:
Voltage and Current
1
0
Sources Generator
(Vref, Iref, ...)
Vdisable2
7.5 V
Startup
14.5 V
CUVLO1
Vdisable1
9.0 V
UVLO1
(to Soft–Start)
As depicted in Figure 47, an undervoltage lockout has
been incorporated to garantee that the IC is fully functional
before allowing system operation.
This block particularly, produces Vref (Pin 16 voltage) and
Iref that is determined by the resistor Rref connected between
Pin 16 and the ground:
+ + Iref
Vref
Rref
where
Vref
2.5 V (typically)
Another resistor is connected to the Reference Block:
RF _Stby that is used to fix the standby frequency.
In addition to this, VCC is compared to a second threshold
level that is nearly equal to 9.0 V (Vdisable1). UVLO1 is
generated to reset the maximum duty cycle and soft–start
block disabling the output stage as soon as VCC becomes
lower than Vdisable1. In this way, the circuit is reset and made
ready for the next startup, before the reference block is
disabled (refer to Figure 29). Finally, the upper limit for the
minimum normal operating voltage is 9.4 V (maximum value
of Vdisable1) and so the minimum hysteresis is 4.2 V.
((Vstup–th) _min = 13.6 V).
The large hysteresis and the low startup current of the
MC44603 make it ideally suited for off–line converter
applications where efficient bootstrap startup techniques are
required.
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MOTOROLA ANALOG IC DEVICE DATA