Datasheet

LT3055
19
3055f
For more information www.linear.com/LT3055
APPLICATIONS INFORMATION
two parallel conductors. In this case, the farther apart the
wires are from each other, the more the self-inductance is
reduced; up to a 50% reduction when placed a few inches
apart. Splitting the wires connects two equal inductors in
parallel, but placing them in close proximity creates mutual
inductance adding to the self-inductance. The second and
most effective way to reduce overall inductance is to place
both forward and return current conductors (the input and
GND wires) in very close proximity. Two 30-AWG wires
separated by only 0.02”, used as forward- and return-
current conductors, reduce the overall self-inductance
to approximately one-fifth that of a single isolated wire.
If a battery, mounted in close proximity, powers the LT3055,
a 10µF input capacitor suffices for stability. However, if a
distant supply powers the LT3055, use a larger value input
capacitor. Use a rough guideline of 1µF (in addition to the
10µF minimum) per 8 inches of wire length. The minimum
input capacitance needed to stabilize the application also
varies with power supply output impedance variations.
Placing additional capacitance on the LT3055’s output also
helps. However, this requires an order of magnitude more
capacitance in comparison with additional LT3055 input
bypassing. Series resistance between the supply and the
LT3055 input also helps stabilize the application; as little
as 0.1Ω to 0.5Ω suffices. This impedance dampens the
LC tank circuit at the expense of dropout voltage. A better
alternative is to use higher ESR tantalum or electrolytic ca-
pacitors at the LT3055 input in place of ceramic capacitors.
Paralleling Devices
Higher output current is obtained by paralleling multiple
LT3055 together. Tie the individual OUT pins together
and tie the individual IN pins together. An external NPN
or NMOS current mirror is used in combination with the
LT3055 I
MON
pins to create a simple amplifier. This ampli-
fier injects current into or out of the feedback divider of
the slave LT3055 in order to ensure that the I
MON
currents
from each LT3055 are equal.
In Figure 10, this is implemented using inexpensive 2N3904
NPN devices. Precision 1k resistors provide 1V emitter
degeneration at full load to guarantee good current mirror
matching. The feedback resistors of the slave LT3055 are
split into sections to ensure adequate headroom for the
slave 2N3904. A 1nF capacitor added to the I
MON
pin of
the slave device frequency compensates the feedback loop.
This circuit architecture is scalable to as many LT3055s
as are needed simply by extending the current mirror and
adding slave LT3055 devices.
+
–
500x
I
MON
600mV
REF
ADJ
LT3055 (MASTER)
IN
OUT
440k
10µF
V
OUT
5V
1A
1x
+
–
60k
+
–
500x
I
MON
600mV
REF
ADJ
LT3055 (SLAVE)
IN
10µF
V
IN
5.6V TO 45V
OUT
300k
1nF
2N3904
1x
+
–
140k
60k 1k 1k
3055 F10
Figure 10. Parallel Devices