FA5604 Datasheet, PDF (16/31 Pages) Fuji Electric – FUJI Power Supply Control IC

FA5604/5605/5606
9. Description of each circuit
(1)Oscillator
A desired oscillation frequency can be set by the value of the resistor connected to the RT pin (Fig.1 recommended
value is 100 kHz to 300 kHz). The built-in capacitor voltage oscillates between about 3V and 1V, with almost the
same charging and discharging gradients (Fig.2).
The oscillator waveform cannot be observed from the outside because the oscillator output is not pinned out. The
oscillator output is connected to a PWM comparator.
OSC
3V
1
RT
Rt
▽ ^GND
1V
Fig.1
Rt=Small ^Rt=Large
Fig.2
This IC has the function that reducing the power consumption by decreasing frequency at light load. It according to
the value of the FB pin voltage at light load (Fig.3). When the FB pin voltage is 1V (typ.), the switching frequency
decreases to about 30% down of the set frequency. When at the overload, it corrects current limiting by decreasing
frequency. The minimum switching frequency is about 10% down of the set frequency.
Extending only the OFF period
(at light load)
3V
Dmax=FA5604:46%
FA5605/06:70%
ON OFF
1V
OFF
Fig.3
As shown in Fig.4 (Fosc-VFB characteristics graph), there is a little difference at the point before the frequency is
reduced. Check that this is not a problem for actual use.
Because this IC adopts the frequency reduction method in which only the off period is extended, if the frequency is
reduced at overload, the ON duty is also reduced, and therefore the output voltage is reduced. Consequently, the load
current is reduced at the point in which it enters the drooping characteristics at overload (Fig 5).
Difference
VthFBS
Volpon
VFB
Fig.4
Fuji Electric Co., Ltd.
AN-033E Rev.1.2
April-2011
The current is
changed by the
changed frequency.
Output current Iout
Fig.5
16
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