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CX1084-3.3 データシートの表示(PDF) - Unspecified

CX1084-3.3 5A Low Dropout Voltage Regulator ETC2
CX1084-3.3 Datasheet PDF : 9 Pages
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needed. Microsecond surge currents of 50A to 100A can be
handled by the internal diode between the input and output
pins of the device. In normal operations it is difficult to get
those values of surge currents even with the use of large
output capacitances. If high value output capacitors are
used, such as 1000uF to 5000uF and the input pin is
instantaneously shorted to ground, damage can occur. A
diode from output to input is recommended, when a crowbar
circuit at the input of the CX1084 is used. Normal
power supply cycling or even plugging and unplugging in
the system will not generate current large enough to do any
damage. The adjustment pin can be driven on a transient
basis  25V, with respect to the output without any device
degradation. As with any IC regulator, none the protection
circuitry will be functional and the internal transistors will
break down if the maximum input to output voltage
differential is exceeded.
Overload Recovery
When the power is first turned on, as the input voltage
rises, the output follows the input, permitting the regulator to
start up into heavy loads. During the start-up, as the input
voltage is rising, the input-to-output voltage differential
remains small, allowing the regulator to supply large output
currents. A problem can occur with a heavy output load
when the input voltage is high and the output voltage is low,
when the removal of an output short will not permit the
output voltage to recover. The load line for such a load may
intersect two points on the output current curve. In this
case, there are two stable output operating points for the
regulator. With this double intersection, the power supply
may need to be cycled down to zero and brought up again
to make the output recover.
Page 5 of 5
Ripple Rejection
The ripple rejection values are measured with the
adjustment pin bypassed. The impedance of the adjust pin
capacitor at the ripple frequency should be less than the
value of R1 (normally 100Ωto120) for a proper bypassing
and ripple rejection approaching the values shown. The size
of the required adjust pin capacitor is a function of the input
ripple frequency. If R1=100at 120Hz the adjust pin
capacitor should be 25uF. At 10kHz only 0.22uF is needed.
The ripple rejection will be a function of output voltage,
in circuits without an adjust pin bypass capacitor. The
output ripple will increase directly as a ratio of the output
voltage to the reference voltage (VOUT / VREF).
Output Voltage
The CX1084 series develops a 1.25V reference
voltage between the output and the adjust terminal. Placing
a resistor between these two terminals causes a constant
current to flow through R1 and down through R2 to set the
overall output voltage.
This current is normally the specified minimum load
current of 10mA. Because IADJ is very small and constant it
represents a small error and it can usually be ignored.
Figure 1. Basic Adjustable Regulator
Load Regulation
True remote load sensing it is not possible to provide,
because the CX1084 is a three terminal device. The
resistance of the wire connecting the regulator to the load
will limit the load regulation. The data sheet specification for
load regulation is measured at the bottom of the package.
Negative side sensing is a true Kelvin connection, with the
bottom of the output divider returned to the negative side of
the load.
The best load regulation is obtained when the top of the
resistor divider R1 is connected directly to the case not to
the load. If R1 were connected to the load, the effective
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