Datasheet

LT3506/LT3506A
15
3506afc
supply or glitching the input supply when the LT3506 is
enabled. The RUN/SS pins do not provide an accurate
delay to start or an accurately controlled ramp at the
output voltage, both of which depend on the output ca-
pacitance and the load current. However, the power good
indicators can be used to sequence the two outputs, as
described below.
Power Good Indicators
The PG pin is the open collector output of an internal
comparator. PG remains low until the FB pin is within
10% of the fi nal regulation voltage. Tie the PG pin to any
supply with a pull-up resistor that will supply less than
250A. Note that this pin will be open when the LT3506 is
placed in shutdown mode (both RUN/SS pins at ground)
regardless of the voltage at the FB pin. Power good is valid
when the LT3506 is enabled (either RUN/SS pin is high)
and V
IN
is greater than ~2.4V.
Output Sequencing
The PG and RUN/SS pins can be used to sequence the
two outputs. Figure 5 shows several circuits to do this. In
each case channel 1 starts fi rst. Note that these circuits
sequence the outputs during start-up. When shut down
the two channels turn off simultaneously. In Figure 5a, a
larger capacitor on RUN/SS2 delays channel 2 with respect
to channel 1. The soft-start capacitor on RUN/SS2 should
be at least twice the value of the capacitor on RUN/SS1.
A larger ratio may be required, depending on the output
capacitance and load on each channel. Make sure to test
the circuit in the system before deciding on fi nal values
for these capacitors. The circuit in Figure 5b requires the
fewest components, with both channels sharing a single
soft-start capacitor. The power good comparator of chan-
nel 1 disables channel 2 until output 1 is in regulation.
For independent control of channel 2, use the circuit in
Figure 5c. The capacitor on RUN/SS1 is smaller than the
Figure 5. Sequencing the Outputs
OFF
3506 F05
RUN/SS1
PG1
ON
GND
OFF
RUN/SS1
LT3506 LT3506
ON
GND
RUN/SS2
OFF
RUN/SS1
ON
OFF2
ON2
GND
RUN/SS2
RUN/SS2
V
C2
PG1
1nF 1nF
1nF
2.2nF
1nF
1.5nF 1.5nF
(5b) Fewest Components
(5c) Independent Control of Channel 2
OFF
RUN/SS1
ON
GND
RUN/SS2
PG1
(5d) Doesn't Work !
(5a) Channel 2 is Delayed
LT3506 LT3506
APPLICATIONS INFORMATION