TS317L
3-Terminal Adjustable Positive Voltage Regulator
Document Number:
DS_P0000264
6
Version: D15
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 (V
REF
)
between its output and adjustment terminals. This reference voltage is converted to a programming current (I
PROG
.) by R
1
(see
Figure 17), and this constant current flows through R
2
to ground. The regulated output voltage is given by:
V
OUT
= V
REF
( 1 + R2 / R1 ) + I
ADJ
* R2
Since the current from the adjustment terminal (I
ADJ
) represents an error term in the equation, the TS317L was designed to control
I
ADJ
to less than 100µA 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 17. Basic Circuit Configuration
Figure 18. Voltage Regulator with Protection Diode
Protection Diode
When external capacitors are used with any I.C. regulator 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 TS317L with the recommended protection diodes for output voltages in excess of 25 V or high capacitance
values (Co > 25µF, C
ADJ
> 10µF). Diode D1 prevents Co from discharging thru the I.C. during an input short circuit. Diode D2
protects against capacitor C
ADJ
discharging through the I.C. during an output short circuit. The combination of diodes D1
and D2
prevents C
ADJ
from discharging through the I.C. during an input short circuit.
Load Regulation
The TS317L 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 regulation. The ground end of R2 can
be returned near the load ground to provide remote ground sensing and improve load regulation.
External Capacitor
A 0.1µF disc or 1µF tantalum input bypass capacitor (C
IN
) is recommended to reduce the sensitivity to input line impedance.
The adjustment terminal may be bypassed to ground to improve ripple rejection. This capacitor (C
ADJ
) prevents ripple from being
amplified as the output voltage is rejection about 15dB at 120H
Z
in a 10V application.
Although the TS317L is stable with no output capacitance, like any feedback circuit, certain values of external capacitance can
cause excessive ringing. An output increased. A 10µF capacitor should improve ripple capacitance (Co) in the form of a 1µF
tantalum or 25µF aluminum electrolytic capacitor on the output swamps this effect and insures stability.
