PJ317
3-Terminal Adjustable Output Positive Voltage Regulator
7-11
2002/10.rev.A
BASIC CIRCUIT OPERATION
The PJ317 is a 3-terminal floating regulator. In
operation, the PJ317 develops and maintains a nominal
1.25 volt reference (Vref) between its output and adjustment
terminals. This reference voltage is converted
to a
programming current ( IPROG ) by R1 (see Figure 17), and this
constant current flows through R2 to ground. The regulated
output voltage is given by:
Vout = Vref (1+ R2 / R1 )+ IAdj R2
Since the current from the adjustment terminal (Iadj)
represents an error term in the equation, the PJ317 was
designed to control Iadj to less than 100 µA and keep it
constant. To do this, all quiescent
operating current is
returned to the output terminal. This imposes the requirement
for a minimum load current. If the load current is less than
this minimum, the output voltage will rise.
Since the PJ317 is a floating regulator, it is only the
voltage differential across the circuit which is important to
performance, and operation at high voltages with respect to
ground is possible.
EXTERNAL CAPACITORS
A 0.1
µF disc or 1µF tantalum input bypass capacitor
(Cin) is recommended to reduce the sensitivity to input line
impedance.
The adjustment terminal may be bypassed to ground
to improve ripple rejection. This capacitor (CADJ) prevents
ripple
from being
amplified as the output
voltage is
rejection about 15dB at 120 HZ in a 10 volt application.
Although the PJ317 is stable with no output capa-
citance, like any feedback circuit ,
certain
values of
external capacitance can cause excessive ringing. An output
increased. A 10
µF capacitor should improve ripple
capacitance (CO) in the form of a 1 µF tantalum or 25 µF
aluminum electrolytic capacitor on the output swamps this
effect and insures stability.
PROTECTION DIODES
When external capacitors are used with any I.C. re-
gulator it is sometimes necessary to add protection diodes to
prevent the capacitors from discharging through low current
points into the regulator.
Figure 18 shows the PJ317 with the recommended
protection diodes for output voltages in excess of 25 V or
high capacitance values (CO > 25 µF, CADJ > 10µF). Diode D1
prevents CO from discharging thru the I.C. during an input
short circuit. Diode
D2 protects against capacitor CADJ
discharging through the I.C. during an output short circuit.
The combination of diodes D1 and D2 prevents CADJ from
discharging through the I.C. during an input short circuit.
The PJ317 is capable of providing extremely good
load regulation, but a few precautions are needed to obtain
maximum
performance.
For
best
performance,
the
programming resistor (R1) should be connected as close to the
regulator as possible to minimize line drops which effectively
appear in series with the reference, thereby degrading
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