Datasheet

LT3090
17
3090fa
For more information www.linear.com/LT3090
applicaTions inForMaTion
Figure 8. Connections for Best Load Regulation
Figure 9. Floating 3-Terminal Adjustable Regulator
It is important to note that in a floating configuration and
with slow V
IN
ramp-up and ramp-down (as shown in
Figures 10 and 11), the LT3090 may exhibit oscillations
during start-up if SHDN is tied to V
IN
. This occurs because
the SHDN comparator’s turn-ON and turn-OFF thresholds
are referenced to the GND pin of LT3090. Since in floating
configuration the GND pin of LT3090 is tied to the OUT
pin, which is slowly increasing as V
IN
is ramping up, the
reference point for the SHDN comparator is changing;
hence, it causes start-up oscillations. This oscillation can
be minimized by placing at least 0.1µF and 15µF capacitor
at the SET and OUT pins, respectively—although it won’t
be eliminated, as per Figures 10 and 11 below. For fast V
IN
ramp-up and ramp-down the LT3090 does not oscillate.
If however, the SHDN pin is tied to a positive supply, 1.3V
and above (as shown in Figure 12), then there are no start-
up oscillations and a 4.7µF minimum output capacitor
can be used—but having some SET pin capacitance is
still
recommended. In addition to tying the GND pin to
the OUT pin (for floating configuration), the GND pin of
LT3090 can also be tied to a positive voltage as shown
in the next section.
Figure 10. Floating Mode: Input Supply Ramp-Up
Floating 3-Terminal Regulator
The LT3090’s rail-to-rail error amp allows the LDO to be
configured as a floating three-terminal regulator. With
proper protection, the LT3090 can be used in arbitrarily
high voltage applications. Figure 9 illustrates this configura-
tion. In this mode, the GND pin current is supplied by the
load; hence, a minimum 1mA load current is required to
maintain regulation. If true zero output voltage operation is
required, return the 1mA load current to a positive supply.
Note that in three terminal operation, the minimum input
voltage is now the device’s dropout voltage. Furthermore,
the ILIM pin is internally regulated to 225mV above IN.
This servo loop will current limit if ILIM is shorted to IN,
thereby causing LT3090’s quiescent current to increase
by about 300µA. Hence, when unused, it is recommended
to tie ILIM to IN through a 10k resistor.
Figure 11. Floating Mode: Input Supply Ramp-Down
V
IN
: –5V TO 0V
C
OUT
: 15µF, C
SET
: 0.1µF
V
OUT
: –3V TO 0V
I
L
: 600mA, SHDN = IN
1ms/DIV
V
IN
2V/DIV
V
OUT
1V/DIV
3090 F11
V
IN
: 0V TO –5V
C
OUT
: 15µF, C
SET
: 0.1µF
V
OUT
: 0V TO –3V
I
L
: 600mA, SHDN = IN
10ms/DIV
V
IN
2V/DIV
V
OUT
1V/DIV
3090 F10
LT3090
V
OUT
–2.5V
MAX I
OUT
600mA
PARASITIC
RESISTANCE
GND
ILIM
SHDN
3090 F08
–
+
50µA
SET
OUT
IMONN
C
IN
4.7µF
R
SET
49.9k
IN
V
IN
–3V TO –10V
IMONP
R
P
R
P
R
P
R
ILIM
10k
LOAD
C
OUT
4.7µF
LT3090
V
OUT
–15V
MAX I
OUT
600mA
GND
ILIM
SHDN
3090 F09
–
+
50µA
SET
OUT
IMONN
C
IN
4.7µF
R
SET
301k
IN
V
IN
–17V TO –22V
IMONP
R
ILIM
10k
C
OUT
15µF
0.1µF
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.