LT1085_15 Datasheet, PDF (12/22 Pages) Linear Technology – 7.5A, 5A, 3A Low Dropout Positive Adjustable Regulators

LT1083/LT1084/LT1085
Applications Information
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low, such
as immediately after 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,
the power supply may need to be cycled down to zero and
brought up again to make the output recover.
Ripple Rejection
The typical curves for ripple rejection reflect values for a
bypassed adjustment 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 frequency should
be less than 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 25µF if R1 = 100Ω. At 10kHz only
0.22µF 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 example,
with the output voltage equal to 5V and no adjust pin capaci-
tor, 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 typical curve.
Output Voltage
The LT1083 develops a 1.25V reference voltage between
the output and the adjust terminal (see Figure 1). By
placing a resistor R1 between these two terminals, a
constant current is caused to flow through R1 and down
through R2 to set the overall output voltage. Normally this
current is the specified minimum load current of 10mA.
Because I_ADJ is very small and constant when compared
with the current through R1, it represents a small error
and can usually be ignored.
V_IN
IN LT1083 OUT
V_OUT
ADJ
V_REF R1
I_ADJ
50µA
( ) ^VOUT ^{= V}REF
1+
R2
R1
+ I_ADJ R2
R2
1083/4/5 ADJ F01
Figure 1. Basic Adjustable Regulator
Load Regulation
Because the LT1083 is a 3-terminal device, it is not pos-
sible to provide true remote load sensing. Load regulation
will be limited by the resistance of the wire connecting
the regulator to the load. The data sheet specification for
load regulation is measured at the bottom of the package.
Negative side sensing is a true Kelvin connection, with the
bottom of the output divider returned to the negative side
of the load. Although it may not be immediately obvious,
best load regulation is obtained when the top of the resis-
tor divider R1 is connected directly to the case not to the
load. This is illustrated in Figure 2. If R1 were connected
to the load, the effective resistance between the regulator
and the load would be:
R_P
•
⎛
⎝⎜
R2R+_1R1⎞⎠⎟_,RP
=
Parasitic
Line
Resistance
R_P
PARASITIC
LINE RESISTANCE
V_IN
IN LT1083 OUT
ADJ
R1*
R_L
R2*
*CONNECT R1 TO CASE
CONNECT R2 TO LOAD
1083/4/5 ADJ F02
Figure 2. Connections for Best Load Regulation
12
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