Datasheet

LT3506/LT3506A
16
3506afc
capacitor on RUN/SS2. This allows the LT3506 to start up
and enable its power good comparator before RUN/SS2
gets high enough to allow channel 2 to start switching.
Channel 2 only operates when it is enabled with the external
control signals and output 1 is in regulation. The circuit in
Figure 5a leaves both power good indicates free. However,
the circuits in Figures 5b and 5c have another advantage.
As well as sequencing the two outputs at start-up, they
also disable channel 2 if output 1 falls out of regulation
(due to a short-circuit or a collapsing input voltage).
Finally, be aware that the circuit in Figure 5d does not
work, because the power good comparators are disabled in
shutdown. When the system is placed in shutdown mode
by pulling down on RUN/SS1, then output 1 will go low,
PG1 will pull down on RUN/SS2, and the LT3506 will enter
its low current shutdown state. This disables PG1, and
RUN/SS2 ramps up again to enable the LT3506. The circuit
will oscillate and pull extra current from the input.
Multiple Input Supplies
The internal supplies of the LT3506 operate from V
IN1
. It is
possible to supply V
IN2
from a different source, provided
V
IN1
is above the minimum supply level whenever V
IN2
is
present. This could be used when a system has two pri-
mary supplies available. It is more effi cient to generate the
desired outputs with the lowest step-down ratio possible.
For example, if a system has 18V and 5V power available
and needs to generate 12V and 2.5V, it would be more
effi cient to generate the 2.5V output from the 5V supply
and the 12V output from the 18V supply. The LT3506 can
step down 18V to 2.5V, but the effi ciency would be lower
than stepping down from 5V to 2.5V.
This feature can also be used when the maximum step-
down ratio is exceeded. In this case, V
IN2
can be tied to
V
OUT1
for applications requiring high V
IN
to V
OUT
ratios. A
dual step-down application steps down the input voltage
(V
IN1
) to the highest output voltage then uses that voltage
to power the second channel (V
IN2
). V
OUT1
must be able
to provide enough current for its output plus the average
current drawn from V
OUT2
. Note that the V
OUT1
must be
above minimum input voltage for V
IN2
when the second
channel starts to switch. Delaying the second channel can
be accomplished by either using independent soft-start
capacitors or sequencing with the PG1 output. The Two
Stage Step-Down circuit in the Applications section shows
an example of the latter approach.
Figure 6. Shorted Input Protection
V
IN
V
IN
V
OUT
SW
LT 3 5 0 6
D4
PARASITIC DIODE
3506 F06
V
IN
SW
GND
(7a)
V
IN
V
SW
C1 D1 C2
3506 F07
L1
SW
GND
(
7c
)
V
IN
SW
GND
(7b)
I
C1
Figure 7. Subtracting the Current when the Switch is ON (a) From the Current when the Switch in OFF (b) Reveals the Path
of the High Frequency Switching current (c) Keep This Loop Small. The Voltage on the SW and BOOST Nodes will also be
Switched; Keep these Nodes as Small as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane.
APPLICATIONS INFORMATION