TS317L Datasheet, PDF (3/6 Pages) Taiwan Semiconductor Company, Ltd – 3-Terminal Adjustable Output Positive Voltage Regulator

Application Information
Basic Circuit Operation
The TS317L is a 3-terminal floating regulator. In
operation, the TS317L develops and maintains a
nominal 1.25V 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 TS317L
was designed to control Iadj to less than 100uA 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 TS317L 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.
Figure 1. basic circuit configuration
Load Regulation
The TS317L is capable of providing extremely good
load regulation, but a few precautions are needed to
obtain maximum performance. The current set resistor
connected between the adjustment therminal and the
output terminal (usually 240Ω) should be tied directly to
the output of the regulator rather than near the load.
This eliminates line drops from appearing effectively in
series with the reference and degrading regulation. For
example, a 15V regulator with 0.05Ω resistance
between the regulator and load will have a load
regulation due to line resistance of 0.05Ω x Io.
If the set resistor is connected near the load the
effective line resistance will be 0.05Ω (1+ R2/R1) or in
this case, 11.5 times worse.
Figure 6 shows the effect of resistance between the
regulator and 240Ω set resistor.
With the TO-92 package, it is easy to minimize the
resistance from the case of the set resistor, by using
two separate leads to the output pin. The ground of R2
can be returned near the ground of the load to provide
remote ground sensing and improve load regulation.
Figure 2. Regulator with line resistance in
output lead
External Capacitor
An input bypass capacitor is recommended in case the
regulator is more than 6 inches away from the usual
large filter capacitor. A 0.1µF 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 and noice. This capacitor
(Cadj) prevents ripple from being amplified as the
output voltage is increased. With a 10uF bypass
capacitor 80dB ripple rejection is obtainable at any
output level. Increased over 10uF 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 25uF in aluminum
electrolytic to equal 1uF solid tantalum at high
frequencies. Ceramic capacitors are also good at high
frequencies; but some types have a large
TS317L
3-6
2003/12 rev. A