LT3080-1
9
30801fc
For more information www.linear.com/LT3080-1
and a lower voltage supply for the collector. This increases
efficiency and reduces dissipation. To further spread the
heat, a resistor can be inserted in series with the collector
to move some of the heat out of the IC and spread it on
the PC board.
The LT3080-1 can be operated in two modes. Three terminal
mode has the control pin connected to the power input pin
which gives a limitation of 1.35V dropout. Alternatively,
the “control” pin can be tied to a higher voltage and the
power IN pin to a lower voltage giving 300mV dropout
on the IN pin and minimizing the power dissipation. This
allows for a 1.1A supply regulating from 2.5V
IN
to 1.8V
OUT
or 1.8V
IN
to 1.2V
OUT
with low dissipation.
Output Voltage
The LT3080-1 generates a 10µA reference current that
flows out of the SET pin. Connecting a resistor from SET
to ground generates a voltage that becomes the reference
point for the error amplifier (see Figure 1). The reference
voltage is a straight
multiplication of the SET pin current
and the value of the resistor. Any voltage can be generated
and there is no minimum output voltage for the regulator.
A minimum load current of 1mA is required to maintain
regulation regardless of output voltage. For true zero voltage
output operation, this 1mA load current must be returned
to a negative supply voltage.
With the low level current used to generate the reference
voltage, leakage paths to or from the SET pin can create
errors in the reference and output voltages. High quality
insulation should be used (e.g., Teflon, Kel-F); cleaning of all
insulating surfaces to remove fluxes and other residues will
probably be required. Surface coating may be necessary to
provide a moisture barrier in high humidity environments.
Board leakage can be minimized by encircling the SET
pin and circuitry with a guard ring operated at a potential
close to itself; the guard ring should be tied to the OUT
pin. Guarding both sides of the circuit board is required.
Bulk leakage reduction depends on the guard ring width.
Ten nanoamperes of leakage into or out of the SET pin and
associated circuitry creates a 0.1% error in the reference
voltage. Leakages of this magnitude, coupled with other
sources of leakage, can cause significant offset voltage
and reference drift, especially over the possible operating
temperature range.
If guardring techniques are used, this bootstraps any
stray capacitance at the SET pin. Since the SET pin is
a high impedance node, unwanted signals may couple
into the SET pin and cause erratic behavior. This will be
most noticeable when operating with minimum output
capacitors at full load current. The easiest way to remedy
this is to bypass the SET pin with a small amount of ca
-
pacitance from SET to ground, 10pF
to 20pF is sufficient.
Stability and Output Capacitance
The LT3080-1 requires an output capacitor for stability.
It is designed to be stable with most low ESR capacitors
(typically ceramic, tantalum or low ESR electrolytic). A
minimum output capacitor of 2.2µF with an ESR of 0.5Ω
or less is recommended to prevent oscillations.
Larger
values of output capacitance decrease peak
deviations
and provide improved transient response for larger load
current changes. Bypass capacitors, used to decouple
individual components powered by the LT3080-1, increase
the effective output capacitor value.
For improvement in transient performance, place a capaci
-
tor across the voltage setting resistor. Capacitors up to
1µ
F can be used. This bypass capacitor reduces system
noise as well, but start-up time is proportional to the time
constant of the voltage setting resistor (R
SET
in Figure 1)
and SET pin bypass capacitor.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
Figure 1. Basic Adjustable Regulator
+
–
LT3080-1
IN
V
CONTROL
V
CONTROL
OUT
30801 F01
SET
C
OUT
R
SET
V
OUT
C
SET
+
V
IN
+
25mΩ
applicaTions inFormaTion
