Datasheet

LT3090
12
3090fa
For more information www.linear.com/LT3090
The LT3090 is a 600mA, rail-to-rail output, negative low
dropout linear regulator featuring very low output noise,
high bandwidth, precision programmable current limit,
precision positive or negative current monitor, and bi
-
directional shutdown.
The LT3090 supplies 600mA at a
typical dropout voltage of 300mV. Unlike other devices,
the LT3090 does not require a separate supply to achieve
low dropout performance. The 1mA quiescent current
drops to well below 1µA in shutdown.
The LT3090 is easy to use and incorporates all of the pro
-
tection features expected in high performance regulators.
Included are short circuit protection, safe operating area
protection, as well as thermal shutdown with hysteresis.
In bipolar supply applications where the regulator’s load
is returned to a positive supply, OUT can be pulled above
GND up to 36V and still allow the LT3090 to safely startup.
Output Voltage
The LT3090 incorporates a zero TC 50µA reference cur
-
rent sour
ce that flows into the SET pin. The SET pin is
the
inverting input of the error amp. 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 resistor value (Ohm’s Law, V = I • R). The
rail-to-rail error amp’s unity gain configuration produces
a low impedance voltage on its noninverting input, i.e.
the OUT pin. Output voltage is programmable from 0V
(using zero Ω resistor) to V
IN
plus dropout. Table 1 lists
many common output voltages and its corresponding 1%
RSET resistance.
Table 1. 1% Resistor for Common Output Voltages
V
OUT
(V) R
SET
(kΩ)
–2.5 49.9
–3 60.4
–3.3 66.5
–5 100
–12 243
–15 301
applicaTions inForMaTion
The benefits of using a current reference, as opposed to a
voltage reference as in conventional regulators such as the
LT1185, LT1175, LT1964 and LT3015, is that the device
always operates in unity gain configuration – regardless of
the programmed output voltage. This allows the LT3090
to have loop gain, frequency response, and bandwidth
independent of the output voltage. Moreover, none of the
error amp gain is needed to amplify the set pin voltage to
a higher output voltage (in magnitude). As a result, output
load regulation is specified in terms of millivolts and not
a fixed percentage of the output voltage.
Since the zero TC current source is very accurate, the SET
pin resistor is the limiting factor in achieving high accu
-
racy; hence, it must be a precision resistor. Moreover, any
leakage
paths to and from the SET pin create errors in the
output voltage. If necessary, use high quality insulation (e.g.
Teflon, Kel-F); moreover, cleaning 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
the guard ring should be tied to the OUT pin. Guarding
both sides of the circuit board is required. Bulk leakage
Figure 1. Basic Adjustable Regulator
LT3090
V
OUT
–2.5V
MAX I
OUT
600mA
SET GND
ILIM
SHDN
3090 F01
–
+
50µA
IMONN
OUT
0.1µF
C
IN
4.7µF
R
SET
49.9k
IN
V
IN
–3V TO –10V
R
ILIM
IMONP
C
OUT
4.7µF
10k
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