FA7764 Datasheet, PDF (15/18 Pages) Fuji Electric – FUJI Power Supply Control IC

FA7764
9. Design tips
(1) Setting an oscillation frequency
The oscillation frequency can be adjusted by
changing the value of the resistor connected to the
RT pin as described under item (2) in "Operation
description of each block." you can set any
oscillation frequency between 30 kHz and 400 kHz.
The oscillation frequency can be determined by the
characteristic curve of "Timing resister vs.
Oscillation frequency" or the RT value can be
approximately calculated as follows:
fosc = 3738× RT ^−0.95
Where, fosc: Oscillation frequency [kHz]
RT: Timing resistance [kΩ]
This expression, however, can be used as rough
calculation, the obtained value is not guaranteed.
The operation frequency varies due to the
conditions such as tolerance of the characteristics of
the ICs, influence of noises, or external discrete
components. When determining the values,
examine the effectiveness of the values in an actual
circuit.
The timing resistor RT should be connected to the
GND pin as shortly as possible because the RT pin
is high impedance pin and is easy affected by
noises.
(2) IC losses
This IC incorporates a switching MOSFET. The
loss generated in this MOSFET accounts for most of
the IC loss, and the loss is determined by the
input/output conditions of the DC-DC converter.
Attention is needed to the permissible loss of the IC
(SOP-8 E-Pad: 3.9 W at 25°C).
The permissible loss in SOP-8 (E-Pad) specified in
this document is under the condition of mounting a
4-layer board (50 mm x 40 mm) at Ta of 25°C when
the exposed pad is connected by solder.
Therefore the permissible loss will be smaller than
the specified loss by using any other board (such as
2-layer board) or the exposed pad is unconnected.
The recommended maximum load current for the
DC-DC converter consisting of this IC is about 1.5A.
This load current depend on the input voltage and
operating ambient temperature.
(3) Restriction of external discrete components
and recommended operating conditions
To achieve a stable operation of the IC, the value
of external discrete components and the voltage and
the current applied to each pin should be within the
recommended operating conditions.
This IC incorporates a P-channel MOSFET
between the OUT pin and the VCC pin. Since the
P-channel MOSFET has a parasitic diode, so if the
voltage of OUT pin becomes higher than the VCC
pin voltage, the current flows from the OUT pin to
the VCC pin.
(4) Preventing the application of negative voltage
If rather large negative voltage is applied to any
pins of this IC, internal parasitic elements start
operating, and they may cause malfunctions.
Therefore the negative voltage which is applied to
each terminal of the ICs must be kept above -0.3V.
(5) Improving the transient response characteristic
The transient response characteristic of a DC-DC
converter generally shows the overshoot in the
output voltage when the power supply voltage is
started, and the overshoot/undershoot in the output
voltage when the load current changes abruptly (Fig.
7).
This IC has few overshoot at startup due to the soft
start function. However, the overshoot/undershoot
may occur when the load changes abruptly,
depending on the conditions.
This overshoot/undershoot can be reduced by
connecting a capacitor to the output voltage
detection resistor as shown in Fig. 2.
No universal constant can be proposed because it
varies with the conditions. However, we believe that
an appropriate effect can be produced when the
constant is between hundreds of pico-Farads and
tens of nano-Farads.
Fuji Electric Systems Co., Ltd.
AN-059E Rev.1.0
15
June.2010
http://www.fujielectric.co.jp/fdt/scd/