PJ2931CS-3.3V データシートの表示（PDF） - Promax Johnton
部品番号
コンポーネント説明
一致するリスト
PJ2931CS-3.3V Datasheet PDF : 5 Pages
| |||
PJ2931
Ultra Low Dorpout Voltator
Application Hints
One of the distinguishing factors of the PJ2931 series
regulators is the requirement of an output capacitor for device
stability. The value required varies greatly depending upon
the application circuit and other factors. Thus some comments
on the characteristics of both capacitors and the regulator are
in order.
High frequency characteristics of electrolytic capacitors
depend greatly on the type and even the manufacturer. As a
result, a value of capacitance that works well with the PJ2931
for one brand or type may not necessary be sufficient with an
electrolytic of different origin. Sometimes actual bench
testing, as described later, will be the only means to determine
the proper capacitor and value. Experience has shown that, as
a rule of thumb, the more expensive and higher quality
electrolytic generally allows a smaller value for regulator
stability. As an example, while a high-quality 100µF
aluminum electrolytic covers all general application circuits,
similar stability can be obtained with a tantalum electrolytic of
only 47µF. This factor of two can generally be applied to any
special application circuit also.
Another critical characteristic of electrolytic is their
performance over temperature. While the PJ2931 is designed
to operate to -20℃, the same is not always true with all
electrolytic(hot is generally not a problem). The electrolyte in
many aluminum types will freeze around -250℃, reducing
their effective value to zero. Since the capacitance is needed
for regulator stability, the natural result is oscillation (and lots
of it) at the regulator output. For all application circuits where
cold operation is necessary, the output capacitor must be rated
to operate at the minimum temperature. By coincidence,
worst-case stability for the PJ2931 also occurs at minimum
temperatures. As a result, in applications where the regulator
junction temperature will never be less than 25℃, the output
capacitor can be reduced approximately by a factor of two
over the value needed for the entire temperature range. To
continue our example with the tantalum electrolytic, a value
of only 22µF would probably thus suffice. For high-quality
aluminum, 47µF would be adequate in such an application.
Another regulator characteristic that is noteworthy is that
stability decreases with higher output currents. This sensible
fact has important connotations. In many applications, the
PJ2931 is operated at only a few milliamps of output current
or less. In such a circuit, the output capacitor can be further
reduced in value. As a rough estimation, a circuit that is
required to deliver a maximum of 10mA of output current
from the regulator would need an output capacitor of only half
the value compared to the same regulator required to deliver
the full output current of 100mA. If the example of the
tantalum capacitor in the circuit rated at 25 ℃ junction
temperature and above were continued to include a maximum
of 10 mA of output current, then the 22 µF output capacitor
could be reduced to only 10 µF.
In the case of the PJ2931CS adjustable regulator in SOP-8
package, the minimum value of output capacitance is a
function of the output voltage. As a general rule, the value
decreases with higher output voltages, since internal loop gain
is reduced.
At this point, the procedure for bench testing the minimum
value of an output capacitor in a special application circuit
should be clear. Since worst-case occurs at minimum
operating temperatures and maximum operating currents, the
entire circuit, including the electrolytic, should be cooled to
the minimum temperature. The input voltage to the regulator
should be maintained at 0.6V above the output to keep
internal power dissipation and die heating to a minimum.
Worst-case occurs just after input power is applied and before
the die has had a chance to heat up. Once the minimum value
of capacitance has been found for the brand and type of
electrolytic in question, the value should be doubled for actual
use to account for production variations both in the capacitor
and the regulator. (All the values in this section and the
remainder of the data sheet were determined in this fashion.)
Definition of Terms
Dropout Voltage: The input-output voltage differential at
which the circuit ceases to regulate against further reduction
in input voltage. Measured when the output voltage has
dropped 100 mV from the nominal value obtained at 14V
input, dropout voltage is dependent upon load current and
junction temperature.
Input Voltage: The DC voltage applied to the input terminals
with respect to ground.
Input-Output Differential: The voltage difference between
the unregulated input voltage and the regulated output voltage
for which the regulator will operate.
Line Regulation: The change in output voltage for a change
input voltage. The measurement is made under conditions of
low dissipation or by using pulse techniques such that the
average chip temperature is not significantly affected.
Load Regulation: The change in output voltage for a change
in load current at constant chip temperature.
Output Noise Voltage: The rms AC voltage at the output,
with constant load and no input ripple, measured over a
specified frequency range.
Quiescent Current: That part of the positive input current
that does not contribute to the positive load current. The
regulator ground lead current.
Ripple Rejection: The ratio of the peak-to-peak input ripple
voltage to the peak-to-peak output ripple voltage.
Temperature Stability of Output Voltage: The percentage
change in output voltage for a thermal variation from room
temperature true to either temperature extreme.
3-5
2003/01 .rev.A-1
Share Link: 