LM217M/LM317M
5/11
APPLICATION INFORMATION
The LM217M/LM317M provide an internal
reference voltage of 1.25V between the output
and adjustment terminals. These devices are
used to set a constant current flow across an
external resistor divider (see fig. 3), giving an
output voltage V
O
of: V
O
=V
REF
(1 + R
2
/R
1
)+
I
ADJ
R
2
The devices were designed to minimize the term
I
ADJ
(100µA max) and to maintain it very constant
in line and load changes. Usually, the error term
I
ADJ
×R
2
can be neglected. To obtain the previous
requirement, all the regulator quiescent current is
returned to the output terminal, imposing a
minimum load current condition. If the load is
insufficient, the output voltage will rise.
Since the LM217M/LM317M devices are floating
regulators and "see" only the input-to-output
differential voltage, supplies of very high voltage
with respect to ground can be regulated as long as
the maximum input-to-output differential is not
exceeded. Furthermore, programmable regulators
are easily obtained and, by connecting a fixed
resistor between the adjustment and output, the
devices can be used as a precision current
regulator. In order to optimize the load regulation,
the current set resistor R
1
(see fig. 3) should be
tied as close as possible to the regulator, while the
ground terminal of R
2
should be near the ground
of the load to provide remote ground sensing.
EXTERNAL CAPACITORS (Fig.5)
Normally no capacitors are needed unless the
devices are situated far from the input filter
capacitors; in which case an input bypass is
needed.
A 0.1µF disc or 1µF tantalum input bypass
capacitor (C
I
) 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 increased. A 10µF capacitor
should improve ripple rejection of about 80dB at
120Hz in a 10V application.
Although the LM217M/LM317M devices are
stable with no output capacitance like any
feedback circuit, certain values of external
capacitance can cause excessive ringing. An
output capacitance (C
O
) in the form of a 1µF
tantalum or 25µF aluminium electrolytic capacitor
on the output swamps this effect and insures
stability.
PROTECTION DIODES (Fig.5)
When external capacitors are used with any IC
regulator it is sometimes necessary to add
protection diodes to prevent the capacitors from
discharging through low current points into the
regulator.
Figure 5 shows the LM217M/LM317M with the
recommended protection diodes for output
voltages in excess of 25V or high capacitance
values (C
3
> 25µF, C
2
> 10µF). Diode D1 prevents
C
3
from discharging through the IC during an input
short-circuit. The combination of diodes D1 and
D2 prevents C
2
from discharging through the
regulator during an input or output short-circuit.
START-UP BLOCK
It’s not guaranteed the Re-Boot of the device
when the junction temperature is over 85°C.
