Datasheet

R
S1
SR
SD
= SR
OUT1
R
S1
+ R
S2
Master Power
Supply
V
OUT1
LM2743
R
S1
R
FB2
R
FB1
FB
V
OUT2
V
FB
SD
R
S2
5V
1.8V
1.8V
V
OUT1
V
OUT2
LM2743
SNVS276G –APRIL 2004–REVISED MARCH 2013 www.ti.com
For the example case of V
OUT1
= 5V and V
OUT2
= 1.8V, with R
T1
set to 150Ω as before, R
T2
is calculated from the
above equation to be 265Ω. A timing diagram for the case of equal slew rates is shown in Figure 25.
Figure 25. Tracking with Equal Slew Rates
SEQUENCING
The start up/soft-start of the LM2743 can be delayed for the purpose of sequencing by connecting a resistor
divider from the output of a master power supply to the SD pin, as shown in Figure 26.
Figure 26. Sequencing Circuit
A desired delay time t
DELAY
between the startup of the master supply output voltage and the LM2743 output
voltage can be set based on the SD pin low-to-high threshold V
SD-IH
and the slew rate of the voltage at the SD
pin, SR
SD
:
t
DELAY
= V
SD-IH
/ SR
SD
Note again, that in Figure 26, the LM2743’s output voltage has been represented symbolically as V
OUT2
, i.e.
without explicitly showing the power components.
V
SD-IH
is typically 1.08V and SR
SD
is the slew rate of the SD pin voltage. The values of the sequencing divider
resistors R
S1
and R
S2
set the SR
SD
based on the master supply output voltage slew rate, SR
OUT1
, using the
following equation:
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