LT3506/LT3506A
U U
W
APPLICATIO S I FOR ATIO
supply or glitching the input supply when the LT3506 is
enabled. The RUN/SS pins do not provide an accurate
delaytostartoranaccuratelycontrolledrampattheoutput
voltage, both of which depend on the output capacitance
and the load current. However, the power good indicators
can be used to sequence the two outputs, as described
below.
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 first. Note that these circuits
sequence the outputs during start-up. When shut down
the two channels turn off simultaneously. In Figure 5a, a
largercapacitoronRUN/SS2delayschannel2withrespect
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 final 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 capaci-
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 final regulation voltage. Tie the PG pin to any
supply with a pull-up resistor that will supply less than
250μA. 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 is greater than ~2.4V.
IN
RUN/SS1
V
C2
RUN/SS1
OFF ON
1nF
1nF
OFF ON
LT3506
LT3506
RUN/SS2
GND
PG1
RUN/SS2
GND
2.2nF
(5a) Channel 2 is Delayed
(5b) Fewest Components
RUN/SS1
RUN/SS1
LT3506
1nF
LT3506
1nF
OFF ON
OFF ON
PG1
PG1
RUN/SS2
RUN/SS2
GND
GND
OFF2 ON2
1.5nF
1.5nF
3506 F05
(5c) Independent Control of Channel 2
Figure 5. Sequencing the Outputs
(5d) Doesn't Work !
3506afb
ꢀꢄ
Linear [ Linear ]
