FA7764 Datasheet, PDF (14/18 Pages) Fuji Electric – FUJI Power Supply Control IC

FA7764
(13) VBIAS circuit
This IC provides the VBIAS pin to increase the
efficiency of the power supply circuit and reduce the
IC power loss.
The internal regulated power supply CREG is
usually generated by the VCC voltage.
If CREG is generated from the output voltage of the
DC-DC converter by the VBIAS pin, it is able to
reduce the power loss at CREG regulator and lower
power consumption operation (Fig. 5).
Fig. 5
When the VBIAS pin is not used, the operating
current consumption of the VCC terminal is 0.75 mA
(typ), on the other hand, when the VBIAS pin is used,
the comparable consumption is 0.4 mA(typ).
This operation switchover is performed at the
VBIAS pin voltage (i.e., output voltage of the DC-DC
converter) of 3.1V. Therefore, the VBIAS operation
will be effective only at the input voltage more than
3.1V.
Furthermore, a 5.5 V Zener diode for protection is
connected to the VBIAS pin. Therefore, if more than
5.5V voltage is applied to the VBIAS pin, a leak
current will be occurred in the Zener diode and also
increases the power loss.
Therefore, apply the voltage from 3.1V to 5.5V to
the VBIAS pin when you use the VBIAS pin function.
Connect a ceramic capacitor close to the VBIAS
pin as shown in Fig. 5 to eliminate noise. The
recommended value of the capacitance is about 0.1
μF.
If you do not use the VBIAS pin and in case of set
the DC-DC converter output voltage to less than
3.1V or more than 5.5V, connect the VBIAS pin to
the GND (Fig. 6).
Fig. 6
Fuji Electric Systems Co., Ltd.
AN-059E Rev.1.0
14
June.2010
http://www.fujielectric.co.jp/fdt/scd/