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LM2748MTC View Datasheet(PDF) - National ->Texas Instruments

Part NameLM2748MTC National-Semiconductor
National ->Texas Instruments National-Semiconductor
DescriptionSynchronous Buck Controller with Pre-bias Startup, and Optional Clock Synchronization
LM2748MTC Datasheet PDF : 23 Pages
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Application Information (Continued)
reach their target values at the same time. For this case, the
equation governing the values of the tracking divider resis-
tors RT1 and RT2 is:
The current through RT1 should be about 4 mA for precise
tracking. The final voltage of the SS/TRACK pin should be
set higher than the feedback voltage of 0.6V (say about
0.65V as in the above equation). If the master supply voltage
was 5V and the LM2745/8 output voltage was 1.8V, for
example, then the value of RT1 needed to give the two
supplies identical soft-start times would be 150. A timing
diagram for the equal soft-start time case is shown in Figure
4.
20137410
FIGURE 5. Tracking with Equal Slew Rates
SEQUENCING
The start up/soft-start of the LM2745/8 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 6.
20137408
FIGURE 4. Tracking with Equal Soft-Start Time
TRACKING A VOLTAGE SLEW RATE
The tracking feature can alternatively be used not to make
both rails reach regulation at the same time but rather to
have similar rise rates (in terms of output dV/dt). This
method ensures that the output voltage of the LM2745/8
always reaches regulation before the output voltage of the
master supply. In this case, the tracking resistors can be
determined based on the following equation:
For the example case of VOUT1 = 5V and VOUT2 = 1.8V, with
RT1 set to 150as before, RT2 is calculated from the above
equation to be 265. A timing diagram for the case of equal
slew rates is shown in Figure 5.
20137414
FIGURE 6. Sequencing Circuit
A desired delay time tDELAY between the startup of the
master supply output voltage and the LM2745/8 output volt-
age can be set based on the SD pin low-to-high threshold
VSD-IH and the slew rate of the voltage at the SD pin, SRSD:
tDELAY = VSD-IH / SRSD
Note again, that in Figure 6, the LM2745/8’s output voltage
has been represented symbolically as VOUT2, i.e. without
explicitly showing the power components.
VSD-IH is typically 1.08V and SRSD is the slew rate of the SD
pin voltage. The values of the sequencing divider resistors
RS1 and RS2 set the SRSD based on the master supply
output voltage slew rate, SROUT1, using the following equa-
tion:
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