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参数资料
| 型号： | FA7738N |
| 厂商： | FUJI ELECTRIC CO LTD |
| 元件分类： | 稳压器 |
| 英文描述： | 5.5 A SWITCHING REGULATOR, 400 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | SOP-8 |
| 文件页数： | 7/19页 |
| 文件大小： | 484K |
| 代理商： | FA7738N |
15
FA7738N/P
8. Design tips
(1) Setting an oscillation frequency
An oscillation frequency can be adjusted to a
desired level by changing the value of the resistor
connected to the RT terminal as described under
item (2) in "Operation description of each block."
Set an oscillation frequency between 30 kHz and
400 kHz.
The oscillation frequency with regard to the RT can
be determined by the "Timing resister vs. Oscillation
frequency" of the characteristic curve or roughly
determined by using the following equation:
95
.
0
3738
×
=
T
R
fosc
Where, fosc: Oscillation frequency [kHz]
RT
: Timing resistance [k]
Note that this equation provides a guide for setting a
rough level and is not intended to guarantee that
level. The operating frequency is determined by
the characteristic dispersion and noise of the IC,
along with external parts and other conditions. In
determining a constant, conduct an operation check
during actual setting.
The RT terminal is a high-impedance terminal and
vulnerable to noise. Therefore, connect the timing
resistor RT to a position close to the RT terminal and
GND terminal by short wiring.
(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 largely determined by the
input/output conditions of the power supply circuit.
When using this IC, be very careful not to let it
exceed the permissible loss of the IC (SOP-8 E-Pad:
3.9 W at 25°C, DIP-8: 1.39 W at 25°C). Note also
that the permissible loss in SOP-8 (E-Pad) specified
in this document is a condition for mounting a
4-layer board (50 mm x 40 mm) at Ta of 25°C when
the exposed pad is connected by solder. Note that
the loss will be smaller than the permissible loss
specified above if any other board (such as 2-layer
board) is used or the exposed pad is unconnected.
Depending on the input voltage and operating
ambient temperature, the recommended maximum
load current for the DC-DC converter consisting of
SOP-8 (E-Pad) of this IC is about 1.5 A.
(3) Constraints and recommended operating
conditions for external parts
For stable operation, set the constants, applied
voltages, and currents of the parts to be connected
to each terminal of this IC within the recommended
range of operating conditions.
The system incorporates a PchMOSFET between
the OUT terminal and the VCC terminal.
This
PchMOSFET
furnishes
a
parasitic
diode.
Therefore, current will flow from the OUT terminal to
the VCC terminal if the OUT terminal voltage
exceeds the Vcc voltage.
(4) Preventing the application of negative voltage
Applying a high negative voltage to each terminal of
the IC activates the parasitic devices in the IC, and
may result in a malfunction.
Make sure that no
terminal is subjected to voltage of less than -0.3 V.
(5) Improving the transient response characteristic
The transient response characteristic of a power
supply circuit generally refers to overshoot in the
output voltage when the power supply voltage is
started, and to overshoot/undershoot in the output
voltage when the load current changes abruptly (Fig.
7).
This IC undergoes hardly any overshoot at startup
due to the soft start of the reference voltage.
However, the overshoot/undershoot may occur
when the load changes abruptly, depending on the
conditions.
This overshoot/undershoot can be
restricted by connecting a capacitor to the output
voltage detection resistor as shown in Fig. 2.
No universal constant can be determined 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.
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