PJ1583CM Ver la hoja de datos (PDF) - Promax Johnton
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PJ1583CM Datasheet PDF : 6 Pages
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PJ1581/PJ1582/PJ1583
Dual Input Low Dropout Voltage Regulator
Figure 2. Remote Load Sensing
handle until the regulator throttles to the load current level.
Because they contain parasitic resistance and inductance,
capacitors are not ideal elements. These parasitic elements
dominate the change in output voltage as the beginning of a
transient load step change. The ESR of the output capacitors
produces and instantaneous step in output voltage
△V=△I(ESR). The ESL of the output capacitors produces a
drop proportional to the rate of change of the output current
V=L(△I/△t). the output capacitance produces a change in
output voltage proportional to the time until the regulator can
respond △V=△t(△I/C).
Figure 4 illustrates there transient effects.
Figure 3. Remote Sensing Improves Load Regulation
Stability
The circuit design used in the PJ1581 Series requires the use
of an output capacitor as part of the device frequency
compensation. The addition of 150uF aluminum electrolytic
or a 22uF solid tantalum on the output will ensure stability for
all operating conditions. In order to meet the transient
performance of the processor larger value capacitors are
needed. To limit the high frequency noise generated by the
processor high quality bypass capacitors must be used. In
order to limit parasitic inductance (ESL) and resistance (ESR)
in capacitors to acceptable limits, multiple small ceramic
capacitors in addition to high quality solid tantalum capacitors
are required.
When the adjustment terminal is bypass to improve the ripple
rejection, the requirement for an output capacitor increases.
The Adjust pin is brought out on the fixed voltage device
specifically to allow this capability. To further improve
stability and transient response of these devices larger values
of output capacitor can be used.
The modern processors generate large high frequency current
transients. The load current step contains higher order
frequency components than the output coupling network must
Output voltage
The PJ1581 (adjustable version) develops a 1.25V reference
voltage between the Sense Pin and the Adjust Pin (Figure 5).
Placing a resistor between these two terminals causes a
constant current to flow though R1 and down though R2 to set
the output voltage. In general R1 is chosen so that this current
is the specified minimum load current of 10mA. The current
out of the Adjust pin is small, typically 50uA and it adds to the
current from R1.For best regulation the top of the resistor
divider should be connected directly to the Sense pin.
Figure 5. Setting Output Voltage
4-6
2002.rev.A
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