SP317L Datasheet, PDF (10/13 Pages) Sipex Corporation – 3-Terminal Adjustable Voltage Regulator

Application Information
In operation, the SP317L develops a nominal
1.25V reference voltage, V_REF, between
the output and the adjustment terminal.
The reference voltage is impressed across
program resistor R1 and, since the voltage
is constant, a constant current I₁ then flows
through the output set resistor R2, giving an
output voltage of:
Vout = Vref _* {1 + R2/R1} + Iadj(R2)
Since the 100μA current from the adjust-
ment terminal represents an error term,
the SP317L was designed to minimize
IADJ and make it very constant with line
and load changes. To do this, all quiescent
operating current is returned to the output
establishing a minimum load current re-
quirement. If there is insufficient load on
the output, the output will rise.
SP317L
V_IN
V _IN
V _OUT
I₁
ADJ
V _REF
V _OUT
R₁
I _ADJ
R₂
Figure 21: R1, R2 and Quiescent Current
EXTERNAL CAPACITORS
Although the SP317L is stable with no
output capacitors, like any feedback circuit,
certain values of external capacitance can
cause excessive ringing. This occurs with
values between 500pF and 5000pF. A
1µF solid tantalum (or 25µF aluminum
electrolytic) on the output swamps this
effect and insures stability.
An input bypass capacitor is recommended
in case the regulator is more than 6 inches
away from the usual large filter capacitor.
A 0.1 µF disc or 1µF solid tantalum on
the input is suitable input bypassing for
almost all applications. The device is more
sensitive to the absence of input bypassing
when adjustment or output capacitors are
used, but the above values will eliminate
the possibility of problems.
The adjustment terminal can be bypassed
to ground on the SP317L to improve ripple
rejection and noise. This bypass capacitor
prevents ripple and noise from being
amplified as the output voltage is increased.
With a 10µF bypass capacitor 80dB ripple
rejection is obtainable at any output level.
Increases over 10µF do not appreciably
improve the ripple rejection at frequencies
above 120Hz. If the bypass capacitor is
used, it is sometimes necessary to include
protection diodes to prevent the capacitor
from discharging through internal low current
paths and damaging the device. In general,
the best type of capacitors to use is solid
tantalum. Solid tantalum capacitors have
low impedance even at high frequencies.
Depending upon capacitor construction, it
takes about 25µF in aluminum electrolytic
to equal 1µF solid tantalum at high
frequencies. Ceramic capacitors are also
good at high frequencies; but some types
have a large decrease in capacitance
at frequencies around 0.5MHz. For this
reason, a 0.01µF disc may seem to work
better than a 0.1µF disc as a bypass.
Date: 8/1/06 Rev H
SP317L 3-Terminal 100mA Adjustable Voltage Regulator
10
© 2006 Sipex Corporation