LM2745 datasheet(10/23 Pages) NSC

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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

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

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

SD-IH and the slew rate of the voltage at the SD pin, SRSD:

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.

SD-IH is typically 1.08V and SRSD is the slew rate of the SD

pin voltage. The values of the sequencing divider resistors

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

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FIGURE 5. Tracking with Equal Slew Rates

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FIGURE 6. Sequencing Circuit

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