LT3506 LINER [Linear Technology], LT3506 Datasheet - Page 15

• Manufacturer Part Number: LT3506
• Description: Dual Monolithic 1.6A Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

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
delay to start or an accurately controlled ramp at the output
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.
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
IN
OFF2 ON2
is greater than ~2.4V.
OFF
OFF
ON
ON
U
U
(5c) Independent Control of Channel 2
(5a) Channel 2 is Delayed
2.2nF
1.5nF
W
1nF
1nF
RUN/SS2
RUN/SS2
RUN/SS1
RUN/SS1
PG1
LT3506
LT3506
GND
GND
Figure 5. Sequencing the Outputs
U
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
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 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-
OFF
OFF
ON
ON
1nF
(5b) Fewest Components
1nF
1.5nF
(5d) Doesn't Work !
RUN/SS1
RUN/SS2
RUN/SS2
RUN/SS1
PG1
LT3506/LT3506A
LT3506
LT3506
GND
GND
PG1
3506 F05
V
C2

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