Datasheet
LT3042
14
3042fb
For more information www.linear.com/LT3042
APPLICATIONS INFORMATION
The LT3042 is a high performance low dropout linear
regulator featuring LTC’s ultralow noise (2nV/√Hz at
10kHz) and ultrahigh PSRR (79dB at 1MHz) architecture
for powering noise sensitive applications. Designed as a
precision current source followed by a high performance
rail-to-rail voltage buffer, the LT3042 can be easily paral
-
leled to further reduce noise, increase output current and
spread heat on the PCB. The device additionally features
programmable current limit, fast start-up capability and
programmable power good.
The LT3042 is easy to use and incorporates all of the
protection features expected in high performance regula
-
tors. Included are short-circuit protection, safe operating
area protection, reverse battery protection, reverse current
protection, and thermal shutdown with hysteresis.
Output Voltage
The LT3042 incorporates a precision 100µA current source
flowing out of the SET pin, which also ties to the error
amplifier’s inverting input. Figure1 illustrates that connect
-
ing a resistor from SET to ground generates a reference
voltage for
the error amplifier. This reference voltage is
simply the product of the SET pin current and the SET
pin resistor. The error amplifier’s unity-gain configuration
produces a low impedance version of this voltage on its
noninverting input, i.e. the OUTS pin, which is externally
tied to the OUT pin.
The LT3042’s rail-to-rail error amplifier and current refer
-
ence allows for a wide output voltage range from 0V (us-
ing a 0Ω resistor) to V
IN
minus dropout — up to 15V. A
PNP-based input pair is active for 0V to 0.6V output and an
NPN-based input pair is active for output voltages greater
than 1.3V, with a smooth transition between the two input
pairs from 0.6V to 1.3V output. While the NPN-based input
pair is designed to offer the best overall performance, refer
to the Electrical Characteristics Table for details on offset
voltage, SET pin current, output noise and PSRR variation
with the error amp input pair. Table 1 lists many common
output voltages and their corresponding 1% R
SET
resistors.
Table 1. 1% Resistor for Common Output Voltages
V
OUT
(V) R
SET
(kΩ)
2.5 24.9
3.3 33.2
5 49.9
12 121
15 150
The benefit of using a current reference compared with
a voltage reference as used in conventional regulators is
that the regulator always operates in unity gain configura
-
tion, independent of the programmed output voltage. This
allows the LT3042 to have loop gain, frequency response
and bandwidth independent of the output voltage. As a
result, noise, PSRR and transient performance do not
change with output voltage. Moreover, since none of the
error amp gain is needed to amplify the SET pin voltage
to a higher output voltage, output load regulation is more
tightly specified in the hundreds of microvolts range and
not as a fixed percentage of the output voltage.
Since the zero TC current source is highly accurate, the
SET pin resistor can become the limiting factor in achiev
-
ing high accuracy. Hence, it should be a precision resistor.
Additionally
, any leakage paths to or from the SET pin
create errors in the output voltage. If necessar
y, use high
quality insulation (e.g., Teflon, Kel-F); moreover, clean
-
ing of all insulating surfaces to remove fluxes and other
residues may be required. High humidity environments
may require a surface coating at the SET pin to provide
a moisture barrier
.
Minimize board leakage by encircling the SET pin with a
guard ring operated at a potential close to itself — ideally
tied to the OUT pin. Guarding both sides of the circuit
board is recommended. Bulk leakage reduction depends
on the guard ring width. Leakages of 100nA into or out of
+
–
100µA
IN
EN/UV
PGFB
GND
OUT
LT3042
ILIM PG
4.7µF
4.7µF
V
IN
5V ±5%
0.47µF
V
OUT,
3.3V
I
OUT(MAX)
, 200mA
3042 F01
33.2k
SET
OUTS
Figure1. Basic Adjustable Regulator
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