Datasheet
LT3090
18
3090fa
For more information www.linear.com/LT3090
applicaTions inForMaTion
GND Pin Versatility of LT3090
For applications requiring very low output voltages such
as below –1V, the minimum input voltage of –1.9V limits
how low V
IN
can drop before the device stops regulating. As
shown in Figure 13, this results in a much higher dropout
voltage set by the minimum V
IN
specification rather than
the actual dropout of the NPN pass device.
set below the LT3090’s –1.9V minimum input voltage. As
long as there is 1.9V between IN and GND pins of LT3090,
the minimum operating voltage is satisfied. Now it can
operate with much lower dropout voltage, with the device
dropout set by the pass device as illustrated in Figure 14.
Figure 12. Floating Mode: Input Supply Ramp-Up and Down
Using Positive SHDN
V
IN
: –5V TO 0V
C
OUT
: 15µF, C
SET
: 0.1µF
V
OUT
: –3V TO 0V
I
L
: 600mA, V
SHDN
= 1.5V
50ms/DIV
V
IN
2V/DIV
V
OUT
1V/DIV
3090 F12
Figure 13. Generating Very Low Output Voltages
Figure 14. Low Dropout Operation for Very Low Output Voltages
Note that if the LT3090’s SHDN capability is not desired,
then tie the SHDN pin to V
IN
. However, if it is desired to
turn the device ON and OFF, then the SHDN logic signal
needs to be referenced to the LT3090’s GND pin. A simple
way to achieve this is shown Figure 15, but the GND pin
needs to be at least +1.4V.
Figure 15. GND Pin Referenced SHDN Signal
A solution to this problem is available from the LT3090
architecture and the flexibility in how its GND pin can be
connected. The GND pin does not need to be connected
to system ground! It can be connected to a positive volt-
age as well. If the GND pin of LT3090 is tied to a positive
voltage that is at least 1.9V above V
IN
, then V
IN
can be
LT3090
GND
ILIM
SHDN
–
+
50µA
SET
OUT
IMONN
C
IN
4.7µF
R
SET
4.02k
IN
V
IN
–1.9V TO –7V
IMONP
10k
C
OUT
4.7µF
V
OUT
–0.2V
MAX I
OUT
600mA
R
ILIM
0.1µF
3090 F13
LT3090
GND
ILIM
SHDN
–
+
50µA
SET
OUT
IMONN
C
IN
4.7µF
R
SET
4.02k
IN
V
IN
–0.7V TO –7V
IMONP
10k
C
OUT
4.7µF
+1.2V OR HIGHER
V
OUT
–0.2V
MAX I
OUT
600mA
R
ILIM
C
GND
0.47µF
0.1µF
3090 F14
LT3090
GND
ILIM
SHDN
SHDN
(ACTIVE
LOW)
–
+
50µA
SET
OUT
IMONN
C
IN
4.7µF
R
SET
4.02k
IN
V
IN
–0.7V TO –7V
IMONP
10k
C
OUT
4.7µF
+1.4V OR HIGHER
V
OUT
–0.2V
MAX I
OUT
600mA
R
ILIM
C
GND
0.47µF
0.1µF
3090 F15
100k
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