LT1086 Series
APPLICATIONS INFORMATION
plugging and unplugging in the system will not generate
current large enough to do any damage.
The adjustment pin can be driven on a transient basis
±25V,
with respect to the output without any device
degradation. Of course as with any IC regulator, exceeding
the maximum input-to-output voltage differential causes
the internal transistors to break down and none of the
protection circuitry is functional.
D1
1N4002
(OPTIONAL)
V
IN
IN
LT1086
ADJ
OUT
R1
+
V
OUT
C
OUT
150µF
+
C
ADJ
10µF
R2
LT1086 • AI01
Overload Recovery
Like any of the IC power regulators, the LT1086 has safe
area protection. The safe area protection decreases the
current limit as input-to-output voltage increases and
keeps the power transistor inside a safe operating region
for all values of input-to-output voltage. The LT1086
protection is designed to provide some output current at
all values of input-to-output voltage up to the device
breakdown.
When power is first turned on, as the input voltage rises,
the output follows the input, allowing the regulator to start
up into very heavy loads. During the start-up, as the input
voltage is rising, the input-to-output voltage differential
remains small, allowing the regulator to supply large
output currents. With high input voltage, a problem can
occur wherein removal of an output short will not allow the
output voltage to recover. Older regulators such as the
7800 series also exhibited this phenomenon, so it is not
unique to the LT1086.
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low, such as
immediately after a removal of a short. The load line for
such a load may intersect the output current curve at two
points. If this happens there are two stable output operat-
ing points for the regulator. With this double intersection
U
W
U
U
the power supply may need to be cycled down to zero and
brought up again to make the output recover.
Ripple Rejection
For the LT1086 the typical curves for ripple rejection
reflect values for a bypassed adjust pin. This curve will be
true for all values of output voltage. For proper bypassing
and ripple rejection approaching the values shown, the
impedance of the adjust pin capacitor at the ripple fre-
quency should equal the value of R1, (normally 100Ω to
120Ω). The size of the required adjust pin capacitor is a
function of the input ripple frequency. At 120Hz the adjust
pin capacitor should be 13µF if R1 = 100Ω; at 10kHz only
0.16µF is needed.
For circuits without an adjust pin bypass capacitor the
ripple rejection will be a function of output voltage. The
output ripple will increase directly as a ratio of the output
voltage to the reference voltage (V
OUT
/V
REF
). For ex-
ample, with the output voltage equal to 5V and no adjust
pin capacitor, the output ripple will be higher by the ratio
of 5V/1.25V or four times larger. Ripple rejection will be
degraded by 12dB from the value shown on the LT1086
curve. Typical curves are provided for the 5V and 12V
devices since the adjust pin is not available.
Output Voltage
The LT1086 develops a 1.25V reference voltage between
the output and the adjust terminal (see Figure 1). By
placing resistor R1 between these two terminals, a con-
stant current is caused to flow through R1 and down
through R2 to set the overall output voltage. Normally this
current is chosen to be the specified minimum load
current of 10mA. Because I
ADJ
is very small and constant
when compared with the current through R1, it repre-
sents a small error and can usually be ignored. For fixed
voltage devices R1 and R2 are included in the device.
V
IN
IN
LT1086
ADJ
I
ADJ
50µA
V
OUT
= V
REF
1 + R2
R1
OUT
V
REF
R1
V
OUT
10µF
TANTALUM
+
(
)
R2
+ I
ADJ
R2
1086 • F01
Figure 1. Basic Adjustable Regulator
sn1086 1086ffs
9
LINER [ LINEAR TECHNOLOGY ]
