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LM2745MTC bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
tên linh kiện LM2745MTC
Giải thích chi tiết về linh kiện  Synchronous Buck Controller with Pre-bias Startup, and Optional Clock Synchronization
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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 R
T1 and RT2 is:
The current through R
T1 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 R
T1 needed to give the two
supplies identical soft-start times would be 150
Ω. A timing
diagram for the equal soft-start time case is shown in Figure
4.
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 V
OUT1 = 5V and VOUT2 = 1.8V, with
R
T1 set to 150
Ω as before, R
T2 is calculated from the above
equation to be 265
Ω. A timing diagram for the case of equal
slew rates is shown in Figure 5.
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.
A desired delay time t
DELAY 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
V
SD-IH and the slew rate of the voltage at the SD pin, SRSD:
t
DELAY =VSD-IH /SRSD
Note again, that in Figure 6, the LM2745/8’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 SRSD is the slew rate of the SD
pin voltage. The values of the sequencing divider resistors
R
S1 and RS2 set the SRSD based on the master supply
output voltage slew rate, SR
OUT1, using the following equa-
tion:
20137408
FIGURE 4. Tracking with Equal Soft-Start Time
20137410
FIGURE 5. Tracking with Equal Slew Rates
20137414
FIGURE 6. Sequencing Circuit
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