LM217M/LM317M
APPLICATION INFORMATION
The LM317M provides an internal reference
voltage of 1.25V between the output and
adjustments terminals. This is used to set a
constant current flow across an external resistor
divider (see fig. 3), giving an output voltage V
O
of:
R
2
V
O
= V
REF
(1 +
)
+ I
ADJ
R
2
R
1
The device was designed to minimize the term
I
ADJ
(100µA max) and to maintain it very constant
with line and load changes. Usually, the error
term IADJ⋅R2 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 LM317M is a floating regulator and
”sees” only the input-to-outputdifferential 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 regulator are easily
obtainable and, by connecting a fixed resistor
between the adjustment and output, the device
can be used as a precision current regulator.
In order to optimise the load regulation, the
current set resistor R1 (see fig. 3) should be tied
as close as possible to the regulator, while the
ground terminal of R2 should be near the ground
of the load to provide remote ground sensing.
EXTERNAL CAPACITORS (Fig.5)
Normally no capacitors are needed unless the
device is situated far from the input filter
capacitors; in which case an input bypass is
needed.
A 0.1µF disc or 1µF tantalium 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
(C
adj
) prevents ripple from being amplified as the
output voltage is increased. A 10µF capacitor
should improve ripple rejection about 80dB at
120Hz an a 10V application.
Although the LM317M is 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 tantalium 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 show the 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 discarging
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
o
C.
5/11
STMICROELECTRONICS [ STMICROELECTRONICS ]
